[musl] [PATCH] pack posix_spawn file actions into extraneous padding

[musl] [PATCH] pack posix_spawn file actions into extraneous padding

[Joakim Sindholt]

[-- Attachment #1: Type: text/plain, Size: 436 bytes --]

I've written this patch that packs posix_spawn file actions into the
posix_spawn_file_actions struct padding. I'm not sure if the way I've
split the code up is desirable. Maybe the faexpand() function needs a
different name and to be in its own .c file; or maybe a different
approach is preferable.

I've only lightly tested it but it seems to work just fine as-is. The
whole explanation of how it works can be found in the patch file.

[-- Attachment #2: 0001-pack-posix_spawn-file-actions-into-extraneous-paddin.patch --]
[-- Type: text/x-diff, Size: 11602 bytes --]

From 94c650b7d6df4ee6592f6d44977b461fc19df4f1 Mon Sep 17 00:00:00 2001
From: Joakim Sindholt <opensource@zhasha.com>
Date: Sat, 29 Jan 2022 14:05:23 +0100
Subject: [PATCH] pack posix_spawn file actions into extraneous padding

The posix_spawn_file_actions struct contains 2 int paddings of 2 and 16
ints respectively, making them ideal for packing file actions into.
The encoding used is simple: __pad0[0] contains a count and __pad
contains the actuals elements. The most important actions, dup2 and
close, are encoded as 2 and 1 int respectively.

If the first element of a sequence is negative, then it's a close and
processing stops there.
If the second element of a sequence is positive, then it's a dup2 and
processing stops there.
If the second element is negative, however, then it contains a command
and requires per-command specific processing.
In total there are 5 file actions. Their encodings are:

close: -fd-1
dup2: fd, srcfd
fchdir: fd, -FDOP_FCHDIR
chdir: 0, -FDOP_CHDIR
open: fd, -FDOP_OPEN, flags, mode

Some file actions need a string. When a string is needed it's allocated
on the tail end of a new file_actions struct. When a string is needed or
an action requires more space than is available then a new struct is
allocated and __actions is set to point to it. All the elements of the
first file_actions struct is copies to the new one. The new file_actions
struct, being the second in the list, always points to the last
file_actions struct in the list. From then on the structs all point back
in reverse order until they reach the second one again, creating a loop.
This rube-goldberg machine exists because there's nowhere to store a
last pointer for appending new actions efficiently.
As a result of this backwards allocation, the actions requiring strings
aren't stored in the same struct as the strings themselves. Rather they
get stored in the logically previous struct when reading.

This packing is far from optimal but it is very simple and it allows for
up to 16 close, 8 dup2, or any combination of them, without needing to
call malloc. At worst it calls malloc as much as before.
---
 src/process/fdop.h                            | 24 +++++--
 src/process/posix_spawn.c                     | 67 ++++++++++++++-----
 .../posix_spawn_file_actions_addchdir.c       | 13 ++--
 .../posix_spawn_file_actions_addclose.c       | 10 +--
 .../posix_spawn_file_actions_adddup2.c        | 12 ++--
 .../posix_spawn_file_actions_addfchdir.c      | 11 ++-
 .../posix_spawn_file_actions_addopen.c        | 18 +++--
 .../posix_spawn_file_actions_destroy.c        | 15 +++--
 src/process/posix_spawn_file_actions_init.c   |  1 +
 9 files changed, 102 insertions(+), 69 deletions(-)

diff --git a/src/process/fdop.h b/src/process/fdop.h
index 7cf733b2..4b25363c 100644
--- a/src/process/fdop.h
+++ b/src/process/fdop.h
@@ -4,14 +4,24 @@
 #define FDOP_CHDIR 4
 #define FDOP_FCHDIR 5
 
-struct fdop {
-	struct fdop *next, *prev;
-	int cmd, fd, srcfd, oflag;
-	mode_t mode;
-	char path[];
-};
-
 #define malloc __libc_malloc
 #define calloc __libc_calloc
 #define realloc undef
 #define free __libc_free
+
+static inline int faexpand(posix_spawn_file_actions_t *fa, size_t str)
+{
+	posix_spawn_file_actions_t *n, *blk;
+	blk = malloc(sizeof(*blk)+str);
+	if (!blk) return -1;
+	*blk = *fa;
+	if (fa->__actions) {
+		n = fa->__actions;
+		blk->__actions = n->__actions;
+		n->__actions = blk;
+	} else
+		blk->__actions = blk;
+	fa->__actions = blk;
+	fa->__pad0[0] = 0;
+	return 0;
+}
diff --git a/src/process/posix_spawn.c b/src/process/posix_spawn.c
index 728551b3..42bd4adb 100644
--- a/src/process/posix_spawn.c
+++ b/src/process/posix_spawn.c
@@ -81,33 +81,65 @@ static int child(void *args_vp)
 		    (ret=__syscall(SYS_setuid, __syscall(SYS_getuid))) )
 			goto fail;
 
