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Date: Sat, 25 Jun 2022 14:51:10 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Nick Peng <pymumu@...il.com>
Cc: musl@...ts.openwall.com
Subject: Re: BUG: Calling readdir/dirfd after vfork will cause
deadlock.
* Nick Peng <pymumu@...il.com> [2022-06-25 11:40:17 +0800]:
> Description: After vfork, calling functions such as readdir/dirfd may
> cause deadlock. GNU C is OK.
why do you think "GNU C is OK"? is this from some real software?
opendir after vfork is documented to be invalid in glibc:
https://www.gnu.org/software/libc/manual/html_mono/libc.html#Low_002dlevel-Directory-Access
the standard is actually much stricter than the glibc manual:
posix conforming code must not call any libc api after vfork
other than _exit or the exec* familiy of functions.
(as-safe is not enough, but opendir is not even as-safe)
since the example is multi-threaded even using fork would
be invalid, but i think both musl and glibc makes that work
(as an extension to the standard).
> Also tested on x86-64 with musl, no deadlock, but
> seems never exit, slower than GNU C.
> Version: latest, musl-1.2.3
> OS: debian bullseye 64bit OS. and asus router
> CPU: raspberrypi aarch64， mips32
> Reproduce Code:
>
> #include <dirent.h>
> #include <pthread.h>
> #include <stdint.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <unistd.h>
> #include <string.h>
>
> pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
> pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
>
> struct tlog_log *logs = NULL;
> int do_exit = 0;
>
> void *test(void *arg)
> {
> int i = 0;
>
> for (i = 0; i < 10000000; i++) {
> char *b = malloc(4096);
> memset(b, 0, 4096);
> free(b);
> }
> do_exit = 1;
> return NULL;
> }
>
> void lockfunc()
> {
> char path_name[4096];
> DIR *dir = NULL;
> struct dirent *ent;
>
> snprintf(path_name, sizeof(path_name), "/proc/self/fd/");
> dir = opendir(path_name);
> if (dir == NULL) {
> goto errout;
> }
>
> while ((ent = readdir(dir)) != NULL) {
> }
>
> closedir(dir);
>
> return;
> errout:
> if (dir) {
> closedir(dir);
> }
>
> return;
> }
>
> void *test_fork(void *arg)
> {
> int count = 0;
> while (do_exit == 0) {
> printf("test fork count %d\n", count++);
> int pid = vfork();
> if (pid < 0) {
> return NULL;
> } else if (pid == 0) {
> lockfunc();
> _exit(0);
> }
>
> int status;
> waitpid(pid, &status, 0);
> }
>
> return NULL;
> }
>
> int main(int argc, char *argv[])
> {
> pthread_attr_t attr;
> pthread_t threads[10];
> pthread_t fork_test;
> int i;
> int ret;
>
> pthread_attr_init(&attr);
>
> ret = pthread_create(&fork_test, &attr, test_fork, NULL);
>
> for (i = 0; i < 10; i++) {
> ret = pthread_create(&threads[i], &attr, test, NULL);
> if (ret != 0) {
> return 1;
> }
> }
>
> for (i = 0; i < 10; i++) {
> void *retval = NULL;
> pthread_join(threads[i], &retval);
> }
>
> void *retval = NULL;
> pthread_join(fork_test, &retval);
> printf("exit\n");
> getchar();
> return 0;
> }
>
>
> Log：
> pi@...pberrypi:~/code/tinylog/test $ ./test
> test fork count 0
> test fork count 1 <-- lock here
> ^C
>
> gdb backtrace:
> x0000000000409524 in __lock ()
> (gdb) bt
> #0 0x0000000000409524 in __lock ()
> #1 0x0000000000406278 in __libc_malloc_impl ()
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