linux-hardening - [RFC v3 5/7] mm: Fix benign off-by-one bugs

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Message-ID: <740755c1a888ae27de3f127c27bf925a91e9b264.1751862634.git.alx@kernel.org>
Date: Mon, 7 Jul 2025 07:06:15 +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>, Jann Horn <jannh@...gle.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [RFC v3 5/7] 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 seprintf(),
the situation isn't different: seprintf() will stop writing *before*
'end' --that is, at most the terminating null byte will be written at
'end-1'--.

Fixes: bc8fbc5f305a (2021-02-26; "kfence: add test suite")
Fixes: 8ed691b02ade (2022-10-03; "kmsan: add tests for KMSAN")
Cc: Kees Cook <kees@...nel.org>
Cc: Christopher Bazley <chris.bazley.wg14@...il.com>
Cc: Alexander Potapenko <glider@...gle.com>
Cc: Marco Elver <elver@...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>
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 ff734c514c03..f02c3e23638a 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 = seprintf(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 a062a46b2d24..882500807db8 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 = seprintf(cur, end, "BUG: KMSAN: %s", r->error_type);
 
-- 
2.50.0