-	if (fa && fa->__actions) {
-		struct fdop *op;
-		int fd;
-		for (op = fa->__actions; op->next; op = op->next);
-		for (; op; op = op->prev) {
+	if (fa) {
+		const posix_spawn_file_actions_t *first;
+		const char *path;
+		int fd, srcfd, flags, cmd, i = 0;
+		mode_t mode;
+
+		first = fa;
+		if (fa->__actions) {
+			fa = fa->__actions;
+			fa = fa->__actions;
+		}
+		while (fa) {
+			if (i == fa->__pad0[0]) {
+				if (fa == first)
+					break;
+				path = (char *)(fa+1);
+				fa = fa==first->__actions ? first : fa->__actions;
+				i = 0;
+				continue;
+			}
+			fd = fa->__pad[i++];
+			if (fd < 0) {
+				fd = -fd-1;
+				cmd = FDOP_CLOSE;
+			} else switch ((cmd = -fa->__pad[i++])) {
+			case FDOP_OPEN:
+				flags = fa->__pad[i++];
+				mode = (mode_t)fa->__pad[i++];
+				break;
+			case FDOP_CHDIR:
+				fd = -1;
+				break;
+			case FDOP_FCHDIR:
+				break;
+			default:
+				srcfd = -cmd;
+				cmd = FDOP_DUP2;
+			}
 			/* It's possible that a file operation would clobber
 			 * the pipe fd used for synchronizing with the
 			 * parent. To avoid that, we dup the pipe onto
 			 * an unoccupied fd. */
-			if (op->fd == p) {
+			if (fd == p) {
 				ret = __syscall(SYS_dup, p);
 				if (ret < 0) goto fail;
 				__syscall(SYS_close, p);
 				p = ret;
 			}
-			switch(op->cmd) {
+			switch(cmd) {
 			case FDOP_CLOSE:
-				__syscall(SYS_close, op->fd);
+				__syscall(SYS_close, fd);
 				break;
 			case FDOP_DUP2:
-				fd = op->srcfd;
-				if (fd == p) {
+				if (srcfd == p) {
 					ret = -EBADF;
 					goto fail;
 				}
-				if (fd != op->fd) {
-					if ((ret=__sys_dup2(fd, op->fd))<0)
+				if (srcfd != fd) {
+					if ((ret=__sys_dup2(srcfd, fd))<0)
 						goto fail;
 				} else {
 					ret = __syscall(SYS_fcntl, fd, F_GETFD);
@@ -118,20 +150,21 @@ static int child(void *args_vp)
 				}
 				break;
 			case FDOP_OPEN:
-				fd = __sys_open(op->path, op->oflag, op->mode);
+				srcfd = fd;
+				fd = __sys_open(path, flags, mode);
 				if ((ret=fd) < 0) goto fail;
-				if (fd != op->fd) {
-					if ((ret=__sys_dup2(fd, op->fd))<0)
+				if (fd != srcfd) {
+					if ((ret=__sys_dup2(fd, srcfd))<0)
 						goto fail;
 					__syscall(SYS_close, fd);
 				}
 				break;
 			case FDOP_CHDIR:
-				ret = __syscall(SYS_chdir, op->path);
+				ret = __syscall(SYS_chdir, path);
 				if (ret<0) goto fail;
 				break;
 			case FDOP_FCHDIR:
-				ret = __syscall(SYS_fchdir, op->fd);
+				ret = __syscall(SYS_fchdir, fd);
 				if (ret<0) goto fail;
 				break;
 			}
diff --git a/src/process/posix_spawn_file_actions_addchdir.c b/src/process/posix_spawn_file_actions_addchdir.c
index 7f2590ae..1aac8198 100644
--- a/src/process/posix_spawn_file_actions_addchdir.c
+++ b/src/process/posix_spawn_file_actions_addchdir.c
@@ -6,13 +6,10 @@
 
 int posix_spawn_file_actions_addchdir_np(posix_spawn_file_actions_t *restrict fa, const char *restrict path)
 {
-	struct fdop *op = malloc(sizeof *op + strlen(path) + 1);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_CHDIR;
-	op->fd = -1;
-	strcpy(op->path, path);
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	size_t len = strlen(path);
+	if (faexpand(fa, len + 1) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = 0;
+	fa->__pad[fa->__pad0[0]++] = -FDOP_CHDIR;
+	memcpy((posix_spawn_file_actions_t *)fa->__actions + 1, path, len + 1);
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addclose.c b/src/process/posix_spawn_file_actions_addclose.c
index 0c2ef8fa..c6942fc0 100644
--- a/src/process/posix_spawn_file_actions_addclose.c
+++ b/src/process/posix_spawn_file_actions_addclose.c
@@ -6,12 +6,8 @@
 int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t *fa, int fd)
 {
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_CLOSE;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (sizeof(fa->__pad)/sizeof(*fa->__pad) - fa->__pad0[0] < 1)
+		if (faexpand(fa, 0) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = -fd-1;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_adddup2.c b/src/process/posix_spawn_file_actions_adddup2.c
index addca4d4..b2945f56 100644
--- a/src/process/posix_spawn_file_actions_adddup2.c
+++ b/src/process/posix_spawn_file_actions_adddup2.c
@@ -6,13 +6,9 @@
 int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t *fa, int srcfd, int fd)
 {
 	if (srcfd < 0 || fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_DUP2;
-	op->srcfd = srcfd;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (sizeof(fa->__pad)/sizeof(*fa->__pad) - fa->__pad0[0] < 2)
+		if (faexpand(fa, 0) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = fd;
+	fa->__pad[fa->__pad0[0]++] = srcfd;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addfchdir.c b/src/process/posix_spawn_file_actions_addfchdir.c
index e89ede8c..a62a35fa 100644
--- a/src/process/posix_spawn_file_actions_addfchdir.c
+++ b/src/process/posix_spawn_file_actions_addfchdir.c
@@ -7,12 +7,9 @@
 int posix_spawn_file_actions_addfchdir_np(posix_spawn_file_actions_t *fa, int fd)
 {
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_FCHDIR;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (sizeof(fa->__pad)/sizeof(*fa->__pad) - fa->__pad0[0] < 2)
+		if (faexpand(fa, 0) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = fd;
+	fa->__pad[fa->__pad0[0]++] = -FDOP_FCHDIR;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addopen.c b/src/process/posix_spawn_file_actions_addopen.c
index 82bbcec9..1c419396 100644
--- a/src/process/posix_spawn_file_actions_addopen.c
+++ b/src/process/posix_spawn_file_actions_addopen.c
@@ -6,16 +6,14 @@
 
 int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t *restrict fa, int fd, const char *restrict path, int flags, mode_t mode)
 {
+	size_t len;
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op + strlen(path) + 1);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_OPEN;
-	op->fd = fd;
-	op->oflag = flags;
-	op->mode = mode;
-	strcpy(op->path, path);
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	len = strlen(path);
+	if (faexpand(fa, len + 1) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = fd;
+	fa->__pad[fa->__pad0[0]++] = -FDOP_OPEN;
+	fa->__pad[fa->__pad0[0]++] = flags;
+	fa->__pad[fa->__pad0[0]++] = mode;
+	memcpy((posix_spawn_file_actions_t *)fa->__actions + 1, path, len + 1);
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_destroy.c b/src/process/posix_spawn_file_actions_destroy.c
index 3251babb..514d2344 100644
--- a/src/process/posix_spawn_file_actions_destroy.c
+++ b/src/process/posix_spawn_file_actions_destroy.c
@@ -4,11 +4,16 @@
 
 int posix_spawn_file_actions_destroy(posix_spawn_file_actions_t *fa)
 {
-	struct fdop *op = fa->__actions, *next;
-	while (op) {
-		next = op->next;
-		free(op);
-		op = next;
+	posix_spawn_file_actions_t *next, *p;
+	if (fa->__actions) {
+		p = fa->__actions;
+		p = p->__actions;
+		while (p != fa->__actions) {
+			next = p->__actions;
+			free(p);
+			p = next;
+		}
+		free(fa->__actions);
 	}
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_init.c b/src/process/posix_spawn_file_actions_init.c
index 89d5e127..49baf889 100644
--- a/src/process/posix_spawn_file_actions_init.c
+++ b/src/process/posix_spawn_file_actions_init.c
@@ -2,6 +2,7 @@
 
 int posix_spawn_file_actions_init(posix_spawn_file_actions_t *fa)
 {
+	fa->__pad0[0] = 0;
 	fa->__actions = 0;
 	return 0;
 }
-- 
2.26.3

Re: [musl] [PATCH] pack posix_spawn file actions into extraneous padding

[Joakim Sindholt]

[-- Attachment #1: Type: text/plain, Size: 2941 bytes --]

On Sat, Jan 29, 2022 at 02:43:28PM +0100, Joakim Sindholt wrote:
> I've written this patch that packs posix_spawn file actions into the
> posix_spawn_file_actions struct padding. I'm not sure if the way I've
> split the code up is desirable. Maybe the faexpand() function needs a
> different name and to be in its own .c file; or maybe a different
> approach is preferable.

