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/aj6btAZqYuv59a8w@redhat.com
Link: https://lore.kernel.org/all/20260622164029.11474-1-include@grrlz.net/ [1]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Bradley Morgan <include@grrlz.net>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/signal.c

@@ -3962,11 +3962,7 @@ static void prepare_kill_siginfo(int sig, struct kernel_siginfo *info,
  */
 SYSCALL_DEFINE2(kill, pid_t, pid, int, sig)
 {
-	struct kernel_siginfo info;
-
-	prepare_kill_siginfo(sig, &info, PIDTYPE_TGID);
-
-	return kill_something_info(sig, &info, pid);
+	return kill_something_info(sig, SEND_SIG_NOINFO, pid);
 }
 
 /*