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* [musl] [PATCH] add close_range() syscall wrapper
@ 2023-09-01  8:02 Natanael Copa
  2023-09-01 13:57 ` Rich Felker
  0 siblings, 1 reply; 8+ messages in thread
From: Natanael Copa @ 2023-09-01  8:02 UTC (permalink / raw)
  To: musl; +Cc: Natanael Copa

close_range() is a syscall present in FreeBSD 8.0 and Linux 5.9. glibc
2.34 added a wrapper.

Expose it under _GNU_SOURCE similar to what GNU libc does. Also expose
it under _BSD_SOURCE since it is also a FreeBSD function.
---

This is a re-take of close_range() syscall wrapper. It was previously
discussed in mailing list: https://www.openwall.com/lists/musl/2022/08/18/5

Difference from previous submission:

- Use correct values for CLOSE_RANGE_UNSHARE and CLOSE_RANGE_CLOEXEC.
- Set errno on errors.
- Drop the (unsigned int) cast for flags, as it raised questions last
  time.

I think it is a good idea to add this to musl because it is difficult to
the close before exec properly without it. 

Most workaounrds currently out there are either parsing /proc or try to
close everything to maxfd reported by getrlimit(), sysconf() or
getdtablesize().

opendir("/proc/self/fd") is problematic becase 1) /proc may not be
mounted and 2) some versions of musl hangs on malloc between fork and 
exec.

Trying to close everything between a maxfd is also problematic. On some
systems (under docker for example) the maxfd can be 1G, which
effectively results in a hang. (See https://github.com/k0sproject/k0s/pull/3436)

I think its better to encourage the use of close_range().

See also: https://github.com/OpenRC/openrc/pull/645

 include/unistd.h        | 3 +++
 src/linux/close_range.c | 8 ++++++++
 2 files changed, 11 insertions(+)
 create mode 100644 src/linux/close_range.c

diff --git a/include/unistd.h b/include/unistd.h
index 5bc7f798..d89e3d4c 100644
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -161,6 +161,9 @@ unsigned ualarm(unsigned, unsigned);
 #define L_INCR 1
 #define L_XTND 2
 int brk(void *);
+#define CLOSE_RANGE_UNSHARE	(1U << 1)
+#define CLOSE_RANGE_CLOEXEC	(1U << 2)
+int close_range(unsigned int, unsigned int, int);
 void *sbrk(intptr_t);
 pid_t vfork(void);
 int vhangup(void);
diff --git a/src/linux/close_range.c b/src/linux/close_range.c
new file mode 100644
index 00000000..258ba8bd
--- /dev/null
+++ b/src/linux/close_range.c
@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <unistd.h>
+#include "syscall.h"
+
+int close_range(unsigned int first, unsigned int last, int flags)
+{
+	return __syscall_ret(syscall(SYS_close_range, first, last, flags));
+}
-- 
2.42.0


^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [musl] [PATCH] add close_range() syscall wrapper
  2023-09-01  8:02 [musl] [PATCH] add close_range() syscall wrapper Natanael Copa
@ 2023-09-01 13:57 ` Rich Felker
  2023-09-01 14:55   ` Natanael Copa
  2023-09-01 14:58   ` [musl] [PATCH v2] " Natanael Copa
  0 siblings, 2 replies; 8+ messages in thread
From: Rich Felker @ 2023-09-01 13:57 UTC (permalink / raw)
  To: Natanael Copa; +Cc: musl