Here's a candidate v2 patch to simplify some things. Changes since the
first one:
* fa->__actions is initialized to point to itself. This makes faexpand a
  little simpler and means that ->__actions->__actions always points to
  the last file_actions struct.
* The previous version caused an increase of 32 bytes in used stack
  space in child() on amd64. This one only causes a 16 byte increase.
  I'm not sure how big of a deal-breaker that is.
* The two switches have been merged into one. This makes it simpler IMO
  but also has some consequences:

The current implementation in musl uses a pipe to communicate
success/failure back to the parent process. This means that the
file_actions struct can accidentally succeed if it tries to use the pipe
fd as a source fd or worse, overwrite the pipe fd when targeting it in
either the dup2 or open actions.
Musl solves this by moving the pipe fd if it's the target of an
operation. Currently musl will move the pipe if it's the source fd of
close or fchdir, or if it's the target fd of dup2 or open. You'll note
that the close and fchdir cases mean that instead of failing
immediately, it will dup the pipe fd, close the old pipe fd, and then do
the actual close/fchdir syscall which then fails. It's presumably
implemented this way because it's simple and doesn't really cause a
problem. The operation will result in failure regardless.

This patch, in an attempt to be very simple, introduces a similar but
perhaps slightly worse bug. The encoding of every operation is
essentially [fd, other stuff], meaning it can do the pipe move check on
the fd in the first slot. It won't work for close since the fd is
actually -fd-1, so close gets its own check. If the check is
unconditional then we might get an unnecessary move with chdir, which is
encoded as [dummy fd, -FDOP_CHDIR]. I've chosen an fd that's unlikely to
collide (INT_MAX) however if you have every fd from 0 to INT_MAX-1 open
then the pipe fd will be on fd INT_MAX and it will not be movable. The
result is that the unnecessary pipe move fails and an otherwise
perfectly legal and possible chdir action will fail.

The fix is very simple but does end up once again causing some
duplication in the op logic:
-if (fd == p) {
+if (fd == p && fa->__pad[i] != -FDOP_CHDIR) {

Alternatively you could do the whole command check twice, only move the
fd if it's the target of dup2 or open, and then have a check in fchdir
for whether it's trying to use the pipe fd as a source, like the ones
already in close and dup2.

Or maybe this bug is acceptable.

Joakim

[-- Attachment #2: 0001-pack-posix_spawn-file-actions-into-extraneous-paddin.patch --]
[-- Type: text/x-diff, Size: 11781 bytes --]

From 9f20a4d5daa19f4508bb18f28e06226b80d49b56 Mon Sep 17 00:00:00 2001
From: Joakim Sindholt <opensource@zhasha.com>
Date: Fri, 4 Feb 2022 19:42:14 +0100
Subject: [PATCH] pack posix_spawn file actions into extraneous padding

The posix_spawn_file_actions struct contains 2 int paddings of 2 and 16
ints respectively, making them ideal for packing file actions into.
The encoding used is simple: __pad0[0] contains a count and __pad
contains the actuals elements. The most important actions, dup2 and
close, are encoded as 2 and 1 int respectively.

If the first element of a sequence is negative, then it's a close and
processing stops there.
If the second element of a sequence is positive, then it's a dup2 and
processing stops there.
If the second element is negative, however, then it contains a command
and requires per-command specific processing.
In total there are 5 file actions. Their encodings are:

close: -fd-1
dup2: fd, srcfd
fchdir: fd, -FDOP_FCHDIR
chdir: INT_MAX, -FDOP_CHDIR
open: fd, -FDOP_OPEN, flags, mode

Some file actions need a string. When a string is needed it's allocated
on the tail end of a new file_actions struct. When a string is needed or
an action requires more space than is available then a new struct is
allocated and __actions is set to point to it. All the elements of the
first file_actions struct are copied to the new one. The new
file_actions struct, being the second in the list, always points to the
last file_actions struct in the list. From then on the structs all point
back in reverse order until they reach the second one again, creating a
loop. This rube-goldberg machine exists because there's nowhere to store
a last pointer for appending new actions efficiently.
As a result of this backwards allocation, the actions requiring strings
aren't stored in the same struct as the strings themselves. Rather the
string gets stored in the logically previous struct when reading.

This packing is far from optimal but it is very simple and it allows for
up to 16 close, 8 dup2, or any combination of them, without needing to
call malloc. At worst it calls malloc as much as before.
---
 src/process/fdop.h                            | 21 ++++--
 src/process/posix_spawn.c                     | 66 ++++++++++++-------
 .../posix_spawn_file_actions_addchdir.c       | 14 ++--
 .../posix_spawn_file_actions_addclose.c       | 10 +--
 .../posix_spawn_file_actions_adddup2.c        | 12 ++--
 .../posix_spawn_file_actions_addfchdir.c      | 11 ++--
 .../posix_spawn_file_actions_addopen.c        | 18 +++--
 .../posix_spawn_file_actions_destroy.c        | 11 ++--
 src/process/posix_spawn_file_actions_init.c   |  3 +-
 9 files changed, 91 insertions(+), 75 deletions(-)

diff --git a/src/process/fdop.h b/src/process/fdop.h
index 7cf733b2..65c3c9f4 100644
--- a/src/process/fdop.h
+++ b/src/process/fdop.h
@@ -4,14 +4,21 @@
 #define FDOP_CHDIR 4
 #define FDOP_FCHDIR 5
 
-struct fdop {
-	struct fdop *next, *prev;
-	int cmd, fd, srcfd, oflag;
-	mode_t mode;
-	char path[];
-};
-
 #define malloc __libc_malloc
 #define calloc __libc_calloc
 #define realloc undef
 #define free __libc_free
+
+static inline int faexpand(posix_spawn_file_actions_t *fa, size_t str)
+{
+	posix_spawn_file_actions_t *n, *blk;
+	blk = malloc(sizeof *blk + str);
+	if (!blk) return -1;
+	*blk = *fa;
+	n = fa->__actions;
+	fa->__actions = blk;
+	blk->__actions = n->__actions;
+	n->__actions = blk;
+	fa->__pad0[0] = 0;
+	return 0;
+}
diff --git a/src/process/posix_spawn.c b/src/process/posix_spawn.c
index 728551b3..c00ab0b4 100644
--- a/src/process/posix_spawn.c
+++ b/src/process/posix_spawn.c
@@ -81,33 +81,53 @@ static int child(void *args_vp)
 		    (ret=__syscall(SYS_setuid, __syscall(SYS_getuid))) )
 			goto fail;
 
