[PATCH -mm 1/1] signal: avoid unconditional siginfo copy in send_signal_locked()

[Oleg Nesterov <oleg@redhat.com>]

send_signal_locked() unconditionally copies siginfo before the namespace
translation to avoid corrupting a shared siginfo.

Not that I think this can actually hurt performance-wise, just it doesn't
look clean to me; the copy is only needed in the unlikely case when the
translation will actually change something.

Defer it to the two cases where si_pid/si_uid are rewritten, and while at
it add #ifdef's just for completeness.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 041498ff835e..0f509ee4f42d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1181,7 +1181,7 @@ static inline bool has_si_pid_and_uid(struct kernel_siginfo *info)
 int send_signal_locked(int sig, struct kernel_siginfo *info,
 		       struct task_struct *t, enum pid_type type)
 {
-	struct kernel_siginfo rewritten;
+	struct kernel_siginfo __maybe_unused rewritten;
 	/* Should SIGKILL or SIGSTOP be received by a pid namespace init? */
 	bool force = false;
 
@@ -1193,27 +1193,34 @@ int send_signal_locked(int sig, struct kernel_siginfo *info,
 		force = true;
 	} else if (has_si_pid_and_uid(info)) {
 		/* SIGKILL and SIGSTOP is special or has ids */
+#ifdef CONFIG_USER_NS
 		struct user_namespace *t_user_ns;
-
-		rewritten = *info;
-		info = &rewritten;
+		kuid_t uid;
 
 		rcu_read_lock();
 		t_user_ns = task_cred_xxx(t, user_ns);
 		if (current_user_ns() != t_user_ns) {
-			kuid_t uid = make_kuid(current_user_ns(), info->si_uid);
-			info->si_uid = from_kuid_munged(t_user_ns, uid);
+			rewritten = *info;
+			info = &rewritten;
+			uid = make_kuid(current_user_ns(), info->si_uid);
+			rewritten.si_uid = from_kuid_munged(t_user_ns, uid);
 		}
 		rcu_read_unlock();
-
+#endif
 		/* A kernel generated signal? */
 		force = (info->si_code == SI_KERNEL);
 
+#ifdef CONFIG_PID_NS
 		/* From an ancestor pid namespace? */
 		if (!task_pid_nr_ns(current, task_active_pid_ns(t))) {
-			info->si_pid = 0;
+			if (info != &rewritten) {
+				rewritten = *info;
+				info = &rewritten;
+			}
+			rewritten.si_pid = 0;
 			force = true;
 		}
+#endif
 	}
 	return __send_signal_locked(sig, info, t, type, force);
 }
-- 
2.52.0