Re: [PATCH v3] posix: Fix system blocks SIGCHLD erroneously [BZ #30163]

[Adam Yi <ayi@janestreet.com>]

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Thanks for reviewing!

On Fri, Mar 3, 2023 at 12:42 AM Adhemerval Zanella Netto <
adhemerval.zanella@linaro.org> wrote:

>
>
> On 24/02/23 12:27, Adam Yi 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.
>
> Thanks for working on this, just some more minor comments below.
>
> > ---
> >  stdlib/tst-system.c          | 28 ++++++++++++++++++++
> >  support/Makefile             |  2 ++
> >  support/dtotimespec-time64.c | 27 +++++++++++++++++++
> >  support/dtotimespec.c        | 50 ++++++++++++++++++++++++++++++++++++
> >  support/shell-container.c    | 13 ++++++++++
> >  support/timespec.h           |  4 +++
> >  sysdeps/posix/system.c       |  6 ++---
> >  7 files changed, 127 insertions(+), 3 deletions(-)
> >  create mode 100644 support/dtotimespec-time64.c
> >  create mode 100644 support/dtotimespec.c
> >
> > diff --git a/stdlib/tst-system.c b/stdlib/tst-system.c
> > index 634acfe264..736d2bf95b 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 +73,20 @@ call_system (void *closure)
> >      }
> >  }
> >
> > +static void *
> > +sleep_and_check_sigchld (void *closure)
> > +{
> > +  double *seconds = (double *) closure;
> > +  char cmd[namemax];
> > +  sprintf (cmd, "sleep %lf" , *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 +170,18 @@ 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, &(double) { 0.2 }), 0);
> > +    TEST_COMPARE (pthread_create (&short_sleep_thread, NULL,
> > +                  sleep_and_check_sigchld, &(double) { 0.1 }), 0);
> > +
> > +    TEST_COMPARE (pthread_join (short_sleep_thread, NULL), 0);
> > +    TEST_COMPARE (pthread_join (long_sleep_thread, NULL), 0);
>
>
> Use xpthread_create and xpthread_join here:
>
>   pthread_t short_sleep_thread = xpthread_create (NULL,
>         sleep_and_check_sigchld, &(double) { 0.2 });
>   pthread_t long_sleep_thread = xpthread_create (NULL,
>         sleep_and_check_sigchld, &(double) { 0.1 });
>   xpthread_join (short_sleep_thread);
>   xpthread_join (sleep_and_check_sigchld);
>

Will do. Thanks!