-	if (fa && fa->__actions) {
-		struct fdop *op;
-		int fd;
-		for (op = fa->__actions; op->next; op = op->next);
-		for (; op; op = op->prev) {
+	if (fa) {
+		const posix_spawn_file_actions_t *first = fa;
+		const char *path;
+		int fd, srcfd, i = 0;
+
+		/* The list of actions, in reverse order, looks like this:
+		 * [0]->[1]<-[2]<-[3]
+		 *        \_______/^
+		 * so to get to the first action we to go to [0]->next->next
+		 * and to get all the way back we have to special case the
+		 * transition from [1]->[0] */
+		fa = fa->__actions;
+		fa = fa->__actions;
+		while (!(fa == first && i == fa->__pad0[0])) {
+			if (i == fa->__pad0[0]) {
+				path = (char *)(fa+1);
+				fa = fa==first->__actions ? first : fa->__actions;
+				i = 0;
+				continue;
+			}
+			fd = fa->__pad[i++];
+
 			/* It's possible that a file operation would clobber
 			 * the pipe fd used for synchronizing with the
 			 * parent. To avoid that, we dup the pipe onto
 			 * an unoccupied fd. */
-			if (op->fd == p) {
+			if (fd == p) {
 				ret = __syscall(SYS_dup, p);
 				if (ret < 0) goto fail;
 				__syscall(SYS_close, p);
 				p = ret;
 			}
-			switch(op->cmd) {
-			case FDOP_CLOSE:
-				__syscall(SYS_close, op->fd);
-				break;
-			case FDOP_DUP2:
-				fd = op->srcfd;
-				if (fd == p) {
+
+			if (fd < 0) { /* FDOP_CLOSE */
+				if (-fd-1 == p) {
+					ret = -EBADF;
+					goto fail;
+				}
+				__syscall(SYS_close, -fd-1);
+			} else switch (-(srcfd = fa->__pad[i++])) {
+			default: /* FDOP_DUP2 */
+				if (srcfd == p) {
 					ret = -EBADF;
 					goto fail;
 				}
-				if (fd != op->fd) {
-					if ((ret=__sys_dup2(fd, op->fd))<0)
+				if (srcfd != fd) {
+					if ((ret=__sys_dup2(srcfd, fd))<0)
 						goto fail;
 				} else {
 					ret = __syscall(SYS_fcntl, fd, F_GETFD);
@@ -118,20 +138,22 @@ static int child(void *args_vp)
 				}
 				break;
 			case FDOP_OPEN:
-				fd = __sys_open(op->path, op->oflag, op->mode);
-				if ((ret=fd) < 0) goto fail;
-				if (fd != op->fd) {
-					if ((ret=__sys_dup2(fd, op->fd))<0)
+				srcfd = __sys_open(path, fa->__pad[i],
+				                   fa->__pad[i+1]);
+				if ((ret=srcfd) < 0) goto fail;
+				if (srcfd != fd) {
+					if ((ret=__sys_dup2(srcfd, fd))<0)
 						goto fail;
-					__syscall(SYS_close, fd);
+					__syscall(SYS_close, srcfd);
 				}
+				i += 2;
 				break;
 			case FDOP_CHDIR:
-				ret = __syscall(SYS_chdir, op->path);
+				ret = __syscall(SYS_chdir, path);
 				if (ret<0) goto fail;
 				break;
 			case FDOP_FCHDIR:
-				ret = __syscall(SYS_fchdir, op->fd);
+				ret = __syscall(SYS_fchdir, fd);
 				if (ret<0) goto fail;
 				break;
 			}
diff --git a/src/process/posix_spawn_file_actions_addchdir.c b/src/process/posix_spawn_file_actions_addchdir.c
index 7f2590ae..01dc52bd 100644
--- a/src/process/posix_spawn_file_actions_addchdir.c
+++ b/src/process/posix_spawn_file_actions_addchdir.c
@@ -2,17 +2,15 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <limits.h>
 #include "fdop.h"
 
 int posix_spawn_file_actions_addchdir_np(posix_spawn_file_actions_t *restrict fa, const char *restrict path)
 {
-	struct fdop *op = malloc(sizeof *op + strlen(path) + 1);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_CHDIR;
-	op->fd = -1;
-	strcpy(op->path, path);
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	size_t len = strlen(path);
+	if (faexpand(fa, len + 1) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = INT_MAX;
+	fa->__pad[fa->__pad0[0]++] = -FDOP_CHDIR;
+	memcpy((posix_spawn_file_actions_t *)fa->__actions + 1, path, len + 1);
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addclose.c b/src/process/posix_spawn_file_actions_addclose.c
index 0c2ef8fa..c6942fc0 100644
--- a/src/process/posix_spawn_file_actions_addclose.c
+++ b/src/process/posix_spawn_file_actions_addclose.c
@@ -6,12 +6,8 @@
 int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t *fa, int fd)
 {
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_CLOSE;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (sizeof(fa->__pad)/sizeof(*fa->__pad) - fa->__pad0[0] < 1)
+		if (faexpand(fa, 0) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = -fd-1;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_adddup2.c b/src/process/posix_spawn_file_actions_adddup2.c
index addca4d4..b2945f56 100644
--- a/src/process/posix_spawn_file_actions_adddup2.c
+++ b/src/process/posix_spawn_file_actions_adddup2.c
@@ -6,13 +6,9 @@
 int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t *fa, int srcfd, int fd)
 {
 	if (srcfd < 0 || fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_DUP2;
-	op->srcfd = srcfd;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (sizeof(fa->__pad)/sizeof(*fa->__pad) - fa->__pad0[0] < 2)
+		if (faexpand(fa, 0) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = fd;
+	fa->__pad[fa->__pad0[0]++] = srcfd;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addfchdir.c b/src/process/posix_spawn_file_actions_addfchdir.c
index e89ede8c..a62a35fa 100644
--- a/src/process/posix_spawn_file_actions_addfchdir.c
+++ b/src/process/posix_spawn_file_actions_addfchdir.c
@@ -7,12 +7,9 @@
 int posix_spawn_file_actions_addfchdir_np(posix_spawn_file_actions_t *fa, int fd)
 {
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_FCHDIR;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (sizeof(fa->__pad)/sizeof(*fa->__pad) - fa->__pad0[0] < 2)
+		if (faexpand(fa, 0) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = fd;
+	fa->__pad[fa->__pad0[0]++] = -FDOP_FCHDIR;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addopen.c b/src/process/posix_spawn_file_actions_addopen.c
index 82bbcec9..1c419396 100644
--- a/src/process/posix_spawn_file_actions_addopen.c
+++ b/src/process/posix_spawn_file_actions_addopen.c
@@ -6,16 +6,14 @@
 
