linux-kernel - SIGCHLD signal sometimes sent with si

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Date:   Sun, 19 Apr 2020 22:13:36 +0200
From:   Christof Meerwald <cmeerw@...erw.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: SIGCHLD signal sometimes sent with si_pid==0 (Linux 5.6.5)

Hi,

this is probably related to commit
7a0cf094944e2540758b7f957eb6846d5126f535 (signal: Correct namespace
fixups of si_pid and si_uid).

With a 5.6.5 kernel I am seeing SIGCHLD signals that don't include a
properly set si_pid field - this seems to happen for multi-threaded
child processes.

A simple test program (based on the sample from the signalfd man page):

#include <sys/signalfd.h>
#include <signal.h>
#include <unistd.h>
#include <spawn.h>
#include <stdlib.h>
#include <stdio.h>

#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

int main(int argc, char *argv[])
{
  sigset_t mask;
  int sfd;
  struct signalfd_siginfo fdsi;
  ssize_t s;

  sigemptyset(&mask);
  sigaddset(&mask, SIGCHLD);

  if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
    handle_error("sigprocmask");

  pid_t chldpid;
  char *chldargv[] = { "./sfdclient", NULL };
  posix_spawn(&chldpid, "./sfdclient", NULL, NULL, chldargv, NULL);

  sfd = signalfd(-1, &mask, 0);
  if (sfd == -1)
    handle_error("signalfd");

  for (;;) {
    s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
    if (s != sizeof(struct signalfd_siginfo))
      handle_error("read");

    if (fdsi.ssi_signo == SIGCHLD) {
      printf("Got SIGCHLD %d %d %d %d\n",
          fdsi.ssi_status, fdsi.ssi_code,
          fdsi.ssi_uid, fdsi.ssi_pid);
      return 0;
    } else {
      printf("Read unexpected signal\n");
    }
  }
}


and a multi-threaded client to test with:

#include <unistd.h>
#include <pthread.h>

void *f(void *arg)
{
  sleep(100);
}

int main()
{
  pthread_t t[8];

  for (int i = 0; i != 8; ++i)
  {
    pthread_create(&t[i], NULL, f, NULL);
  }
}


I tried to do a bit of debugging and what seems to be happening is
that

		/* From an ancestor pid namespace? */
		if (!task_pid_nr_ns(current, task_active_pid_ns(t))) {

fails inside task_pid_nr_ns because the check for "pid_alive" fails.

This code seems to be called from do_notify_parent and there we
actually have "tsk != current" (I am assuming both are threads of the
current process?)


Christof

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