On Fri, Sep 01, 2023 at 10:02:00AM +0200, Natanael Copa wrote:
> close_range() is a syscall present in FreeBSD 8.0 and Linux 5.9. glibc
> 2.34 added a wrapper.
> 
> Expose it under _GNU_SOURCE similar to what GNU libc does. Also expose
> it under _BSD_SOURCE since it is also a FreeBSD function.
> ---
> 
> This is a re-take of close_range() syscall wrapper. It was previously
> discussed in mailing list: https://www.openwall.com/lists/musl/2022/08/18/5
> 
> Difference from previous submission:
> 
> - Use correct values for CLOSE_RANGE_UNSHARE and CLOSE_RANGE_CLOEXEC.
> - Set errno on errors.
> - Drop the (unsigned int) cast for flags, as it raised questions last
>   time.
> 
> I think it is a good idea to add this to musl because it is difficult to
> the close before exec properly without it. 
> 
> Most workaounrds currently out there are either parsing /proc or try to
> close everything to maxfd reported by getrlimit(), sysconf() or
> getdtablesize().
> 
> opendir("/proc/self/fd") is problematic becase 1) /proc may not be
> mounted and 2) some versions of musl hangs on malloc between fork and 
> exec.
> 
> Trying to close everything between a maxfd is also problematic. On some
> systems (under docker for example) the maxfd can be 1G, which
> effectively results in a hang. (See https://github.com/k0sproject/k0s/pull/3436)
> 
> I think its better to encourage the use of close_range().
> 
> See also: https://github.com/OpenRC/openrc/pull/645
> 
>  include/unistd.h        | 3 +++
>  src/linux/close_range.c | 8 ++++++++
>  2 files changed, 11 insertions(+)
>  create mode 100644 src/linux/close_range.c
> 
> diff --git a/include/unistd.h b/include/unistd.h
> index 5bc7f798..d89e3d4c 100644
> --- a/include/unistd.h
> +++ b/include/unistd.h
> @@ -161,6 +161,9 @@ unsigned ualarm(unsigned, unsigned);
>  #define L_INCR 1
>  #define L_XTND 2
>  int brk(void *);
> +#define CLOSE_RANGE_UNSHARE	(1U << 1)
> +#define CLOSE_RANGE_CLOEXEC	(1U << 2)
> +int close_range(unsigned int, unsigned int, int);
>  void *sbrk(intptr_t);
>  pid_t vfork(void);
>  int vhangup(void);
> diff --git a/src/linux/close_range.c b/src/linux/close_range.c
> new file mode 100644
> index 00000000..258ba8bd
> --- /dev/null
> +++ b/src/linux/close_range.c
> @@ -0,0 +1,8 @@
> +#define _GNU_SOURCE
> +#include <unistd.h>
> +#include "syscall.h"
> +
> +int close_range(unsigned int first, unsigned int last, int flags)
> +{
> +	return __syscall_ret(syscall(SYS_close_range, first, last, flags));
> +}
> -- 
> 2.42.0

This is double-processing errno. You need either return
__syscall_ret(__syscall(...)) (note the second __) or just return
syscall(...) (the syscall macro without __ automatically does the
__syscall_ret).

Aside from that, I think there's a question whether, if we support
this as a function rather than leaving it to the application to use
the syscall, we should provide a fallback for ENOSYS. I'm not sure,
but it's something that should be considered before adding it.

Rich

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [musl] [PATCH] add close_range() syscall wrapper
  2023-09-01 13:57 ` Rich Felker
@ 2023-09-01 14:55   ` Natanael Copa
  2023-09-01 15:06     ` Rich Felker
  2023-09-01 14:58   ` [musl] [PATCH v2] " Natanael Copa
  1 sibling, 1 reply; 8+ messages in thread
From: Natanael Copa @ 2023-09-01 14:55 UTC (permalink / raw)
  To: Rich Felker; +Cc: musl

On Fri, 1 Sep 2023 09:57:34 -0400
Rich Felker <dalias@libc.org> wrote:


> > +int close_range(unsigned int first, unsigned int last, int flags)
> > +{
> > +	return __syscall_ret(syscall(SYS_close_range, first, last, flags));
> > +}
> > -- 
> > 2.42.0  
> 
> This is double-processing errno. You need either return
> __syscall_ret(__syscall(...)) (note the second __) or just return
> syscall(...) (the syscall macro without __ automatically does the
> __syscall_ret).

Ah, ok, I'll send a v2 patch.

> Aside from that, I think there's a question whether, if we support
> this as a function rather than leaving it to the application to use
> the syscall, we should provide a fallback for ENOSYS. I'm not sure,
> but it's something that should be considered before adding it.

