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Exit of the last `clone(CLONE_VM)` child ends the parent process

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c, linux
Domain
operating-systems

Research direction

Start in src/runtime/forkipc.c at vm_clone_thread_run around line 1446 and read the proc_request_exit_group call and its comment. Compare the clone child’s exit handling with the Linux behavior described in the issue, including how the parent is woken and how waitpid receives the child status. Done means the parent remains alive to wait for the child and return its own exit code.

Written by the indexing model from the issue text.

Description

bug

When the last clone(CLONE_VM) child (without CLONE_THREAD) of an elfuse process exits, vm_clone_thread_run (src/runtime/forkipc.c:1446, at 173b23b) calls proc_request_exit_group with the child's exit code. The parent ends with it, even when the parent is still running and has more work to do.

On Linux the child is a separate process. Its exit raises SIGCHLD in the parent, waitpid returns its status, and the parent keeps running.

Reproduce (static aarch64 binary):

#define _GNU_SOURCE
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

static volatile int go;
static int child(void *a) { (void) a; while (!go) usleep(1000); return 0; }
int main(void) {
    char *stk = malloc(1 << 16);
    pid_t c = clone(child, stk + (1 << 16), CLONE_VM | SIGCHLD, NULL);
    go = 1;
    int st;
    pid_t w = waitpid(c, &st, 0);
    dprintf(2, "waited %d st=%x\n", w, st);
    return 3;
}
  • Linux 6.18.57 (qemu reference VM): prints waited <pid> st=0, exits 3.
  • elfuse at 173b23b: prints nothing after the clone and exits 0, the child's code.

The comment at the call says the exit_group is there so that a main thread blocked in futex_wait or its run loop wakes up. To match Linux, that main thread would need another way to wake, and the parent would need to exit only through its own exit or exit_group.

test-proc-limits works around this in #448 by running its VM-clone check in a fork child.

Checked, not affected: posix_spawn, which glibc implements with clone(CLONE_VM | CLONE_VFORK). A static glibc guest that spawns busybox true, waits for it and returns 3 prints spawn rc=0 waited <pid> st=0 and exits 3, under elfuse at 173b23b and on Linux 6.18.57 alike.

Dominant language
C
Stars
275
Forks
29
Avg merge
2d 8h
Merged PRs (30d)
17

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