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From: Alexey Izbyshev <izbyshev@ispras.ru>
To: musl@lists.openwall.com
Subject: [musl] Potentially infinite loop in posix_spawn'ed child
Date: Mon, 24 May 2021 13:09:21 +0300	[thread overview]
Message-ID: <985e15962c164eb3076752d6ee4c05fe@ispras.ru> (raw)

[-- Attachment #1: Type: text/plain, Size: 912 bytes --]

Hi,

I've noticed the following loop at 
https://git.musl-libc.org/cgit/musl/tree/src/process/posix_spawn.c#n159:

     exec(args->path, args->argv, args->envp);
     ret = -errno;

fail:
     /* Since sizeof errno < PIPE_BUF, the write is atomic. */
     ret = -ret;
     if (ret) while (__syscall(SYS_write, p, &ret, sizeof ret) < 0);
     _exit(127);

Is there any reason that write is done in a loop? If SIGPIPE is blocked 
or ignored and the parent dies before this point, the child will spin in 
it forever.

A test case is attached. It overrides execve() to abuse it as a 
callback, avoiding reliance on timings.

As an aside, perhaps it would make sense to call execve() in 
posix_spawn() implementation via a hidden symbol? This would both make 
it consistent with posix_spawnp() and avoid any trouble with user code 
executing in a vfork'ed child if execve() is overridden via e.g. 
LD_PRELOAD.

Alexey

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: test.c --]
[-- Type: text/x-c; name=test.c, Size: 440 bytes --]

#include <errno.h>
#include <signal.h>
#include <spawn.h>
#include <unistd.h>

static int p[2];

int execve(const char *path, char *const argv[], char *const envp[]) {
  close(p[1]);
  kill(getppid(), SIGKILL);
  read(p[0], &(char){0}, 1);
  errno = ENOENT;
  return -1;
}

int main(int argc, char *argv[], char *envp[]) {
    signal(SIGPIPE, SIG_IGN);
    pipe(p);
    posix_spawn(0L, "/non-existing", 0L, 0L, argv, envp);
    return 0;
}

             reply	other threads:[~2021-05-24 10:09 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-24 10:09 Alexey Izbyshev [this message]
2021-05-24 15:40 ` Szabolcs Nagy
2021-05-24 16:50   ` Alexey Izbyshev
2021-05-24 20:33 ` Rich Felker
2021-05-25  6:30   ` Alexey Izbyshev
2021-05-25 14:32     ` Rich Felker
2024-02-29 14:03       ` Alexey Izbyshev
2024-02-29 15:35         ` Rich Felker

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