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From: Rich Felker <dalias@libc.org>
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Subject: Re: Hangup calling setuid() from vfork() child
Date: Mon, 30 Sep 2019 13:41:08 -0400
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On Mon, Sep 30, 2019 at 08:29:16AM -0700, Joshua Hudson wrote:
> If there is more than one thread and vfork() calls setuid(), musl libc hangs up.
> 
> void *thfunction(void*ig) {sleep(1000);returnNULL;}
> 
> int main()
> {
>     pthread_t id;
>     pthread_create(&id, NULL, thfunction, NULL);
>     if (vfork() == 0) {
>         setuid(0); /* hangup */
>         _exit(0);
>     }
> }

This is expected; the only legal action after vfork is _exit or
execve. In practice you could probably get by with
syscall(SYS_setuid,0) or similar in the child, but this isn't
supported usage and the specification for vfork has always been clear
that you can't do arbitrary stuff in the child. If you need to, you
should be using fork.

Rich