Re: [PATCH] posix: Fix system blocks SIGCHLD erroneously [BZ #30163]

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 24/02/23 15:00, Adam Yi wrote:
> Looks like some spaces/tabs got mixed up in the email. Please use the
> attachment instead.

Next time just send a v2, it is better to keep track with you patchwork
instance [1].

[1] https://patchwork.sourceware.org/project/glibc/list/

> 
> On Sat, Feb 25, 2023 at 1:31 AM Adam Yi <ayi@janestreet.com> wrote:
>>
>> Fix bug that SIGCHLD is erroneously blocked forever in the following
>> scenario:
>>
>> 1. Thread A calls system but hasn't returned yet
>> 2. Thread B calls another system but returns
>>
>> SIGCHLD would be blocked forever in thread B after its system() returns,
>> even after the system() in thread A returns.
>>
>> Although POSIX does not require, glibc system implementation aims to be
>> thread and cancellation safe. This bug was introduced in
>> 5fb7fc96350575c9adb1316833e48ca11553be49 when we moved reverting signal
>> mask to happen when the last concurrently running system returns,
>> despite that signal mask is per thread. This commit reverts this logic
>> and adds a test.

Patch looks ok, I would recommend a v3 to enhance the testing a bit.

>> ---
>>  stdlib/tst-system.c       | 27 +++++++++++++++++++++++++++
>>  support/shell-container.c | 10 ++++++++++
>>  sysdeps/posix/system.c    |  6 +++---
>>  3 files changed, 40 insertions(+), 3 deletions(-)
>>
>> diff --git a/stdlib/tst-system.c b/stdlib/tst-system.c
>> index 634acfe264..be95c2de44 100644
>> --- a/stdlib/tst-system.c
>> +++ b/stdlib/tst-system.c
>> @@ -20,6 +20,8 @@
>>  #include <string.h>
>>  #include <signal.h>
>>  #include <paths.h>
>> +#include <pthread.h>
>> +#include <inttypes.h>
>>
>>  #include <support/capture_subprocess.h>
>>  #include <support/check.h>
>> @@ -71,6 +73,19 @@ call_system (void *closure)
>>      }
>>  }
>>
>> +static void *
>> +sleep_and_check_sigchld (void *seconds)
>> +{
>> +  char cmd[namemax];
>> +  sprintf(cmd, "sleep %" PRIdPTR, (intptr_t) seconds);
>> +  TEST_COMPARE (system (cmd), 0);
>> +
>> +  sigset_t blocked = {0};
>> +  TEST_COMPARE (sigprocmask (SIG_BLOCK, NULL, &blocked), 0);
>> +  TEST_COMPARE (sigismember (&blocked, SIGCHLD), 0);
>> +  return NULL;
>> +}
>> +
>>  static int
>>  do_test (void)
>>  {
>> @@ -154,6 +169,18 @@ do_test (void)
>>      xchmod (_PATH_BSHELL, st.st_mode);
>>    }
>>
>> +  {
>> +    pthread_t long_sleep_thread, short_sleep_thread;
>> +
>> +    TEST_COMPARE (pthread_create (&long_sleep_thread, NULL,
>> +                  sleep_and_check_sigchld, (void *) 2), 0);
>> +    TEST_COMPARE (pthread_create (&short_sleep_thread, NULL,
>> +                  sleep_and_check_sigchld, (void *) 1), 0);
>> +
>> +    TEST_COMPARE (pthread_join (short_sleep_thread, NULL), 0);
>> +    TEST_COMPARE (pthread_join (long_sleep_thread, NULL), 0);
>> +  }
>> +
>>    TEST_COMPARE (system (""), 0);
>>
>>    return 0;

Do we really need to sleep for 2 seconds here? It adds more latency on
testcase run.

>> diff --git a/support/shell-container.c b/support/shell-container.c
>> index e9ac9b6d04..083426550b 100644
>> --- a/support/shell-container.c
>> +++ b/support/shell-container.c
>> @@ -171,6 +171,15 @@ kill_func (char **argv)
>>    return 0;
>>  }
>>
>> +/* Emulate the "/bin/sleep" command.  Options are ignored.  */
>> +static int
>> +sleep_func (char **argv)
>> +{
>> +  int secs = atoi (argv[0]);

I think it would be better add support for split second, similar to
coreutils.  Something like:

#include <support/check.h>
#include <support/timespec.h>

/* I shamelessly copied it from gnulib, move it to support/dtotimespec.c.  */
struct timespec
dtotimespec (double sec)
{
  if (! (TYPE_MINIMUM (time_t) < sec))
    return make_timespec (TYPE_MINIMUM (time_t), 0);
  else if (! (sec < 1.0 + TYPE_MAXIMUM (time_t)))
    return make_timespec (TYPE_MAXIMUM (time_t), TIMESPEC_HZ - 1);
  else
    {
      time_t s = sec;
      double frac = TIMESPEC_HZ * (sec - s);
      long ns = frac;
      ns += ns < frac;
      s += ns / TIMESPEC_HZ;
      ns %= TIMESPEC_HZ;

      if (ns < 0)
        {
          s--;
          ns += TIMESPEC_HZ;
        }

      return make_timespec (s, ns);
    }
}

static int
sleep_func (char **argv)
{
   TEST_VERIFY_EXIT  (argv[0] != NULL);
   char *endptr = NULL;
   double sec = strtod (argv[0], &endptr);
   TEST_VERIFY_EXIT (endptr != argv[0] && errno != ERANGE);
   /* No suffix support.  */
   TEST_VERIFY_EXIT (sec >= 0.0);
   
   struct timespec ts = dtotimespec (sec);
   /* We don't expect any signal here.  */
   TEST_VERIFY_EXIT (nanosleep (&ts, NULL) == 0);
}   

>> +  sleep (secs);
>> +  return 0;
>> +}
>> +
>>  /* This is a list of all the built-in commands we understand.  */
>>  static struct {
>>    const char *name;
>> @@ -181,6 +190,7 @@ static struct {
>>    { "cp", copy_func },
>>    { "exit", exit_func },
>>    { "kill", kill_func },
>> +  { "sleep", sleep_func },
>>    { NULL, NULL }
>>  };
>>
>> diff --git a/sysdeps/posix/system.c b/sysdeps/posix/system.c
>> index 2335a99184..d77720a625 100644
>> --- a/sysdeps/posix/system.c
>> +++ b/sysdeps/posix/system.c
>> @@ -179,16 +179,16 @@ do_system (const char *line)
>>        as if the shell had terminated using _exit(127).  */
>>     status = W_EXITCODE (127, 0);
>>
>> +  /* sigaction can not fail with SIGINT/SIGQUIT used with old
>> +     disposition.  Same applies for sigprocmask.  */
>>    DO_LOCK ();
>>    if (SUB_REF () == 0)
>>      {
>> -      /* sigaction can not fail with SIGINT/SIGQUIT used with old
>> - disposition.  Same applies for sigprocmask.  */
>>        __sigaction (SIGINT, &intr, NULL);
>>        __sigaction (SIGQUIT, &quit, NULL);
>> -      __sigprocmask (SIG_SETMASK, &omask, NULL);
>>      }
>>    DO_UNLOCK ();
>> +  __sigprocmask (SIG_SETMASK, &omask, NULL);
>>
>>    if (ret != 0)
>>      __set_errno (ret);

Ok, thanks for spotting it.

>> --
>> 2.31.1