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

Aperta
  • Qualcosa non va?
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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
c, linux
Ambito
operating-systems

Direzione di ricerca

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.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

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.

Lingua principale
C
Stelle
275
Fork
29
Merge medio
2g 22h
PR unite (30g)
18

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