Bug: Crash after forked child exits

[haael]

Contact Details

haael@interia.pl

What happened?

First fork a child. Then return from that child. The parent proces crashes with SIGSEGV.

#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	pid_t pid;
	printf("enter\n");
	
	pid = vfork();
	
	if(pid < 0)
	{
		printf("error %d\n", pid);
		return 1;
	}
	
	if(pid == 0)
	{
		printf("parent\n");
	}
	else
	{
		printf("child %d\n", pid);
	}
	
	printf("exit\n");
	return 0;
}

What gives?

Version

cosmocc (GCC) 14.1.0

What operating system are you seeing the problem on?

Linux

Relevant log output

The stack trace (from .com.dbg) shows null pointer at the top of the stack.


enter
[Detaching after vfork from child process 11666]
parent
exit
child 11666
exit

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) bt
#0  0x0000000000000000 in ?? ()
#1  0x00000000042015de in _start () at libc/crt/crt.S:128


In non-debug binary there are 5 pointers without labels, the top one is null.


Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) bt
#0  0x0000000000000000 in ?? ()
#1  0x00000000042015de in ?? ()
#2  0x0000000000000001 in ?? ()
#3  0x00007fffffffe318 in ?? ()
#4  0x0000000000000000 in ?? ()

[GabrielRavier]

Does the reproducer also fail if e.g.:

1. It uses fork instead of vfork ?
2. It doesn't return in the child from the function that called vfork ? (doing so is some pretty massive UB - the child unwinds the shared stack and clobbers the parent's frame - I'm genuinely surprised it can run on glibc)

I should like to note that while this somehow works on glibc, it crashes hard on musl - as a whole it seems like somewhat of a NOTABUG if either 1) or 2) makes it not crash.

(also fyi you've swapped the parent and child when printing)

[haael]

Swapping fork for vfork indeed solves the problem. Also, _exit(0) instead of return.

[jart]

If you use vfork() then be sure to build cosmo at head rather than using the last release. The 5ddb5c2 change added a lot of vfork() improvements to windows in particular, which now simulates vfork the same way that Cygwin does. To build cosmo from sauce you have to use that tool/cosmocc script.