It was mentioned earlier that CPython expects close_range() to
async-safe, and that glibc does not provide fallback. I would prefer
that musl does not provide fallback.

https://www.openwall.com/lists/musl/2022/08/18/4

-nc

^ permalink raw reply	[flat|nested] 8+ messages in thread

* [musl] [PATCH v2] add close_range() syscall wrapper
  2023-09-01 13:57 ` Rich Felker
  2023-09-01 14:55   ` Natanael Copa
@ 2023-09-01 14:58   ` Natanael Copa
  2024-08-01  9:43     ` Alexey Izbyshev
  1 sibling, 1 reply; 8+ messages in thread
From: Natanael Copa @ 2023-09-01 14:58 UTC (permalink / raw)
  To: musl; +Cc: Natanael Copa

close_range() is a syscall present in FreeBSD 8.0 and Linux 5.9. glibc
2.34 added a wrapper.

Expose it under _GNU_SOURCE similar to what GNU libc does. Also expose
it under _BSD_SOURCE since it is also a FreeBSD function.
---

v2: use syscall without __syscall_ret

 include/unistd.h        | 3 +++
 src/linux/close_range.c | 8 ++++++++
 2 files changed, 11 insertions(+)
 create mode 100644 src/linux/close_range.c

diff --git a/include/unistd.h b/include/unistd.h
index 5bc7f798..d89e3d4c 100644
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -161,6 +161,9 @@ unsigned ualarm(unsigned, unsigned);
 #define L_INCR 1
 #define L_XTND 2
 int brk(void *);
+#define CLOSE_RANGE_UNSHARE	(1U << 1)
+#define CLOSE_RANGE_CLOEXEC	(1U << 2)
+int close_range(unsigned int, unsigned int, int);
 void *sbrk(intptr_t);
 pid_t vfork(void);
 int vhangup(void);
diff --git a/src/linux/close_range.c b/src/linux/close_range.c
new file mode 100644
index 00000000..3f1378a0
--- /dev/null
+++ b/src/linux/close_range.c
@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <unistd.h>
+#include "syscall.h"
+
+int close_range(unsigned int first, unsigned int last, int flags)
+{
+	return syscall(SYS_close_range, first, last, flags);
+}
-- 
2.42.0


^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [musl] [PATCH] add close_range() syscall wrapper
  2023-09-01 14:55   ` Natanael Copa
@ 2023-09-01 15:06     ` Rich Felker
  0 siblings, 0 replies; 8+ messages in thread
From: Rich Felker @ 2023-09-01 15:06 UTC (permalink / raw)
  To: Natanael Copa; +Cc: musl

On Fri, Sep 01, 2023 at 04:55:53PM +0200, Natanael Copa wrote:
> On Fri, 1 Sep 2023 09:57:34 -0400
> Rich Felker <dalias@libc.org> wrote:
> 
> 
> > > +int close_range(unsigned int first, unsigned int last, int flags)
> > > +{
> > > +	return __syscall_ret(syscall(SYS_close_range, first, last, flags));
> > > +}
> > > -- 
> > > 2.42.0  
> > 
> > This is double-processing errno. You need either return
> > __syscall_ret(__syscall(...)) (note the second __) or just return
> > syscall(...) (the syscall macro without __ automatically does the
> > __syscall_ret).
> 
> Ah, ok, I'll send a v2 patch.
> 
> > Aside from that, I think there's a question whether, if we support
> > this as a function rather than leaving it to the application to use
> > the syscall, we should provide a fallback for ENOSYS. I'm not sure,
> > but it's something that should be considered before adding it.
> 
> It was mentioned earlier that CPython expects close_range() to
> async-safe, and that glibc does not provide fallback. I would prefer
> that musl does not provide fallback.
> 
> https://www.openwall.com/lists/musl/2022/08/18/4

If musl were to provide a fallback it would be AS-safe.

