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[2a02:3100:a133:e401:bc8e:d36c:2d08:c5e0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4814a5be29fsm4464099f8f.12.2026.08.11.07.12.45 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 11 Aug 2026 07:12:45 -0700 (PDT) From: Karl Mehltretter To: Vlastimil Babka , Harry Yoo , Andrew Morton Cc: Karl Mehltretter , Rasmus Villemoes , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Catalin Marinas , Kees Cook , "Gustavo A. R. Silva" , linux-hardening@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: [PATCH v2 1/3] slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN Date: Tue, 11 Aug 2026 16:12:38 +0200 Message-Id: <20260811141240.62519-2-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20260811141240.62519-1-kmehltretter@gmail.com> References: <20260811141240.62519-1-kmehltretter@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The kmalloc entry points are annotated with __assume_kmalloc_alignment but return ZERO_SIZE_PTR, currently (void *)16, for zero-size requests. This violates the annotation when ARCH_KMALLOC_MINALIGN exceeds 16. Clang's UBSAN_ALIGNMENT detects the violation on armv5. GCC and Clang retain the ZERO_OR_NULL_PTR() range check but eliminate an exact ZERO_SIZE_PTR comparison after an annotated allocation. Define ZERO_SIZE_PTR as the greater of 16 and ARCH_KMALLOC_MINALIGN, retaining the existing value where it is already aligned. Assert that ARCH_KMALLOC_MINALIGN remains below 0x100, the value of LIST_POISON1 when POISON_POINTER_DELTA is zero, so the sentinel remains distinct from that poison pointer. Fixes: 94a58c360a45 ("slab.h: sprinkle __assume_aligned attributes") Assisted-by: Claude:claude-fable-5 Signed-off-by: Karl Mehltretter --- include/linux/slab.h | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/include/linux/slab.h b/include/linux/slab.h index 32c9f8ed7ae20..3e012cc4f001e 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -259,13 +259,16 @@ enum _slab_flag_bits { /* * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests. + * It satisfies the alignment promised by __assume_kmalloc_alignment + * and keeps the historic value 16 where that is already aligned. * * Dereferencing ZERO_SIZE_PTR will lead to a distinct access fault. * * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can. * Both make kfree a no-op. */ -#define ZERO_SIZE_PTR ((void *)16) +#define ZERO_SIZE_PTR ((void *)(ARCH_KMALLOC_MINALIGN > 16 ? \ + ARCH_KMALLOC_MINALIGN : 16)) #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \ (unsigned long)ZERO_SIZE_PTR) @@ -622,6 +625,13 @@ static inline bool kmem_dump_obj(void *object) { return false; } #define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE) #endif +/* + * Keep ZERO_SIZE_PTR at most 128, i.e. below 0x100: LIST_POISON1 is + * 0x100 when POISON_POINTER_DELTA is 0, and no architecture currently + * has an ARCH_KMALLOC_MINALIGN above 128. + */ +static_assert(ARCH_KMALLOC_MINALIGN < 0x100); + /* * Setting ARCH_SLAB_MINALIGN in arch headers allows a different alignment. * Intended for arches that get misalignment faults even for 64 bit integer -- 2.53.0