[PATCH] signal: always allocate siginfo for SI_TKILL

[Jack Andersen <jackoalan@gmail.com>]

The patch titled
`signal: Never allocate siginfo for SIGKILL or SIGSTOP`
created a regression for users of PTRACE_GETSIGINFO needing to
discern signals that were raised via the tgkill syscall.

A notable user of this tgkill+ptrace combination is lldb while
debugging a multithreaded program. Without the ability to detect a
SIGSTOP originating from tgkill, lldb does not have a way to
synchronize on a per-thread basis and falls back to SIGSTOP-ing the
entire process.

This patch allocates the siginfo as it did previously whenever the
SI_TKILL code is present.

Signed-off-by: Jack Andersen <jackoalan@gmail.com>
---
 kernel/signal.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 9a32bc2088c9..7a810aefb5df 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1058,9 +1058,11 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
 	result = TRACE_SIGNAL_DELIVERED;
 	/*
 	 * Skip useless siginfo allocation for SIGKILL SIGSTOP,
-	 * and kernel threads.
+	 * and kernel threads. SI_TKILL is an exception to allow
+	 * processes to discern signals originating from tgkill.
 	 */
-	if (sig_kernel_only(sig) || (t->flags & PF_KTHREAD))
+	if ((sig_kernel_only(sig) && info->si_code != SI_TKILL) ||
+	    (t->flags & PF_KTHREAD))
 		goto out_set;
 
 	/*
-- 
2.20.1