Rich

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [musl] [PATCH v2] add close_range() syscall wrapper
  2023-09-01 14:58   ` [musl] [PATCH v2] " Natanael Copa
@ 2024-08-01  9:43     ` Alexey Izbyshev
  2024-08-01 16:25       ` Rich Felker
  0 siblings, 1 reply; 8+ messages in thread
From: Alexey Izbyshev @ 2024-08-01  9:43 UTC (permalink / raw)
  To: musl; +Cc: Natanael Copa

On 2023-09-01 17:58, Natanael Copa wrote:
> close_range() is a syscall present in FreeBSD 8.0 and Linux 5.9. glibc
> 2.34 added a wrapper.
> 
> Expose it under _GNU_SOURCE similar to what GNU libc does. Also expose
> it under _BSD_SOURCE since it is also a FreeBSD function.
> ---
> 
> v2: use syscall without __syscall_ret
> 
>  include/unistd.h        | 3 +++
>  src/linux/close_range.c | 8 ++++++++
>  2 files changed, 11 insertions(+)
>  create mode 100644 src/linux/close_range.c
> 
> diff --git a/include/unistd.h b/include/unistd.h
> index 5bc7f798..d89e3d4c 100644
> --- a/include/unistd.h
> +++ b/include/unistd.h
> @@ -161,6 +161,9 @@ unsigned ualarm(unsigned, unsigned);
>  #define L_INCR 1
>  #define L_XTND 2
>  int brk(void *);
> +#define CLOSE_RANGE_UNSHARE	(1U << 1)
> +#define CLOSE_RANGE_CLOEXEC	(1U << 2)
> +int close_range(unsigned int, unsigned int, int);
>  void *sbrk(intptr_t);
>  pid_t vfork(void);
>  int vhangup(void);
> diff --git a/src/linux/close_range.c b/src/linux/close_range.c
> new file mode 100644
> index 00000000..3f1378a0
> --- /dev/null
> +++ b/src/linux/close_range.c
> @@ -0,0 +1,8 @@
> +#define _GNU_SOURCE
> +#include <unistd.h>
> +#include "syscall.h"
> +
> +int close_range(unsigned int first, unsigned int last, int flags)
> +{
> +	return syscall(SYS_close_range, first, last, flags);
> +}

Regarding FreeBSD, close_range was added not in 8.0, but in 13.0 [1], 
and also backported to 12.2 [2].

Otherwise, this patch looks good to me.

Rich, is it possible to consider close_range wrapper inclusion again? 
Apart from FreeBSD and glibc, bionic has it too. A cursory Debian code 
search shows that close_range libc wrapper can be used at least by 
openssh, libvirt, network-manager, openrc, qemu, lxc, rsyslog packages 
(in addition to CPython that I mentioned ealier).

As for having a fallback in case the syscall is unavailable, I'm not 
aware of anybody implementing it, so I'd expect all close_range users to 
implement their own fallback/error handling. For example, Debian's 
openssh migrated from closefrom to close_range with their own fallback 
because of too aggressive closefrom fallback in glibc[3].

Thanks,
Alexey

[1] 
https://cgit.freebsd.org/src/commit/?h=releng/13.0&id=472ced39efb537374068f06b348fe5dac389c45a
[2] 
https://cgit.freebsd.org/src/commit/?h=releng/12.2&id=a80adba5ab46ba6d44d5abfc9b7f3b6de8afda55
[3] 
https://sources.debian.org/src/openssh/1%3A9.8p1-1/debian/changelog/#L895

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [musl] [PATCH v2] add close_range() syscall wrapper
  2024-08-01  9:43     ` Alexey Izbyshev
@ 2024-08-01 16:25       ` Rich Felker
  2024-08-01 20:24         ` Alexey Izbyshev
  0 siblings, 1 reply; 8+ messages in thread
From: Rich Felker @ 2024-08-01 16:25 UTC (permalink / raw)
  To: musl

