How can kill(-1, 0) return EPERM?

[Dmitry Marakasov]

Hi!

I'm helping to investigate some userspace issue [1], where kill(-1, SIGKILL)
fails with EPERM. I've managed to isolate this case in a small program:


```
#include <err.h>
#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int main() {
    if (setuid(66) == -1)  // uucp, just for the test
        err(1, "setuid");

    int res = kill(-1, 0);  // <- fails with EPERM
    fprintf(stderr, "kill(-1, 0) result=%d, errno=%s\n", res, strerror(errno));

    return 0;
}
```

when run from root on 12.1 kill call fails with EPERM. However I cannot
comprehend what it is caused by and how it's even possible: kill(2) manpage
says that with pid=-1 kill should only send (and in this case of sig=0,
/not/ send) signals to the processes belonging to the current uid, so there
should be no permission problems. I've also looked into the kernel code
(sys_kill, killpg1), and it matches to what manpage says, I see no way
for it to return EPERM: sys_kill() should fall through to the switch, call
killpg1() with all=1 and killpg1() if(all) branch may only set `ret` to
either 0 or ESRCH. Am I missing something, or is there a problem somewhere?

[1] https://github.com/NixOS/nix/issues/3250

-- 
Dmitry Marakasov   .   55B5 0596 FF1E 8D84 5F56  9510 D35A 80DD F9D2 F77D
amdmi3 at amdmi3.ru  ..:              https://github.com/AMDmi3