 int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t *restrict fa, int fd, const char *restrict path, int flags, mode_t mode)
 {
+	size_t len;
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op + strlen(path) + 1);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_OPEN;
-	op->fd = fd;
-	op->oflag = flags;
-	op->mode = mode;
-	strcpy(op->path, path);
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	len = strlen(path);
+	if (faexpand(fa, len + 1) != 0) return ENOMEM;
+	fa->__pad[fa->__pad0[0]++] = fd;
+	fa->__pad[fa->__pad0[0]++] = -FDOP_OPEN;
+	fa->__pad[fa->__pad0[0]++] = flags;
+	fa->__pad[fa->__pad0[0]++] = mode;
+	memcpy((posix_spawn_file_actions_t *)fa->__actions + 1, path, len + 1);
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_destroy.c b/src/process/posix_spawn_file_actions_destroy.c
index 3251babb..dbb9ac52 100644
--- a/src/process/posix_spawn_file_actions_destroy.c
+++ b/src/process/posix_spawn_file_actions_destroy.c
@@ -4,11 +4,12 @@
 
 int posix_spawn_file_actions_destroy(posix_spawn_file_actions_t *fa)
 {
-	struct fdop *op = fa->__actions, *next;
-	while (op) {
-		next = op->next;
-		free(op);
-		op = next;
+	posix_spawn_file_actions_t *next, *p = fa->__actions;
+	for (p = p->__actions; p != fa->__actions; p = next) {
+		next = p->__actions;
+		free(p);
 	}
+	if (p != fa)
+		free(p);
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_init.c b/src/process/posix_spawn_file_actions_init.c
index 89d5e127..ee9c8687 100644
--- a/src/process/posix_spawn_file_actions_init.c
+++ b/src/process/posix_spawn_file_actions_init.c
@@ -2,6 +2,7 @@
 
 int posix_spawn_file_actions_init(posix_spawn_file_actions_t *fa)
 {
-	fa->__actions = 0;
+	fa->__pad0[0] = 0;
+	fa->__actions = fa;
 	return 0;
 }
-- 
2.26.3

Re: [musl] [PATCH] pack posix_spawn file actions into extraneous padding

[Rich Felker]

On Fri, Feb 04, 2022 at 08:09:13PM +0100, Joakim Sindholt wrote:
> On Sat, Jan 29, 2022 at 02:43:28PM +0100, Joakim Sindholt wrote:
> > I've written this patch that packs posix_spawn file actions into the
> > posix_spawn_file_actions struct padding. I'm not sure if the way I've
> > split the code up is desirable. Maybe the faexpand() function needs a
> > different name and to be in its own .c file; or maybe a different
> > approach is preferable.
> 
> Here's a candidate v2 patch to simplify some things. Changes since the
> first one:
> * fa->__actions is initialized to point to itself. This makes faexpand a
>   little simpler and means that ->__actions->__actions always points to
>   the last file_actions struct.
> * The previous version caused an increase of 32 bytes in used stack
>   space in child() on amd64. This one only causes a 16 byte increase.
>   I'm not sure how big of a deal-breaker that is.
> * The two switches have been merged into one. This makes it simpler IMO
>   but also has some consequences:
> 
> The current implementation in musl uses a pipe to communicate
> success/failure back to the parent process. This means that the
> file_actions struct can accidentally succeed if it tries to use the pipe
> fd as a source fd or worse, overwrite the pipe fd when targeting it in
> either the dup2 or open actions.
> Musl solves this by moving the pipe fd if it's the target of an
> operation. Currently musl will move the pipe if it's the source fd of
> close or fchdir, or if it's the target fd of dup2 or open. You'll note
> that the close and fchdir cases mean that instead of failing
> immediately, it will dup the pipe fd, close the old pipe fd, and then do
> the actual close/fchdir syscall which then fails. It's presumably
> implemented this way because it's simple and doesn't really cause a
> problem. The operation will result in failure regardless.
> 
> This patch, in an attempt to be very simple, introduces a similar but
> perhaps slightly worse bug. The encoding of every operation is
> essentially [fd, other stuff], meaning it can do the pipe move check on
> the fd in the first slot. It won't work for close since the fd is
> actually -fd-1, so close gets its own check. If the check is
> unconditional then we might get an unnecessary move with chdir, which is
> encoded as [dummy fd, -FDOP_CHDIR]. I've chosen an fd that's unlikely to
> collide (INT_MAX) however if you have every fd from 0 to INT_MAX-1 open
> then the pipe fd will be on fd INT_MAX and it will not be movable. The
> result is that the unnecessary pipe move fails and an otherwise
> perfectly legal and possible chdir action will fail.
> 
> The fix is very simple but does end up once again causing some
> duplication in the op logic:
> -if (fd == p) {
> +if (fd == p && fa->__pad[i] != -FDOP_CHDIR) {
> 
> Alternatively you could do the whole command check twice, only move the
> fd if it's the target of dup2 or open, and then have a check in fchdir
> for whether it's trying to use the pipe fd as a source, like the ones
> already in close and dup2.
> 
> Or maybe this bug is acceptable.

Since we can already have "spurious" failures anyway from insufficient
free fd space to shuffle the pipe fd, I don't think the thing with
FDOP_CHDIR matters. If we do ever decide we care, you've already
presented a way we could solve the problem.

(I noted on IRC that we could also solve the problem of the "density"
of the coding space by using the 32 bits of __pad0[1] to store type
information for the 16 slots, but this does not seem necessary.)

One change I think I would like to make is not using the names __pad0
and __pad directly in code. It's ugly. Instead, fdop.h could do:

#define fa_cnt __pad0[0]
#define fa_ops __pad

It might also make sense to define a macro for the fa_ops array limit

#define FA_OPS_MAX_CNT ...

using sizeof on a compound literal as appropriate.

We could also for consistency (not using __ junk names from the public
header directly) do

#define fa_chain __actions

Not sure if that's the best name for it but it seems to make things
make more sense than writing __actions in the code that's no longer
using the pointed-to data as the actions list.

I think I'm happy at this point with just doing "strace tests", but
sometime between now and next release I would like to get a better set
of tests into libc-test that cover usage of sufficiently many ops to
require allocations. Something like a test function that takes a list
of operations and builds both the file actions object and a shell
command to evaluate that the file actions were executed correctly,
using:

	read x <&%d && test "$x" = %s

and:

	! 2>/dev/null <&%d

to assert that each fd is either closed or reads the expected content
(expected content could be temp files with different content in each).

