Re: [PATCH] signal: change sys_kill() to use SEND_SIG_NOINFO

[Oleg Nesterov]

On 06/26, Andrew Morton wrote:
>
> On Fri, 26 Jun 2026 17:33:08 +0200 Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> > prepare_kill_siginfo(PIDTYPE_TGID) fills si_code = SI_USER and sets
> > si_pid/si_uid in the sender's namespace. Then send_signal_locked()
> > translates si_pid/si_uid to the target's namespace.
> >
> > SEND_SIG_NOINFO produces the same result: si_code = SI_USER, and
> > __send_signal_locked() computes si_pid/si_uid directly in the target's
> > namespace. The force computation is also the same: both check if the
> > sender is visible in the target's pid namespace.
>
> The above paragraphs contain no description of any flaw.  What's wrong
> here?

Ah, sorry. nothing wrong if we forget about the fix from Bradley, but we
need this fix with or without this cleanup...

> > Note: this also fixes the kill(-1, sig) case where send_signal_locked()
> > rewrites si_pid/si_uid in the shared siginfo, corrupting it for subsequent
> > recipients. But for other group senders like __kill_pgrp_info() we still
> > need the fix from Bradley Morgan [1] who found this problem.
>
> "also fixes".  Again, what was the first fix?

Agreed. This is confusing.

> Thanks, I'll queue this for testing.

Thanks,

> Please send along some changelog
> edits sometime?

Please see the new changelog below. Does it look more clear?

Bradley, do you agree?

Oleg.
---

signal: change sys_kill() to use SEND_SIG_NOINFO

prepare_kill_siginfo(PIDTYPE_TGID) fills si_code = SI_USER and sets
si_pid/si_uid in the sender's namespace. Then send_signal_locked()
translates si_pid/si_uid to the target's namespace.

SEND_SIG_NOINFO exists precisely for the case when si_code == SI_USER
and si_pid/si_uid are the sender's ids; this is exactly what sys_kill()
does via prepare_kill_siginfo(PIDTYPE_TGID). Change sys_kill() to use
it directly.

SEND_SIG_NOINFO produces the same result: si_code = SI_USER, and
__send_signal_locked() computes si_pid/si_uid directly in the target's
namespace. The force computation is also the same: both check if the
sender is visible in the target's pid namespace.

This is just a cleanup and microoptimization (especially with [1]), this
skips the has_si_pid_and_uid() block in send_signal_locked() and offloads
the namespace translation logic to __send_signal_locked(SEND_SIG_NOINFO)
which uses the simpler computations.

NOTE: As a "side effect" this also fixes the kill(pid < 0, sig) case where
send_signal_locked() rewrites si_pid/si_uid in the shared siginfo,
corrupting it for subsequent recipients. But for other group senders like
__kill_pgrp_info() we still need the fix from Bradley Morgan [1] who found
this problem.

TODO: kill prepare_kill_siginfo() and change other users to use
SEND_SIG_NOINFO too. This needs trivial changes in __send_signal_locked()
and TP_STORE_SIGINFO().

Link: https://lore.kernel.org/all/20260622164029.11474-1-include@xxxxxxxxx/ [1]
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>