linux-kernel - [RFC v6 7/8] mm: Fix benign off-by-one bugs

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Message-ID: <c88780354e13d8531f4f4118f251a070de7ef13e.1752193588.git.alx@kernel.org>
Date: Fri, 11 Jul 2025 03:57:18 +0200
From: Alejandro Colomar <alx@...nel.org>
To: linux-mm@...ck.org, linux-hardening@...r.kernel.org
Cc: Alejandro Colomar <alx@...nel.org>, Kees Cook <kees@...nel.org>, 
	Christopher Bazley <chris.bazley.wg14@...il.com>, shadow <~hallyn/shadow@...ts.sr.ht>, 
	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, 
	kasan-dev@...glegroups.com, Dmitry Vyukov <dvyukov@...gle.com>, 
	Alexander Potapenko <glider@...gle.com>, Marco Elver <elver@...gle.com>, Christoph Lameter <cl@...ux.com>, 
	David Rientjes <rientjes@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>, 
	Andrew Clayton <andrew@...ital-domain.net>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Michal Hocko <mhocko@...e.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Al Viro <viro@...iv.linux.org.uk>, Martin Uecker <uecker@...raz.at>, Sam James <sam@...too.org>, 
	Andrew Pinski <pinskia@...il.com>, Jann Horn <jannh@...gle.com>
Subject: [RFC v6 7/8] mm: Fix benign off-by-one bugs

We were wasting a byte due to an off-by-one bug.  s[c]nprintf()
doesn't write more than $2 bytes including the null byte, so trying to
pass 'size-1' there is wasting one byte.  Now that we use sprintf_end(),
the situation isn't different: sprintf_end() will stop writing *before*
'end' --that is, at most the terminating null byte will be written at
'end-1'--.

Acked-by: Marco Elver <elver@...gle.com>
Cc: Kees Cook <kees@...nel.org>
Cc: Christopher Bazley <chris.bazley.wg14@...il.com>
Cc: Alexander Potapenko <glider@...gle.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Alexander Potapenko <glider@...gle.com>
Cc: Jann Horn <jannh@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Marco Elver <elver@...gle.com>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Alejandro Colomar <alx@...nel.org>
---
 mm/kfence/kfence_test.c | 4 ++--
 mm/kmsan/kmsan_test.c   | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
index bae382eca4ab..c635aa9d478b 100644
--- a/mm/kfence/kfence_test.c
+++ b/mm/kfence/kfence_test.c
@@ -110,7 +110,7 @@ static bool report_matches(const struct expect_report *r)
 
 	/* Title */
 	cur = expect[0];
-	end = &expect[0][sizeof(expect[0]) - 1];
+	end = ENDOF(expect[0]);
 	switch (r->type) {
 	case KFENCE_ERROR_OOB:
 		cur = sprintf_end(cur, end, "BUG: KFENCE: out-of-bounds %s",
@@ -140,7 +140,7 @@ static bool report_matches(const struct expect_report *r)
 
 	/* Access information */
 	cur = expect[1];
-	end = &expect[1][sizeof(expect[1]) - 1];
+	end = ENDOF(expect[1]);
 
 	switch (r->type) {
 	case KFENCE_ERROR_OOB:
diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
index e48ca1972ff3..9bda55992e3d 100644
--- a/mm/kmsan/kmsan_test.c
+++ b/mm/kmsan/kmsan_test.c
@@ -105,7 +105,7 @@ static bool report_matches(const struct expect_report *r)
 
 	/* Title */
 	cur = expected_header;
-	end = &expected_header[sizeof(expected_header) - 1];
+	end = ENDOF(expected_header);
 
 	cur = sprintf_end(cur, end, "BUG: KMSAN: %s", r->error_type);
 
-- 
2.50.0