Re: [musl] [PATCH] pack posix_spawn file actions into extraneous padding

[Joakim Sindholt]

[-- Attachment #1: Type: text/plain, Size: 3207 bytes --]

On Wed, 4 May 2022 16:10:10 -0400, Rich Felker <dalias@libc.org> wrote:
> On Fri, Feb 04, 2022 at 08:09:13PM +0100, Joakim Sindholt wrote:
> > On Sat, Jan 29, 2022 at 02:43:28PM +0100, Joakim Sindholt wrote:
> > > I've written this patch that packs posix_spawn file actions into the
> > > posix_spawn_file_actions struct padding. I'm not sure if the way I've
> > > split the code up is desirable. Maybe the faexpand() function needs a
> > > different name and to be in its own .c file; or maybe a different
> > > approach is preferable.
> 
> (I noted on IRC that we could also solve the problem of the "density"
> of the coding space by using the 32 bits of __pad0[1] to store type
> information for the 16 slots, but this does not seem necessary.)

Since close and dup2 are as densely packed as possible without getting
into bit packing, and those two are by far the most common cases, and
since open, the only remaining action that you might reasonably want to
call many times, also requires malloc every time (for the path), I don't
see a need to add complexity to the reader.

We should also consider the complexity of adding new actions.

> One change I think I would like to make is not using the names __pad0
> and __pad directly in code. It's ugly. Instead, fdop.h could do:
> 
> #define fa_cnt __pad0[0]
> #define fa_ops __pad
> 
> It might also make sense to define a macro for the fa_ops array limit
> 
> #define FA_OPS_MAX_CNT ...
> 
> using sizeof on a compound literal as appropriate.

I agree and I have integrated all of these into this new patch. On
multiple occasions I had to hunt down bugs related to confusing __pad
and __pad0 but I wasn't sure if this solution was acceptable.

> We could also for consistency (not using __ junk names from the public
> header directly) do
> 
> #define fa_chain __actions
> 
> Not sure if that's the best name for it but it seems to make things
> make more sense than writing __actions in the code that's no longer
> using the pointed-to data as the actions list.

Visually it's nicer just to be consistent with the other 2 fields being
fa_*.

Furthermore I have split faexpand into __faexpand in its own .c file to
avoid increasing the .text size of all the add* functions.

> I think I'm happy at this point with just doing "strace tests", but
> sometime between now and next release I would like to get a better set
> of tests into libc-test that cover usage of sufficiently many ops to
> require allocations. Something like a test function that takes a list
> of operations and builds both the file actions object and a shell
> command to evaluate that the file actions were executed correctly,
> using:
> 
> 	read x <&%d && test "$x" = %s
> 
> and:
> 
> 	! 2>/dev/null <&%d
> 
> to assert that each fd is either closed or reads the expected content
> (expected content could be temp files with different content in each).

Testing manually with strace is still a fair bit of work if you're
trying to hit the weird corner cases. I think it makes sense to just
make the tests now. It might take a little while before I get to it
though.

This patch still appears to work fine. I have also attached the skeleton
program I manually modify to test it.

[-- Attachment #2: 0001-pack-posix_spawn-file-actions-into-extraneous-paddin.patch --]
[-- Type: application/octet-stream, Size: 12766 bytes --]

From b25d5c8305409c34a874b527001996219e1337c7 Mon Sep 17 00:00:00 2001
From: Joakim Sindholt <opensource@zhasha.com>
Date: Fri, 4 Feb 2022 19:42:14 +0100
Subject: [PATCH] pack posix_spawn file actions into extraneous padding

The posix_spawn_file_actions struct contains 2 int paddings of 2 and 16
ints respectively, making them ideal for packing file actions into.
The encoding used is simple: __pad0[0] contains a count and __pad
contains the actuals elements. The most important actions, dup2 and
close, are encoded as 2 and 1 int respectively.

If the first element of a sequence is negative, then it's a close and
processing stops there.
If the second element of a sequence is positive, then it's a dup2 and
processing stops there.
If the second element is negative, however, then it contains a command
and requires per-command specific processing.
In total there are 5 file actions. Their encodings are:

close: -fd-1
dup2: fd, srcfd
fchdir: fd, -FDOP_FCHDIR
chdir: INT_MAX, -FDOP_CHDIR
open: fd, -FDOP_OPEN, flags, mode

Some file actions need a string. When a string is needed it's allocated
on the tail end of a new file_actions struct. When a string is needed or
an action requires more space than is available then a new struct is
allocated and __actions is set to point to it. All the elements of the
first file_actions struct are copied to the new one. The new
file_actions struct, being the second in the list, always points to the
last file_actions struct in the list. From then on the structs all point
back in reverse order until they reach the second one again, creating a
loop. This rube-goldberg machine exists because there's nowhere to store
a last pointer for appending new actions efficiently.
As a result of this backwards allocation, the actions requiring strings
aren't stored in the same struct as the strings themselves. Rather the
string gets stored in the logically previous struct when reading.

This packing is far from optimal but it is very simple and it allows for
up to 16 close, 8 dup2, or any combination of them, without needing to
call malloc. At worst it calls malloc as much as before.
---
 src/process/__faexpand.c                      | 17 +++++
 src/process/fdop.h                            | 18 +++--
 src/process/posix_spawn.c                     | 66 ++++++++++++-------
 .../posix_spawn_file_actions_addchdir.c       | 15 ++---
 .../posix_spawn_file_actions_addclose.c       | 11 +---
 .../posix_spawn_file_actions_adddup2.c        | 13 ++--
 .../posix_spawn_file_actions_addfchdir.c      | 13 ++--
 .../posix_spawn_file_actions_addopen.c        | 19 +++---
 .../posix_spawn_file_actions_destroy.c        | 11 ++--
 src/process/posix_spawn_file_actions_init.c   |  4 +-
 10 files changed, 106 insertions(+), 81 deletions(-)
 create mode 100644 src/process/__faexpand.c

diff --git a/src/process/__faexpand.c b/src/process/__faexpand.c
new file mode 100644
index 00000000..f1d8c6fc
--- /dev/null
+++ b/src/process/__faexpand.c
@@ -0,0 +1,17 @@
+#include <spawn.h>
+#include <stdlib.h>
+#include "fdop.h"
+
+int __faexpand(posix_spawn_file_actions_t *fa, size_t str)
+{
+	posix_spawn_file_actions_t *n, *blk;
+	blk = malloc(sizeof *blk + str);
+	if (!blk) return -1;
+	*blk = *fa;
+	n = fa->fa_chain;
+	fa->fa_chain = blk;
+	blk->fa_chain = n->fa_chain;
+	n->fa_chain = blk;
+	fa->fa_cnt = 0;
+	return 0;
+}
diff --git a/src/process/fdop.h b/src/process/fdop.h
index 7cf733b2..5696e4ab 100644
--- a/src/process/fdop.h
+++ b/src/process/fdop.h
@@ -1,17 +1,21 @@
+#include <stddef.h>
+
 #define FDOP_CLOSE 1
 #define FDOP_DUP2 2
 #define FDOP_OPEN 3
 #define FDOP_CHDIR 4
 #define FDOP_FCHDIR 5
 