On Thu, Aug 01, 2024 at 12:43:00PM +0300, Alexey Izbyshev wrote:
> On 2023-09-01 17:58, Natanael Copa wrote:
> >close_range() is a syscall present in FreeBSD 8.0 and Linux 5.9. glibc
> >2.34 added a wrapper.
> >
> >Expose it under _GNU_SOURCE similar to what GNU libc does. Also expose
> >it under _BSD_SOURCE since it is also a FreeBSD function.
> >---
> >
> >v2: use syscall without __syscall_ret
> >
> > include/unistd.h        | 3 +++
> > src/linux/close_range.c | 8 ++++++++
> > 2 files changed, 11 insertions(+)
> > create mode 100644 src/linux/close_range.c
> >
> >diff --git a/include/unistd.h b/include/unistd.h
> >index 5bc7f798..d89e3d4c 100644
> >--- a/include/unistd.h
> >+++ b/include/unistd.h
> >@@ -161,6 +161,9 @@ unsigned ualarm(unsigned, unsigned);
> > #define L_INCR 1
> > #define L_XTND 2
> > int brk(void *);
> >+#define CLOSE_RANGE_UNSHARE	(1U << 1)
> >+#define CLOSE_RANGE_CLOEXEC	(1U << 2)
> >+int close_range(unsigned int, unsigned int, int);
> > void *sbrk(intptr_t);
> > pid_t vfork(void);
> > int vhangup(void);
> >diff --git a/src/linux/close_range.c b/src/linux/close_range.c
> >new file mode 100644
> >index 00000000..3f1378a0
> >--- /dev/null
> >+++ b/src/linux/close_range.c
> >@@ -0,0 +1,8 @@
> >+#define _GNU_SOURCE
> >+#include <unistd.h>
> >+#include "syscall.h"
> >+
> >+int close_range(unsigned int first, unsigned int last, int flags)
> >+{
> >+	return syscall(SYS_close_range, first, last, flags);
> >+}
> 
> Regarding FreeBSD, close_range was added not in 8.0, but in 13.0
> [1], and also backported to 12.2 [2].
> 
> Otherwise, this patch looks good to me.
> 
> Rich, is it possible to consider close_range wrapper inclusion
> again? Apart from FreeBSD and glibc, bionic has it too. A cursory
> Debian code search shows that close_range libc wrapper can be used
> at least by openssh, libvirt, network-manager, openrc, qemu, lxc,
> rsyslog packages (in addition to CPython that I mentioned ealier).
> 
> As for having a fallback in case the syscall is unavailable, I'm not
> aware of anybody implementing it, so I'd expect all close_range
> users to implement their own fallback/error handling. For example,
> Debian's openssh migrated from closefrom to close_range with their
> own fallback because of too aggressive closefrom fallback in
> glibc[3].
> 
> Thanks,
> Alexey
> 
> [1] https://cgit.freebsd.org/src/commit/?h=releng/13.0&id=472ced39efb537374068f06b348fe5dac389c45a
> [2] https://cgit.freebsd.org/src/commit/?h=releng/12.2&id=a80adba5ab46ba6d44d5abfc9b7f3b6de8afda55
> [3] https://sources.debian.org/src/openssh/1%3A9.8p1-1/debian/changelog/#L895

Thanks for looking into this.

Generally, I try to follow a principle that if an interface is
genuinely new functionality, managing some new kind of kernel object
or something previously not in the data model, that it's fine not to
have fallback, but that if it's just a new way to act on existing
things (like adding a missing flags argument to an existing
operation), there should be fallback at least in cases where no new
underlying functionality is needed (like in that example, if flags
value is 0).

However, close_range really isn't an improved/generalized way to do
close, but something intended for its own purposes, and if
applications which use it are prepared for it to fail with ENOSYS
(this is important! not just prepared for it to be missing at
configure-time link check) then omitting fallback and letting them do
their own fallbacks seems like it'd be okay.

Unlike closefrom, whose *only* use is invoking UB by closing fds you
don't own, close_range at least admits well-defined uses where you've
tracked ranges you do own and want to close them quickly. That's not
to say folks will use it this way, but having at least some valid use
is something going for it.

Without a fallback, I'm not sure there's a lot of value to providing a
wrapper rather than just having applications use it via
syscall(SYS_close_range, ...), but there's also not any significant
cost to having the wrapper if that's what programs expect.

