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From: Arnd Bergmann <arnd@kernel.org>
To: SeongJae Park <sj@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
	damon@lists.linux.dev,
	linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: [PATCH] mm/damon/dbgfs: fix bogus string length
Date: Fri,  2 Feb 2024 13:43:26 +0100
Message-Id: <20240202124339.892862-1-arnd@kernel.org>
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From: Arnd Bergmann <arnd@arndb.de>

gcc correctly points out that using strnlen() on a fixed size array
is nonsense with an overlong limit:

mm/damon/dbgfs.c: In function 'damon_dbgfs_deprecated_read':
mm/damon/dbgfs.c:814:19: error: 'strnlen' specified bound 1024 exceeds source size 512 [-Werror=stringop-overread]
  814 |         int len = strnlen(kbuf, 1024);
      |                   ^~~~~~~~~~~~~~~~~~~
mm/damon/dbgfs.c:813:14: note: source object allocated here
  813 |         char kbuf[512] = DAMON_DBGFS_DEPRECATION_NOTICE;
      |              ^~~~

In fact, neither of the arbitrary limits are needed here: The first
one can just be a static const string and avoid wasting any more
space then necessary, and the strnlen() can be either strlen() or
sizeof(kbuf)-1, both of which the compiler turns into the same
constant here.

Fixes: adf9047adfff ("mm/damon/dbgfs: implement deprecation notice file")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 mm/damon/dbgfs.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index f7abbc0633aa..4a189eb65d14 100644
--- a/mm/damon/dbgfs.c
+++ b/mm/damon/dbgfs.c
@@ -810,10 +810,9 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx)
 static ssize_t damon_dbgfs_deprecated_read(struct file *file,
 		char __user *buf, size_t count, loff_t *ppos)
 {
-	char kbuf[512] = DAMON_DBGFS_DEPRECATION_NOTICE;
-	int len = strnlen(kbuf, 1024);
+	static const char kbuf[] = DAMON_DBGFS_DEPRECATION_NOTICE;
 
-	return simple_read_from_buffer(buf, count, ppos, kbuf, len);
+	return simple_read_from_buffer(buf, count, ppos, kbuf, strlen(kbuf));
 }
 
 /*
-- 
2.39.2