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

[Petr Skocik <pskocik@gmail.com>]

On 5/4/23 19:52, Rich Felker wrote:
> 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

Hi. I think Markus Wichmann makes a valid point.

Unless it's valid behavior for

#include <stdio.h>
#include <stdlib.h>
void sigprocmask(void){
     puts("Hi, I'm a user-defined sigprocmask");
     system("echo hello world");
}
int main(void){ sigprocmask(); }

to keep outputting "Hi, I'm a user-defined sigprocmask" until it crashes 
from stack overflow (musl-gcc -static).

Cheers, Petr Skocik