Are there any other things we should weigh on this topic?

Rich

^ permalink raw reply	[flat|nested] 8+ messages in thread

* Re: [musl] [PATCH v2] add close_range() syscall wrapper
  2024-08-01 16:25       ` Rich Felker
@ 2024-08-01 20:24         ` Alexey Izbyshev
  0 siblings, 0 replies; 8+ messages in thread
From: Alexey Izbyshev @ 2024-08-01 20:24 UTC (permalink / raw)
  To: musl

On 2024-08-01 19:25, Rich Felker wrote:
> On Thu, Aug 01, 2024 at 12:43:00PM +0300, Alexey Izbyshev wrote:
>> On 2023-09-01 17:58, Natanael Copa wrote:
>> >close_range() is a syscall present in FreeBSD 8.0 and Linux 5.9. glibc
>> >2.34 added a wrapper.
>> >
>> >Expose it under _GNU_SOURCE similar to what GNU libc does. Also expose
>> >it under _BSD_SOURCE since it is also a FreeBSD function.
>> >---
>> >
>> >v2: use syscall without __syscall_ret
>> >
>> > include/unistd.h        | 3 +++
>> > src/linux/close_range.c | 8 ++++++++
>> > 2 files changed, 11 insertions(+)
>> > create mode 100644 src/linux/close_range.c
>> >
>> >diff --git a/include/unistd.h b/include/unistd.h
>> >index 5bc7f798..d89e3d4c 100644
>> >--- a/include/unistd.h
>> >+++ b/include/unistd.h
>> >@@ -161,6 +161,9 @@ unsigned ualarm(unsigned, unsigned);
>> > #define L_INCR 1
>> > #define L_XTND 2
>> > int brk(void *);
>> >+#define CLOSE_RANGE_UNSHARE	(1U << 1)
>> >+#define CLOSE_RANGE_CLOEXEC	(1U << 2)
>> >+int close_range(unsigned int, unsigned int, int);
>> > void *sbrk(intptr_t);
>> > pid_t vfork(void);
>> > int vhangup(void);
>> >diff --git a/src/linux/close_range.c b/src/linux/close_range.c
>> >new file mode 100644
>> >index 00000000..3f1378a0
>> >--- /dev/null
>> >+++ b/src/linux/close_range.c
>> >@@ -0,0 +1,8 @@
>> >+#define _GNU_SOURCE
>> >+#include <unistd.h>
>> >+#include "syscall.h"
>> >+
>> >+int close_range(unsigned int first, unsigned int last, int flags)
>> >+{
>> >+	return syscall(SYS_close_range, first, last, flags);
>> >+}
>> 
>> Regarding FreeBSD, close_range was added not in 8.0, but in 13.0
>> [1], and also backported to 12.2 [2].
>> 
>> Otherwise, this patch looks good to me.
>> 
>> Rich, is it possible to consider close_range wrapper inclusion
>> again? Apart from FreeBSD and glibc, bionic has it too. A cursory
>> Debian code search shows that close_range libc wrapper can be used
>> at least by openssh, libvirt, network-manager, openrc, qemu, lxc,
>> rsyslog packages (in addition to CPython that I mentioned ealier).
>> 
>> As for having a fallback in case the syscall is unavailable, I'm not
>> aware of anybody implementing it, so I'd expect all close_range
>> users to implement their own fallback/error handling. For example,
>> Debian's openssh migrated from closefrom to close_range with their
>> own fallback because of too aggressive closefrom fallback in
>> glibc[3].
>> 
>> Thanks,
>> Alexey
>> 
>> [1] 
>> https://cgit.freebsd.org/src/commit/?h=releng/13.0&id=472ced39efb537374068f06b348fe5dac389c45a
>> [2] 
>> https://cgit.freebsd.org/src/commit/?h=releng/12.2&id=a80adba5ab46ba6d44d5abfc9b7f3b6de8afda55
>> [3] 
>> https://sources.debian.org/src/openssh/1%3A9.8p1-1/debian/changelog/#L895
> 
> Thanks for looking into this.
> 
> Generally, I try to follow a principle that if an interface is
> genuinely new functionality, managing some new kind of kernel object
> or something previously not in the data model, that it's fine not to
> have fallback, but that if it's just a new way to act on existing
> things (like adding a missing flags argument to an existing
> operation), there should be fallback at least in cases where no new
> underlying functionality is needed (like in that example, if flags
> value is 0).
> 
> However, close_range really isn't an improved/generalized way to do
> close, but something intended for its own purposes, and if
> applications which use it are prepared for it to fail with ENOSYS
> (this is important! not just prepared for it to be missing at
> configure-time link check) then omitting fallback and letting them do
> their own fallbacks seems like it'd be okay.

