[PATCH v2 1/2] clone3: add CLONE3_CLEAR_SIGHAND

* [PATCH v2 1/2] clone3: add CLONE3_CLEAR_SIGHAND
@ 2019-10-11 10:25 ` Christian Brauner
  0 siblings, 0 replies; 4+ messages in thread
From: Christian Brauner @ 2019-10-11 10:25 UTC (permalink / raw)
  To: linux-kernel, Oleg Nesterov, Florian Weimer, libc-alpha
  Cc: Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot,
	Dietmar Eggemann, Steven Rostedt, Ben Segall, Mel Gorman,
	Shuah Khan, Andrew Morton, Michal Hocko, Elena Reshetova,
	Thomas Gleixner, Roman Gushchin, Andrea Arcangeli, Al Viro,
	Aleksa Sarai, Dmitry V. Levin, linux-kselftest,
	Christian Brauner, linux-api

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 */
- Florian Weimer <fweimer@redhat.com>:
  - update comment in clone3_args_valid()
---
 include/uapi/linux/sched.h |  3 +++
 kernel/fork.c              | 16 +++++++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 99335e1f4a27..c583720f689f 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 CLONE3_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..0a0269cb2c18 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 & CLONE3_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 | CLONE3_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 | CLONE3_CLEAR_SIGHAND)) ==
+	    (CLONE_SIGHAND | CLONE3_CLEAR_SIGHAND))
+		return false;
+
 	if ((kargs->flags & (CLONE_THREAD | CLONE_PARENT)) &&
 	    kargs->exit_signal)
 		return false;
-- 
2.23.0


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