Subject: system* does not honor SIGINT restoration in child

From: Olivier Dion <[email-masked]>
Date: Tue, 01 Oct 2024 16:51:03 -0400

Hi,

Given the following C program:
--8<---------------cut here---------------start------------->8---
#include <stdio.h>
#include <signal.h>

int main(void)
{
	struct sigaction act;

	sigaction(SIGINT, NULL, &act);

	printf("%p %p %p\n", act.sa_handler, SIG_IGN, SIG_DFL);
}
--8<---------------cut here---------------end--------------->8---

and its ouput from various executions:

  1) $ ./a.out => (nil) 0x1 (nil)
  2) $ guile -c '(system "./a.out")' => (nil) 0x1 (nil)
  3) $ guile -c '(system* "./a.out")' => 0x1 0x1 (nil)

We can see that 3) does not honor restoration of `SIGINT' to `SIG_DFL'
like `system(3)' does.  This seems to be because of the following
sigaction before the creation of the process:

--8<---------------cut here---------------start------------->8---
1697  scm_dynwind_sigaction (SIGINT,
1698                         scm_from_uintptr_t ((uintptr_t) SIG_IGN),
1699                         SCM_UNDEFINED);
1700 #ifdef SIGQUIT
1701  scm_dynwind_sigaction (SIGQUIT,
1702                         scm_from_uintptr_t ((uintptr_t) SIG_IGN),
1703                         SCM_UNDEFINED);
1704 #endif
1705
1706  err = piped_process (&pid, prog, args,
1707                       SCM_UNDEFINED, SCM_UNDEFINED);
--8<---------------cut here---------------end--------------->8---

>From execve(2):

   POSIX.1 specifies that the dispositions of any signals that are
   ignored or set to the default are left unchanged.  POSIX.1 specifies
   one exception: if SIGCHLD is being ignored, then an implementation
   may leave the disposition unchanged or reset it to the default; Linux
   does the former.

Therefore, setting `SIG_IGN' for `SIGINT' before the fork/execve results
in ignoring the signals in the child, which is unexpected as a user of
`system(3)'.  The solution would be to restore `SIG_DFL' before
`execve(2)' if before the call to `system*' the action was not
`SIG_IGN'.

In the mean time, I have a solution that works for single-threaded
application:
--8<---------------cut here---------------start------------->8---
(define (system* . args)
  (let ((handler+flags (sigaction SIGINT)))
    (dynamic-wind
      (lambda ()
        (sigaction SIGINT SIG_IGN))
      (lambda ()
        (let ((cpid (primitive-fork)))
          (if (zero? cpid)
              (catch #t
                (lambda ()
                  (sigaction SIGINT SIG_DFL)
                  (apply execlp (car args) args))
                (lambda _
                  (primitive-exit EXIT_FAILURE)))
              (waitpid cpid))))
      (lambda ()
        (sigaction SIGINT
          (car handler+flags)
          (cdr handler+flags))))))
--8<---------------cut here---------------end--------------->8---

Thanks,
Olivier

-- 
Olivier Dion
oldiob.ca