Date: Fri, 29 Nov 2019 19:45:09 +0300 From: Dmitry Marakasov <amdmi3@amdmi3.ru> To: freebsd-stable@freebsd.org Subject: Re: How can kill(-1, 0) return EPERM? Message-ID: <20191129164509.GE4071@hades.panopticon> In-Reply-To: <20191129151606.GD4071@hades.panopticon> References: <20191129151606.GD4071@hades.panopticon>
next in thread | previous in thread | raw e-mail | index | archive | help
* Dmitry Marakasov (amdmi3@amdmi3.ru) wrote:
> 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?
It looks like I have misread the `else if' path of this core.
if (all) {
/*
* broadcast
*/
sx_slock(&allproc_lock);
FOREACH_PROC_IN_SYSTEM(p) {
if (p->p_pid <= 1 || p->p_flag & P_SYSTEM ||
p == td->td_proc || p->p_state == PRS_NEW) {
continue;
}
PROC_LOCK(p);
err = p_cansignal(td, p, sig);
if (err == 0) {
if (sig)
pksignal(p, sig, ksi);
ret = err;
}
else if (ret == ESRCH)
ret = err;
PROC_UNLOCK(p);
}
sx_sunlock(&allproc_lock);
} ...
so it's clear now where EPERM comes from. However it looks like the
behavior contradicts the manpage - there are no signs of check that
the signalled process has the same uid as the caller.
--
Dmitry Marakasov . 55B5 0596 FF1E 8D84 5F56 9510 D35A 80DD F9D2 F77D
amdmi3@amdmi3.ru ..: https://github.com/AMDmi3
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20191129164509.GE4071>