Re: [PATCH 1/2] clone3: add CLONE3_CLEAR_SIGHAND

Michal Hocko
2019-10-11T08:21:26Z

[Cc linux-api]

On Thu 10-10-19 15:35:17, Christian Brauner wrote:
> 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
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> ---
> include/uapi/linux/sched.h | 3 +++
> kernel/fork.c | 11 ++++++++++-
> 2 files changed, 13 insertions(+), 1 deletion(-)
>
> 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..661f8d1f3881 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;
> }
>
> @@ -2567,7 +2572,7 @@ 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)
> + if (kargs->flags & ~(CLONE_LEGACY_FLAGS | CLONE3_CLEAR_SIGHAND))
> return false;
>
> /*
> @@ -2577,6 +2582,10 @@ 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

--
Michal Hocko
SUSE Labs