-struct fdop {
-	struct fdop *next, *prev;
-	int cmd, fd, srcfd, oflag;
-	mode_t mode;
-	char path[];
-};
-
 #define malloc __libc_malloc
 #define calloc __libc_calloc
 #define realloc undef
 #define free __libc_free
+
+#define fa_chain __actions
+#define fa_ops __pad
+#define fa_cnt __pad0[0]
+#define FA_CNT_MAX \
+	(sizeof((posix_spawn_file_actions_t){0}.fa_ops) \
+	/sizeof((posix_spawn_file_actions_t){0}.fa_ops[0]))
+
+int __faexpand(posix_spawn_file_actions_t *, size_t);
diff --git a/src/process/posix_spawn.c b/src/process/posix_spawn.c
index 728551b3..eab09191 100644
--- a/src/process/posix_spawn.c
+++ b/src/process/posix_spawn.c
@@ -81,33 +81,53 @@ static int child(void *args_vp)
 		    (ret=__syscall(SYS_setuid, __syscall(SYS_getuid))) )
 			goto fail;
 
-	if (fa && fa->__actions) {
-		struct fdop *op;
-		int fd;
-		for (op = fa->__actions; op->next; op = op->next);
-		for (; op; op = op->prev) {
+	if (fa) {
+		const posix_spawn_file_actions_t *first = fa;
+		const char *path;
+		int fd, srcfd, i = 0;
+
+		/* The list of actions, in reverse order, looks like this:
+		 * [0]->[1]<-[2]<-[3]
+		 *        \_______/^
+		 * so to get to the first action we to go to [0]->next->next
+		 * and to get all the way back we have to special case the
+		 * transition from [1]->[0] */
+		fa = fa->fa_chain;
+		fa = fa->fa_chain;
+		while (!(fa == first && i == fa->fa_cnt)) {
+			if (i == fa->fa_cnt) {
+				path = (char *)(fa+1);
+				fa = fa==first->fa_chain ? first : fa->fa_chain;
+				i = 0;
+				continue;
+			}
+			fd = fa->fa_ops[i++];
+
 			/* It's possible that a file operation would clobber
 			 * the pipe fd used for synchronizing with the
 			 * parent. To avoid that, we dup the pipe onto
 			 * an unoccupied fd. */
-			if (op->fd == p) {
+			if (fd == p) {
 				ret = __syscall(SYS_dup, p);
 				if (ret < 0) goto fail;
 				__syscall(SYS_close, p);
 				p = ret;
 			}
-			switch(op->cmd) {
-			case FDOP_CLOSE:
-				__syscall(SYS_close, op->fd);
-				break;
-			case FDOP_DUP2:
-				fd = op->srcfd;
-				if (fd == p) {
+
+			if (fd < 0) { /* FDOP_CLOSE */
+				if (-fd-1 == p) {
+					ret = -EBADF;
+					goto fail;
+				}
+				__syscall(SYS_close, -fd-1);
+			} else switch (-(srcfd = fa->fa_ops[i++])) {
+			default: /* FDOP_DUP2 */
+				if (srcfd == p) {
 					ret = -EBADF;
 					goto fail;
 				}
-				if (fd != op->fd) {
-					if ((ret=__sys_dup2(fd, op->fd))<0)
+				if (srcfd != fd) {
+					if ((ret=__sys_dup2(srcfd, fd))<0)
 						goto fail;
 				} else {
 					ret = __syscall(SYS_fcntl, fd, F_GETFD);
@@ -118,20 +138,22 @@ static int child(void *args_vp)
 				}
 				break;
 			case FDOP_OPEN:
-				fd = __sys_open(op->path, op->oflag, op->mode);
-				if ((ret=fd) < 0) goto fail;
-				if (fd != op->fd) {
-					if ((ret=__sys_dup2(fd, op->fd))<0)
+				srcfd = __sys_open(path, fa->fa_ops[i],
+				                   fa->fa_ops[i+1]);
+				if ((ret=srcfd) < 0) goto fail;
+				if (srcfd != fd) {
+					if ((ret=__sys_dup2(srcfd, fd))<0)
 						goto fail;
-					__syscall(SYS_close, fd);
+					__syscall(SYS_close, srcfd);
 				}
+				i += 2;
 				break;
 			case FDOP_CHDIR:
-				ret = __syscall(SYS_chdir, op->path);
+				ret = __syscall(SYS_chdir, path);
 				if (ret<0) goto fail;
 				break;
 			case FDOP_FCHDIR:
-				ret = __syscall(SYS_fchdir, op->fd);
+				ret = __syscall(SYS_fchdir, fd);
 				if (ret<0) goto fail;
 				break;
 			}
diff --git a/src/process/posix_spawn_file_actions_addchdir.c b/src/process/posix_spawn_file_actions_addchdir.c
index 7f2590ae..636bfba7 100644
--- a/src/process/posix_spawn_file_actions_addchdir.c
+++ b/src/process/posix_spawn_file_actions_addchdir.c
@@ -1,18 +1,15 @@
 #include <spawn.h>
-#include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <limits.h>
 #include "fdop.h"
 
 int posix_spawn_file_actions_addchdir_np(posix_spawn_file_actions_t *restrict fa, const char *restrict path)
 {
-	struct fdop *op = malloc(sizeof *op + strlen(path) + 1);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_CHDIR;
-	op->fd = -1;
-	strcpy(op->path, path);
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	size_t len = strlen(path);
+	if (__faexpand(fa, len + 1) != 0) return ENOMEM;
+	fa->fa_ops[fa->fa_cnt++] = INT_MAX;
+	fa->fa_ops[fa->fa_cnt++] = -FDOP_CHDIR;
+	memcpy((posix_spawn_file_actions_t *)fa->fa_chain + 1, path, len + 1);
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addclose.c b/src/process/posix_spawn_file_actions_addclose.c
index 0c2ef8fa..5f496c06 100644
--- a/src/process/posix_spawn_file_actions_addclose.c
+++ b/src/process/posix_spawn_file_actions_addclose.c
@@ -1,17 +1,12 @@
 #include <spawn.h>
-#include <stdlib.h>
 #include <errno.h>
 #include "fdop.h"
 
