Unverified Commit b612e5df authored Oct 14, 2019 by Christian Brauner

clone3: add CLONE_CLEAR_SIGHAND

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: 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>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20191014104538.3096-1-christian.brauner@ubuntu.com

parent 1e1d0f0b

include/uapi/linux/sched.h

+3 −0

Original line number	Diff line number	Diff line
		@@ -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

kernel/fork.c

+11 −5

Original line number	Diff line number	Diff line
		@@ -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;
		}

		@@ -2619,11 +2624,8 @@ 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;

		/*
		@@ -2633,6 +2635,10 @@ 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;

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