Patchwork [v3,1/2] clone3: add CLONE_CLEAR_SIGHAND

login
register
mail settings
Submitter Christian Brauner
Date Oct. 14, 2019, 10:45 a.m.
Message ID <20191014104538.3096-1-christian.brauner@ubuntu.com>
Download mbox | patch
Permalink /patch/34934/
State New
Headers show

Comments

Christian Brauner - Oct. 14, 2019, 10:45 a.m.
Reset all signal handlers of the child not set to SIG_IGN to SIG_DFL.
Mutually exclusive with CLONE_SIGHAND to not disturb other thread's
signal handler.

In the spirit of closer cooperation between glibc developers and kernel
developers (cf. [2]) this patchset came out of a discussion on the glibc
mailing list for improving posix_spawn() (cf. [1], [3], [4]). Kernel
support for this feature has been explicitly requested by glibc and I
see no reason not to help them with this.

The child helper process on Linux posix_spawn must ensure that no signal
handlers are enabled, so the signal disposition must be either SIG_DFL
or SIG_IGN. However, it requires a sigprocmask to obtain the current
signal mask and at least _NSIG sigaction calls to reset the signal
handlers for each posix_spawn call or complex state tracking that might
lead to data corruption in glibc. Adding this flags lets glibc avoid
these problems.

[1]: https://www.sourceware.org/ml/libc-alpha/2019-10/msg00149.html
[3]: https://www.sourceware.org/ml/libc-alpha/2019-10/msg00158.html
[4]: https://www.sourceware.org/ml/libc-alpha/2019-10/msg00160.html
[2]: https://lwn.net/Articles/799331/
     '[...] by asking for better cooperation with the C-library projects
     in general. They should be copied on patches containing ABI
     changes, for example. I noted that there are often times where
     C-library developers wish the kernel community had done things
     differently; how could those be avoided in the future? Members of
     the audience suggested that more glibc developers should perhaps
     join the linux-api list. The other suggestion was to "copy Florian
     on everything".'
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: libc-alpha@sourceware.org
Cc: linux-api@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v1 */
Link: https://lore.kernel.org/r/20191010133518.5420-1-christian.brauner@ubuntu.com

/* v2 */
Link: https://lore.kernel.org/r/20191011102537.27502-1-christian.brauner@ubuntu.com
- Florian Weimer <fweimer@redhat.com>:
  - update comment in clone3_args_valid()

/* v3 */
- "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>:
  - s/CLONE3_CLEAR_SIGHAND/CLONE_CLEAR_SIGHAND/g
---
 include/uapi/linux/sched.h |  3 +++
 kernel/fork.c              | 16 +++++++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)
Christian Brauner - Oct. 21, 2019, 7:40 p.m.
On Mon, Oct 21, 2019 at 05:12:55PM +0200, Oleg Nesterov wrote:
> On 10/21, Oleg Nesterov wrote:
> >
> > On 10/14, Christian Brauner wrote:
> > >
> > > The child helper process on Linux posix_spawn must ensure that no signal
> > > handlers are enabled, so the signal disposition must be either SIG_DFL
> > > or SIG_IGN. However, it requires a sigprocmask to obtain the current
> > > signal mask and at least _NSIG sigaction calls to reset the signal
> > > handlers for each posix_spawn call
> >
> > Plus the caller has to block/unblock all signals around clone(VM|VFORK).
>        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> just in case... I meant that posix_spawn() has to block/unblock, not its
> caller.

Yeah, I think that is what's currently happening.

Christian
Christian Brauner - Oct. 21, 2019, 7:42 p.m.
On Mon, Oct 21, 2019 at 04:46:33PM +0200, Oleg Nesterov wrote:
> On 10/14, Christian Brauner wrote:
> >
> > The child helper process on Linux posix_spawn must ensure that no signal
> > handlers are enabled, so the signal disposition must be either SIG_DFL
> > or SIG_IGN. However, it requires a sigprocmask to obtain the current
> > signal mask and at least _NSIG sigaction calls to reset the signal
> > handlers for each posix_spawn call
> 
> Plus the caller has to block/unblock all signals around clone(VM|VFORK).
> 
> Can this justify the new CLONE_ flag? Honestly, I have no idea. But the
> patch is simple and looks technically correct to me. FWIW,
> 
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>

The problem is not just the number of syscalls but also that the
sigaction logic is already quite complicated and would need to be even
more complicated without this flag. That's covered mostly in the glibc
thread though. Even just the ability to avoid potentially _NSIG syscalls
is enough justification especially since were not scarce on flags.

Thanks! I'll pick this up for 5.5.
Christian

Patch

diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 99335e1f4a27..1d500ed03c63 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -33,6 +33,9 @@ 
 #define CLONE_NEWNET		0x40000000	/* New network namespace */
 #define CLONE_IO		0x80000000	/* Clone io context */
 
+/* Flags for the clone3() syscall. */
+#define CLONE_CLEAR_SIGHAND 0x100000000ULL /* Clear any signal handler and reset to SIG_DFL. */
+
 #ifndef __ASSEMBLY__
 /**
  * struct clone_args - arguments for the clone3 syscall
diff --git a/kernel/fork.c b/kernel/fork.c
index 1f6c45f6a734..aa5b5137f071 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1517,6 +1517,11 @@  static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
 	spin_lock_irq(&current->sighand->siglock);
 	memcpy(sig->action, current->sighand->action, sizeof(sig->action));
 	spin_unlock_irq(&current->sighand->siglock);
+
+	/* Reset all signal handler not set to SIG_IGN to SIG_DFL. */
+	if (clone_flags & CLONE_CLEAR_SIGHAND)
+		flush_signal_handlers(tsk, 0);
+
 	return 0;
 }
 
@@ -2563,11 +2568,8 @@  noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 
 static bool clone3_args_valid(const struct kernel_clone_args *kargs)
 {
-	/*
-	 * All lower bits of the flag word are taken.
-	 * Verify that no other unknown flags are passed along.
-	 */
-	if (kargs->flags & ~CLONE_LEGACY_FLAGS)
+	/* Verify that no unknown flags are passed along. */
+	if (kargs->flags & ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND))
 		return false;
 
 	/*
@@ -2577,6 +2579,10 @@  static bool clone3_args_valid(const struct kernel_clone_args *kargs)
 	if (kargs->flags & (CLONE_DETACHED | CSIGNAL))
 		return false;
 
+	if ((kargs->flags & (CLONE_SIGHAND | CLONE_CLEAR_SIGHAND)) ==
+	    (CLONE_SIGHAND | CLONE_CLEAR_SIGHAND))
+		return false;
+
 	if ((kargs->flags & (CLONE_THREAD | CLONE_PARENT)) &&
 	    kargs->exit_signal)
 		return false;