[v2,1/2] clone3: add CLONE3_CLEAR_SIGHAND
Commit Message
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(-)
@@ -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
@@ -1517,6 +1517,11 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
spin_lock_irq(¤t->sighand->siglock);
memcpy(sig->action, current->sighand->action, sizeof(sig->action));
spin_unlock_irq(¤t->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;