[PATCH] drm: fix off-by-one in logger

[Norbert Manthey]

The current implementation will leak a byte to the log via memmove. The
specified 27 bytes are off-by-one, as the payload is 25 bytes, and the
termination character is only one byte large. To avoid this, factor out
the error message, and furthermore make the second parameter of the
append_entry function const.

Fixes: 4562236b3bc0 ("drm/amd/dc: Add dc display driver (v2)")

The full trace is as follows:

In function ‘memmove’,
   from ‘append_entry’ at
        drivers/gpu/drm/amd/display/dc/basics/logger.c:257:2,
   from ‘dm_logger_append_va’ at
        drivers/gpu/drm/amd/display/dc/basics/logger.c:348:4
   detected read beyond size of object passed as 2nd parameter

Signed-off-by: Norbert Manthey <nmanthey at amazon.de>

Cc: Alex Deucher <alexander.deucher at amd.com>
Cc: "Christian König" <christian.koenig at amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou at amd.com>
Cc: David Airlie <airlied at linux.ie>
Cc: Harry Wentland <harry.wentland at amd.com>
Cc: Tony Cheng <tony.cheng at amd.com>
Cc: Yongqiang Sun <yongqiang.sun at amd.com>
Cc: Aric Cyr <Aric.Cyr at amd.com>
Cc: Colin Ian King <colin.king at canonical.com>
Cc: Corbin McElhanney <corbin.mcelhanney at amd.com>
Cc: Jordan Lazare <Jordan.Lazare at amd.com>
Cc: Dmytro Laktyushkin <Dmytro.Laktyushkin at amd.com>
Cc: amd-gfx at lists.freedesktop.org
Cc: dri-devel at lists.freedesktop.org
Cc: linux-kernel at vger.kernel.org

---
 drivers/gpu/drm/amd/display/dc/basics/logger.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/basics/logger.c b/drivers/gpu/drm/amd/display/dc/basics/logger.c
index 180a9d6..958070c 100644
--- a/drivers/gpu/drm/amd/display/dc/basics/logger.c
+++ b/drivers/gpu/drm/amd/display/dc/basics/logger.c
@@ -243,7 +243,7 @@ static void log_heading(struct log_entry *entry)
 
 static void append_entry(
 		struct log_entry *entry,
-		char *buffer,
+		const char *buffer,
 		uint32_t buf_size)
 {
 	if (!entry->buf ||
@@ -345,7 +345,9 @@ void dm_logger_append_va(
 		if (size < LOG_MAX_LINE_SIZE - 1) {
 			append_entry(entry, buffer, size);
 		} else {
-			append_entry(entry, "LOG_ERROR, line too long\n", 27);
+			static const char msg[] = "LOG_ERROR, line too long\n";
+
+			append_entry(entry, msg, sizeof(msg));
 		}
 	}
 }
-- 
2.7.4

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