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`wait4` and `waitid` never return `EINTR` when a handled signal arrives

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
54/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c
Domain
operating-systems

Research direction

Start in src/syscall/proc.c at the polling loops in sys_wait4 and sys_waitid, then compare how the poll, futex and pipe paths use signal_claim_interruption(). The issue describes the required behavior for handlers with and without SA_RESTART and provides a reproducer. Add regression coverage for wait4(pid), wait4(-1) and waitid, checking EINTR without SA_RESTART and successful reap with it.

Written by the indexing model from the issue text.

Description

bug

On Linux, a blocking wait4 or waitid sleeps in do_wait (kernel/exit.c) with TASK_INTERRUPTIBLE and leaves the loop with -ERESTARTSYS once a signal is pending. Signal delivery turns that into EINTR when the handler lacks SA_RESTART, and restarts the wait when it has it. Under elfuse the wait ignores the signal and returns only when the child exits: the polling loops in sys_wait4 and sys_waitid (src/syscall/proc.c) check thread_stop_requested() but never signal_claim_interruption(), which the poll, futex and pipe paths use for the same purpose.

Reproducer (the child signals the parent at 0.2 s and exits 1 s later; the handler has no SA_RESTART):

#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

static void on_usr1(int sig)
{
    (void) sig;
}

static double now(void)
{
    struct timespec t;
    clock_gettime(CLOCK_MONOTONIC, &t);
    return t.tv_sec + t.tv_nsec / 1e9;
}

static void run(const char *name, int mode)
{
    pid_t child = fork();
    if (child == 0) {
        usleep(200000);
        kill(getppid(), SIGUSR1);
        sleep(1);
        _exit(0);
    }
    double t0 = now();
    int status;
    siginfo_t si;
    long ret;
    if (mode == 0)
        ret = waitpid(child, &status, 0);
    else if (mode == 1)
        ret = waitpid(-1, &status, 0);
    else
        ret = waitid(P_PID, child, &si, WEXITED);
    int err = errno;
    printf("%-10s ret=%ld %s after %.2fs\n", name, ret,
           ret < 0 ? strerror(err) : "", now() - t0);
    if (ret < 0)
        waitpid(child, &status, 0);
}

int main(void)
{
    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));
    sa.sa_handler = on_usr1;
    sigaction(SIGUSR1, &sa, NULL);
    run("wait4(pid)", 0);
    run("wait4(-1)", 1);
    run("waitid", 2);
    return 0;
}
wait4(pid) wait4(-1) waitid
elfuse c2785c7 child pid after 1.2 s child pid after 1.2 s 0 after 1.2 s
Linux 6.12 under qemu -1, EINTR after 0.2 s -1, EINTR after 0.2 s -1, EINTR after 0.2 s

The SA_RESTART half needs its own handling: elfuse has no general ERESTARTSYS path, so a loop that starts honoring the claimed signal has to keep waiting when that signal's handler has SA_RESTART. A regression test needs the child to stay alive until the handler releases it, and should cover EINTR without SA_RESTART and a successful reap with it, for wait4(pid), wait4(-1) and waitid.

Found by jserv in https://github.com/sysprog21/elfuse/pull/460#discussion_r4348510702: the race fixed there never showed under elfuse for this reason.

Dominant language
C
Stars
275
Forks
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Avg merge
2d 22h
Merged PRs (30d)
18

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