I've checked 8 projects mentioned above, and all of them are prepared to 
handle close_range errors. Usually any error leads to a fallback, though 
there are cases where certain errors are treated differently, e.g. in 
network-manager[1], lxc[2].

The glibc manual explicitly warns that the caller has to be prepared for 
ENOSYS[3], although the Linux man-pages project doesn't mention that[4].

I've also discovered that half of those projects (libvirt, 
network-manager, openrc, lxc) can use close_range via syscall in absence 
of the wrapper, and libvirt isn't even aware of the wrapper (i.e. it 
uses syscall() only).
> 
> Unlike closefrom, whose *only* use is invoking UB by closing fds you
> don't own, close_range at least admits well-defined uses where you've
> tracked ranges you do own and want to close them quickly. That's not
> to say folks will use it this way, but having at least some valid use
> is something going for it.
> 
> Without a fallback, I'm not sure there's a lot of value to providing a
> wrapper rather than just having applications use it via
> syscall(SYS_close_range, ...), but there's also not any significant
> cost to having the wrapper if that's what programs expect.
> 
Indeed, the value of the wrapper can't be called huge, but it's still a 
convenience:

* There is no danger of invoking the usual UB by passing int variadic 
arguments to libc's syscall implementation that expects long (even 
though I'm not aware of an arch where this matters in practice).

* FreeBSD had the wrapper since day 1, so for projects that support both 
FreeBSD and Linux (and don't need close_range so desperately that they 
can't wait for the wrapper in libc) it'd be nicer to just use the 
wrapper in both cases.

> Are there any other things we should weigh on this topic?
> 
One thing that comes to mind is that CLOSE_RANGE_UNSHARE flag is an 
extra way to create an execution environment that is unexpected by musl. 
However, given that musl provides the wrapper for unshare, I don't 
believe that  breaking compatibility with glibc and bionic by rejecting 
some close_range flags (like in clone) is worth it.

I also have to add that I'm somewhat biased in the favor of the wrapper 
because it was me who enabled its usage in CPython's subprocess on Linux 
(based on pre-existing work for FreeBSD), but this code doesn't work on 
musl yet.

Thanks,
Alexey

[1] 
https://sources.debian.org/src/network-manager/1.48.6-1/src/libnm-systemd-shared/src/basic/fd-util.c/?hl=314#L314
[2] 
https://sources.debian.org/src/lxc/1:6.0.1-1/src/lxc/initutils.c/?hl=612#L612
[3] 
https://www.gnu.org/software/libc/manual/html_node/Opening-and-Closing-Files.html#index-close_005frange
[4] https://man7.org/linux/man-pages/man2/close_range.2.html

^ permalink raw reply	[flat|nested] 8+ messages in thread

end of thread, other threads:[~2024-08-01 20:24 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-09-01  8:02 [musl] [PATCH] add close_range() syscall wrapper Natanael Copa
2023-09-01 13:57 ` Rich Felker
2023-09-01 14:55   ` Natanael Copa
2023-09-01 15:06     ` Rich Felker
2023-09-01 14:58   ` [musl] [PATCH v2] " Natanael Copa
2024-08-01  9:43     ` Alexey Izbyshev
2024-08-01 16:25       ` Rich Felker
2024-08-01 20:24         ` Alexey Izbyshev

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