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

[Oleg Nesterov <oleg@redhat.com>]

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.

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.

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@grrlz.net/ [1]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 077effd21582..12edbf43d678 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3966,11 +3966,7 @@ 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);
 }
 
 /*
-- 
2.52.0