SIGCHLD signal sometimes sent with si_pid==0 (Linux 5.6.5)
From: Christof Meerwald
Date: Sun Apr 19 2020 - 16:20:05 EST
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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