Re: [linus:master] [mm] ba42b524a0: segfault_at_ip_sp_error

[Ingo Saitz <ingo@xxxxxxxxxxxxxxx>]

On Tue, May 28, 2024 at 10:52:41PM +0200, Ingo Saitz wrote:
> I hit the same error on devuan unstable, with startpar crashing on boot:
> 
> [   10.025881] dracut: Switching root
> [   10.339687] startpar[720]: segfault at 0 ip 0000000000000000 sp 00007fffbdca9c38 error 14 likely on CPU 1 (core 0, socket 0)
> [   10.340225] Code: Unable to access opcode bytes at 0xffffffffffffffd6.
> [   10.349516] startpar[820]: segfault at 0 ip 0000000000000000 sp 00007ffd52c34e48 error 14 likely on CPU 3 (core 1, socket 0)
> [   10.350086] Code: Unable to access opcode bytes at 0xffffffffffffffd6.

I can reproduce the error with the attached c program.

[ 1325.980596] thoughts[2127]: segfault at 0 ip 0000000000000000 sp 00007fff183d66e8 error 14 likely on CPU 0 (core 0, socket 0)
[ 1325.981180] Code: Unable to access opcode bytes at 0xffffffffffffffd6.

Simply calling mlockall(MCL_CURRENT) and then fork() seems to be enough
to trigger the error.

    Ingo
-- 
const_cast<long double>(Λ)

#define _GNU_SOURCE

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/select.h>
#include <unistd.h>

int main() {
    mlockall(MCL_CURRENT);

    pid_t pid = fork();
    if (pid == 0) {
        _exit(0);
    }

    const struct timespec zero = {2, 0};

    sigset_t smask;
    sigemptyset(&smask);

    pselect(0, 0, 0, 0, &zero, &smask);

    printf("Success on pid %d\n", pid);
}