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(¤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; }
@@ -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;