 int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t *fa, int fd)
 {
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_CLOSE;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (FA_CNT_MAX - fa->fa_cnt < 1)
+		if (__faexpand(fa, 0) != 0) return ENOMEM;
+	fa->fa_ops[fa->fa_cnt++] = -fd-1;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_adddup2.c b/src/process/posix_spawn_file_actions_adddup2.c
index addca4d4..d83a99b0 100644
--- a/src/process/posix_spawn_file_actions_adddup2.c
+++ b/src/process/posix_spawn_file_actions_adddup2.c
@@ -1,18 +1,13 @@
 #include <spawn.h>
-#include <stdlib.h>
 #include <errno.h>
 #include "fdop.h"
 
 int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t *fa, int srcfd, int fd)
 {
 	if (srcfd < 0 || fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_DUP2;
-	op->srcfd = srcfd;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (FA_CNT_MAX - fa->fa_cnt < 2)
+		if (__faexpand(fa, 0) != 0) return ENOMEM;
+	fa->fa_ops[fa->fa_cnt++] = fd;
+	fa->fa_ops[fa->fa_cnt++] = srcfd;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addfchdir.c b/src/process/posix_spawn_file_actions_addfchdir.c
index e89ede8c..fe248a5d 100644
--- a/src/process/posix_spawn_file_actions_addfchdir.c
+++ b/src/process/posix_spawn_file_actions_addfchdir.c
@@ -1,18 +1,13 @@
 #include <spawn.h>
-#include <stdlib.h>
-#include <string.h>
 #include <errno.h>
 #include "fdop.h"
 
 int posix_spawn_file_actions_addfchdir_np(posix_spawn_file_actions_t *fa, int fd)
 {
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_FCHDIR;
-	op->fd = fd;
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	if (FA_CNT_MAX - fa->fa_cnt < 2)
+		if (__faexpand(fa, 0) != 0) return ENOMEM;
+	fa->fa_ops[fa->fa_cnt++] = fd;
+	fa->fa_ops[fa->fa_cnt++] = -FDOP_FCHDIR;
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_addopen.c b/src/process/posix_spawn_file_actions_addopen.c
index 82bbcec9..8840dbd6 100644
--- a/src/process/posix_spawn_file_actions_addopen.c
+++ b/src/process/posix_spawn_file_actions_addopen.c
@@ -1,21 +1,18 @@
 #include <spawn.h>
-#include <stdlib.h>
 #include <string.h>
 #include <errno.h>
 #include "fdop.h"
 
 int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t *restrict fa, int fd, const char *restrict path, int flags, mode_t mode)
 {
+	size_t len;
 	if (fd < 0) return EBADF;
-	struct fdop *op = malloc(sizeof *op + strlen(path) + 1);
-	if (!op) return ENOMEM;
-	op->cmd = FDOP_OPEN;
-	op->fd = fd;
-	op->oflag = flags;
-	op->mode = mode;
-	strcpy(op->path, path);
-	if ((op->next = fa->__actions)) op->next->prev = op;
-	op->prev = 0;
-	fa->__actions = op;
+	len = strlen(path);
+	if (__faexpand(fa, len + 1) != 0) return ENOMEM;
+	fa->fa_ops[fa->fa_cnt++] = fd;
+	fa->fa_ops[fa->fa_cnt++] = -FDOP_OPEN;
+	fa->fa_ops[fa->fa_cnt++] = flags;
+	fa->fa_ops[fa->fa_cnt++] = mode;
+	memcpy((posix_spawn_file_actions_t *)fa->fa_chain + 1, path, len + 1);
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_destroy.c b/src/process/posix_spawn_file_actions_destroy.c
index 3251babb..0ac48480 100644
--- a/src/process/posix_spawn_file_actions_destroy.c
+++ b/src/process/posix_spawn_file_actions_destroy.c
@@ -4,11 +4,12 @@
 
 int posix_spawn_file_actions_destroy(posix_spawn_file_actions_t *fa)
 {
-	struct fdop *op = fa->__actions, *next;
-	while (op) {
-		next = op->next;
-		free(op);
-		op = next;
+	posix_spawn_file_actions_t *next, *p = fa->fa_chain;
+	for (p = p->fa_chain; p != fa->fa_chain; p = next) {
+		next = p->fa_chain;
+		free(p);
 	}
+	if (p != fa)
+		free(p);
 	return 0;
 }
diff --git a/src/process/posix_spawn_file_actions_init.c b/src/process/posix_spawn_file_actions_init.c
index 89d5e127..a26d6f41 100644
--- a/src/process/posix_spawn_file_actions_init.c
+++ b/src/process/posix_spawn_file_actions_init.c
@@ -1,7 +1,9 @@
 #include <spawn.h>
+#include "fdop.h"
 
 int posix_spawn_file_actions_init(posix_spawn_file_actions_t *fa)
 {
-	fa->__actions = 0;
+	fa->fa_cnt = 0;
+	fa->fa_chain = fa;
 	return 0;
 }
-- 
2.26.3


[-- Attachment #3: test.c --]
[-- Type: text/plain, Size: 1132 bytes --]

#include <spawn.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>

#define A(x) \
    if ((errno = (x)) != 0) \
        perror(#x)
int
main(int argc, char *argv[])
{
    posix_spawn_file_actions_t fa;
    pid_t pid;

    posix_spawn_file_actions_init(&fa);
    A(posix_spawn_file_actions_addclose(&fa, 0));
    A(posix_spawn_file_actions_adddup2(&fa, 2, 3));
    A(posix_spawn_file_actions_addopen(&fa, 5, "test.c", O_RDONLY, 0));
    A(posix_spawn_file_actions_addopen(&fa, 4, "/usr", O_DIRECTORY, 0));
    A(posix_spawn_file_actions_addchdir_np(&fa, "/"));
    A(posix_spawn_file_actions_addfchdir_np(&fa, 4));
    A(posix_spawn_file_actions_addclose(&fa, 5));
    A(posix_spawn_file_actions_adddup2(&fa, 2, 10));
    A(posix_spawn_file_actions_adddup2(&fa, 2, 11));
    A(posix_spawn_file_actions_adddup2(&fa, 2, 12));
    A(posix_spawn_file_actions_adddup2(&fa, 2, 13));
    A(posix_spawn_file_actions_adddup2(&fa, 2, 14));
    A(posix_spawn_file_actions_adddup2(&fa, 2, 15));
    posix_spawn(&pid, "/bin/false", &fa, 0, (char*[]){"false", 0}, (char*[]){0});
    posix_spawn_file_actions_destroy(&fa);
    return 0;
}