Re: [musl] Namespace violation in system()?

[Rich Felker <dalias@libc.org>]

On Thu, May 04, 2023 at 06:33:27PM +0200, Markus Wichmann wrote:
> Hi all,
> 
> I stumbled upon the source code of system() today. It is this at the
> moment:
> 
> |int system(const char *cmd)
> |{
> |	pid_t pid;
> |	sigset_t old, reset;
> |	struct sigaction sa = { .sa_handler = SIG_IGN }, oldint, oldquit;
> |	int status = -1, ret;
> |	posix_spawnattr_t attr;
> |
> |	pthread_testcancel();
> |
> |	if (!cmd) return 1;
> |
> |	sigaction(SIGINT, &sa, &oldint);
> |	sigaction(SIGQUIT, &sa, &oldquit);
> |	sigaddset(&sa.sa_mask, SIGCHLD);
> |	sigprocmask(SIG_BLOCK, &sa.sa_mask, &old);
> |
> |	sigemptyset(&reset);
> |	if (oldint.sa_handler != SIG_IGN) sigaddset(&reset, SIGINT);
> |	if (oldquit.sa_handler != SIG_IGN) sigaddset(&reset, SIGQUIT);
> |	posix_spawnattr_init(&attr);
> |	posix_spawnattr_setsigmask(&attr, &old);
> |	posix_spawnattr_setsigdefault(&attr, &reset);
> |	posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGDEF|POSIX_SPAWN_SETSIGMASK);
> |	ret = posix_spawn(&pid, "/bin/sh", 0, &attr,
> |		(char *[]){"sh", "-c", (char *)cmd, 0}, __environ);
> |	posix_spawnattr_destroy(&attr);
> |
> |	if (!ret) while (waitpid(pid, &status, 0)<0 && errno == EINTR);
> |	sigaction(SIGINT, &oldint, NULL);
> |	sigaction(SIGQUIT, &oldquit, NULL);
> |	sigprocmask(SIG_SETMASK, &old, NULL);
> |
> |	if (ret) errno = ret;
> |	return status;
> |}
> 
> Aren't all of those calls namespace violations? system() is an ISO-C
> function, so the only symbols it is allowed to pull into the link are
> other ISO-C functions or hidden double-underscore symbols, right? But
> all the functions called here POSIX functions. And while POSIX contains
> the rule that posix_* functions are reserved, that is in POSIX, not
> ISO-C. And even with that rule, there are all the other calls.
> 
> Does someone need to pour out a bucket of underscores over this
> function?

The behavior of system() is implementation-defined, so we define it as
calling those functions. :-)

Rich