[PATCH v2] watchdog/perf: fix off by one in the raw event config copy

[Bradley Morgan <brads@mainlining.org>]

You were right, the truncation was the bigger half of the bug and my
v1 only closed the corner. Fixed both with your len + 1 shape.

Commit 6164be01f179 ("watchdog/perf: optimize bytes copied and remove
manual NUL-termination") replaced strscpy(buf, str, sizeof(buf)) plus a
manual buf[len] = 0 with strscpy(buf, str, len). That count is one less
than the code needs, strscpy() always reserves the last byte of the
destination for the NUL, so the config loses its final digit.
nmi_watchdog=r300,panic for example ends up with buf = "30" and arms
the raw event with the wrong config.

The same count also drops the empty case on the floor, strscpy() with
a zero count writes nothing at all, so nmi_watchdog=r,1 leaves buf
uninitialized and kstrtoull() reads stack garbage.

The old code was safe on both counts by accident, strscpy() filled the
whole buffer before buf[len] = 0 overwrote the comma position, so the
worst outcome was a truncated parse failure.

Pass len + 1 so the copy includes the character the NUL replaces, and
reject len >= sizeof(buf) like the code did before the optimization,
which also makes an empty config a clean parse failure again.

Suggested-by: Doug Anderson <dianders@chromium.org>
Fixes: 6164be01f179 ("watchdog/perf: optimize bytes copied and remove manual NUL-termination")
Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 kernel/watchdog_perf.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/watchdog_perf.c b/kernel/watchdog_perf.c
index cca0485ba28c..a4f677c16b20 100644
--- a/kernel/watchdog_perf.c
+++ b/kernel/watchdog_perf.c
@@ -301,10 +301,10 @@ void __init hardlockup_config_perf_event(const char *str)
 	} else {
 		unsigned int len = comma - str;
 
-		if (!len || len > sizeof(buf))
+		if (len >= sizeof(buf))
 			return;
 
-		strscpy(buf, str, len);
+		strscpy(buf, str, len + 1);
 		if (kstrtoull(buf, 16, &config))
 			return;
 	}
-- 
2.53.0