>
> > +  }
> > +
> >    TEST_COMPARE (system (""), 0);
> >
> >    return 0> diff --git a/support/Makefile b/support/Makefile
> > index b29b7eb505..48cd74581d 100644
> > --- a/support/Makefile
> > +++ b/support/Makefile
> > @@ -32,6 +32,8 @@ libsupport-routines = \
> >    check_hostent \
> >    check_netent \
> >    delayed_exit \
> > +  dtotimespec \
> > +  dtotimespec-time64 \
> >    ignore_stderr \
> >    next_to_fault \
> >    oom_error \
>
> Ok.
>
> > diff --git a/support/dtotimespec-time64.c b/support/dtotimespec-time64.c
> > new file mode 100644
> > index 0000000000..b3d5e351e3
> > --- /dev/null
> > +++ b/support/dtotimespec-time64.c
> > @@ -0,0 +1,27 @@
> > +/* Convert double to timespec.  64-bit time support.
> > +   Copyright (C) 2011-2023 Free Software Foundation, Inc.
> > +   This file is part of the GNU C Library and is also part of gnulib.
> > +   Patches to this file should be submitted to both projects.
> > +
> > +   The GNU C Library is free software; you can redistribute it and/or
> > +   modify it under the terms of the GNU Lesser General Public
> > +   License as published by the Free Software Foundation; either
> > +   version 2.1 of the License, or (at your option) any later version.
> > +
> > +   The GNU C Library is distributed in the hope that it will be useful,
> > +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> > +   Lesser General Public License for more details.
> > +
> > +   You should have received a copy of the GNU Lesser General Public
> > +   License along with the GNU C Library; if not, see
> > +   <https://www.gnu.org/licenses/>.  */
> > +
> > +#include <time.h>
> > +
> > +#if __TIMESIZE != 64
> > +# define timespec      __timespec64
> > +# define time_t        __time64_t
> > +# define dtotimespec   dtotimespec_time64
> > +# include "dtotimespec.c"
> > +#endif
>
> Ok.
>
> > diff --git a/support/dtotimespec.c b/support/dtotimespec.c
> > new file mode 100644
> > index 0000000000..860403b83c
> > --- /dev/null
> > +++ b/support/dtotimespec.c
> > @@ -0,0 +1,50 @@
> > +/* Convert double to timespec.
> > +   Copyright (C) 2011-2023 Free Software Foundation, Inc.
> > +   This file is part of the GNU C Library and is also part of gnulib.
> > +   Patches to this file should be submitted to both projects.
> > +
> > +   The GNU C Library is free software; you can redistribute it and/or
> > +   modify it under the terms of the GNU Lesser General Public
> > +   License as published by the Free Software Foundation; either
> > +   version 2.1 of the License, or (at your option) any later version.
> > +
> > +   The GNU C Library is distributed in the hope that it will be useful,
> > +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> > +   Lesser General Public License for more details.
> > +
> > +   You should have received a copy of the GNU Lesser General Public
> > +   License along with the GNU C Library; if not, see
> > +   <https://www.gnu.org/licenses/>.  */
> > +
> > +/* Convert the double value SEC to a struct timespec.  Round toward
> > +   positive infinity.  On overflow, return an extremal value.  */
> > +
> > +#include <support/timespec.h>
> > +#include <intprops.h>
> > +
> > +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);
> > +    }
> > +}
>
> Ok.
>
> > diff --git a/support/shell-container.c b/support/shell-container.c
> > index e9ac9b6d04..a2be314ad9 100644
> > --- a/support/shell-container.c
> > +++ b/support/shell-container.c
> > @@ -39,6 +39,7 @@
> >  #include <error.h>
> >
> >  #include <support/support.h>
> > +#include <support/timespec.h>
> >
> >  /* Design considerations
> >
> > @@ -171,6 +172,17 @@ kill_func (char **argv)
> >    return 0;
> >  }
> >
> > +/* Emulate the "/bin/sleep" command.  No suffix support.  Options are
> > +   ignored.  */
> > +static int
> > +sleep_func (char **argv)
> > +{
> > +  double sec = atof (argv[0]);
>
> Add some parsing check here:
>
>    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 and only positive values.  */
>    TEST_VERIFY_EXIT (sec >= 0.0);
>

I'm not sure if TEST_VERIFY_EXIT is the correct thing to do here, since
it's the mocked /bin/sh,
not directly part of the test. It's weird that providing invalid argument
to `sleep` would crash the shell.
Would it make more sense to print an error to stderr and return 1? FWIW, I
didn't add parsing check
here because I saw that other functions here do not have any sanity checks.
exit and kill are just
calling atoi without checking for errors and they don't check return values
either. There are some
checks in copy_func but it doesn't even check that there are two arguments
before referencing them.



> > +  struct timespec ts = dtotimespec (sec);
> > +  nanosleep (&ts, NULL);
> > +  return 0;
> > +}
> > +
> >  /* This is a list of all the built-in commands we understand.  */
> >  static struct {
> >    const char *name;
> > @@ -181,6 +193,7 @@ static struct {
> >    { "cp", copy_func },
> >    { "exit", exit_func },
> >    { "kill", kill_func },
> > +  { "sleep", sleep_func },
> >    { NULL, NULL }
> >  };
> >
> > diff --git a/support/timespec.h b/support/timespec.h
> > index 77b1e4e8d6..9559836d4c 100644
> > --- a/support/timespec.h
> > +++ b/support/timespec.h
> > @@ -57,6 +57,8 @@ int support_timespec_check_in_range (struct timespec
> expected,
> >                                    struct timespec observed,
> >                                    double lower_bound, double
> upper_bound);
> >
> > +struct timespec dtotimespec (double sec) __attribute__((const));
> > +
> >  #else
> >  struct timespec __REDIRECT (timespec_add, (struct timespec, struct
> timespec),
> >                           timespec_add_time64);
> > @@ -82,6 +84,8 @@ int __REDIRECT (support_timespec_check_in_range,
> (struct timespec expected,
> >                                                 double lower_bound,
> >                                                 double upper_bound),
> >               support_timespec_check_in_range_time64);
> > +
> > +struct timespec __REDIRECT (dtotimespec, (double sec),
> dtotimespec_time64);
> >  #endif
> >
> >  /* Check that the timespec on the left represents a time before the
> > 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);
>