* [PATCH 1/1] openpty: use TIOCGPTPEER to open slave side fd
2017-09-01 15:35 [PATCH 0/1] openpty: use TIOCGPTPEER to open slave side fd Christian Brauner
@ 2017-09-01 15:35 ` Christian Brauner
2017-09-01 16:00 ` [PATCH 0/1] " Rich Felker
1 sibling, 0 replies; 4+ messages in thread
From: Christian Brauner @ 2017-09-01 15:35 UTC (permalink / raw)
To: musl; +Cc: Christian Brauner
Newer kernels expose the ioctl TIOCGPTPEER [1] call to userspace which allows to
safely allocate a file descriptor for a pty slave based solely on the master
file descriptor. This allows us to avoid path-based operations and makes this
function a lot safer in the face of devpts mounts in different mount namespaces.
[1]: https://patchwork.kernel.org/patch/9760743/
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
src/misc/openpty.c | 34 +++++++++++++++++++++++++++++++---
1 file changed, 31 insertions(+), 3 deletions(-)
diff --git a/src/misc/openpty.c b/src/misc/openpty.c
index c1074060..9eab7a37 100644
--- a/src/misc/openpty.c
+++ b/src/misc/openpty.c
@@ -3,13 +3,14 @@
#include <unistd.h>
#include <pty.h>
#include <stdio.h>
+#include <string.h>
#include <pthread.h>
/* Nonstandard, but vastly superior to the standard functions */
int openpty(int *pm, int *ps, char *name, const struct termios *tio, const struct winsize *ws)
{
- int m, s, n=0, cs;
+ int m, s = -1, cs;
char buf[20];
m = open("/dev/ptmx", O_RDWR|O_NOCTTY);
@@ -17,13 +18,38 @@ int openpty(int *pm, int *ps, char *name, const struct termios *tio, const struc
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+ if (!name) name = buf;
+
+#ifdef TIOCGPTPEER
+ int ret;
+
+ /* Try to allocate slave fd solely based on the master fd in case the
+ * kernel supports it.
+ */
+ s = ioctl(m, TIOCGPTPEER, O_RDWR|O_NOCTTY);
+ if (s < 0)
+ goto fail;
+
+ /* Check the contents of the symlink in case devpts has been mounted in
+ * a non-standard location.
+ */
+ ret = ttyname_r(s, name, sizeof buf);
+ if (ret)
+ goto fail;
+
+ if (strncmp(name, "/dev/pts/", 9))
+ goto fail;
+#else
+ int n = 0;
+
if (ioctl(m, TIOCSPTLCK, &n) || ioctl (m, TIOCGPTN, &n))
goto fail;
- if (!name) name = buf;
snprintf(name, sizeof buf, "/dev/pts/%d", n);
- if ((s = open(name, O_RDWR|O_NOCTTY)) < 0)
+ s = open(name, O_RDWR|O_NOCTTY);
+ if (s < 0)
goto fail;
+#endif
if (tio) tcsetattr(s, TCSANOW, tio);
if (ws) ioctl(s, TIOCSWINSZ, ws);
@@ -35,6 +61,8 @@ int openpty(int *pm, int *ps, char *name, const struct termios *tio, const struc
return 0;
fail:
close(m);
+ if (s >= 0)
+ close(s);
pthread_setcancelstate(cs, 0);
return -1;
}
--
2.14.1
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH 0/1] openpty: use TIOCGPTPEER to open slave side fd
2017-09-01 15:35 [PATCH 0/1] openpty: use TIOCGPTPEER to open slave side fd Christian Brauner
2017-09-01 15:35 ` [PATCH 1/1] " Christian Brauner
@ 2017-09-01 16:00 ` Rich Felker
2017-09-01 16:07 ` Rich Felker
1 sibling, 1 reply; 4+ messages in thread
From: Rich Felker @ 2017-09-01 16:00 UTC (permalink / raw)
To: musl
On Fri, Sep 01, 2017 at 05:35:57PM +0200, Christian Brauner wrote:
> Hi,
>
> Newer kernels expose the ioctl TIOCGPTPEER [1] call to userspace which allows to
> safely allocate a file descriptor for a pty slave based solely on the master
> file descriptor. This allows us to avoid path-based operations and makes this
> function a lot safer in the face of devpts mounts in different mount namespaces.
>
> [1]: https://patchwork.kernel.org/patch/9760743/
>
> The way I wrote this patch it doesn't use the snprintf() and open() routine as a
> fallback in case the ioctl() call fails. If this is a compatibility issue for
> non-Linux systems I can rewrite. But the musl documentation gave me the
> impression that this is not really a concern.
It's a compatibility issue for everything but bleeding-edge Linux.
musl supports all the way back to 2.6.0 and possibly farther, but
especially needs to support actual modern versions people run; you
can't just add dependencies on newly-added features.
There's also no reason for the #ifdef TIOCGPTPEER, as musl decides if
TIOCGPTPEER is defined or not (it has to provide the definition).
Instead, you just need to try the ioctl and fall back if it fails.
Is the TIOCGPTPEER thing upstream in the kernel yet? If not it's not a
stable API and possibly subject to change (even reassignment of the
ioctl number which could be dangerous) so this patch can't be merged
until it's official/permanent on the kernel side.
Otherwise this looks like a good change, but I do wonder a bit about
how the cases where it would help are intended to work, since the
POSIX interfaces for opening a pty require using a pathname
(ptsname[_r]). It seems like only programs using the nonstandard
openpty() function could benefit, and while this interface is nicer in
many ways, it's fundamentally broken in that it lacks a way to
atomically set the FD_CLOEXEC flag. The POSIX functions posix_openpt
and ptsname+open both allow O_CLOEXEC which solves this problem.
Rich
^ permalink raw reply [flat|nested] 4+ messages in thread