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[2a02:3100:a133:e401:bc8e:d36c:2d08:c5e0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4814a5be29fsm4464099f8f.12.2026.08.11.07.12.42 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 11 Aug 2026 07:12:43 -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 0/3] slab: ZERO_SIZE_PTR alignment and exact matching Date: Tue, 11 Aug 2026 16:12:37 +0200 Message-Id: <20260811141240.62519-1-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) 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 promise ARCH_KMALLOC_MINALIGN alignment but return (void *)16 for zero-size requests. Patch 1 aligns the sentinel to that promise. Patch 2 makes ZERO_OR_NULL_PTR() match only NULL and the sentinel, so increasing the sentinel does not widen the set it matches. Patch 3 adds KUnit coverage. In hardened usercopy, patch 2 changes check_bogus_address() so nonzero addresses below ZERO_SIZE_PTR no longer cause its null-address abort. v1 -> v2: - express the poison-pointer bound as 0x100 instead of 128 (Catalin) - make ZERO_OR_NULL_PTR() use exact matches - add KUnit coverage Tested on bcc44b6785f21 with the series applied: - GCC 15.2 QEMU boots on ten configurations spanning sentinel values 16, 32, 64 and 128, all passing the zero-size allocation selftest - the x86_64 test also passes with GCC 8.1, the minimum supported compiler - the new KUnit test passes on UML - Clang UBSAN_ALIGNMENT is clean on armv5 and the generated code retains the exact sentinel comparison - Clang builds the macro for 21 targets at -O2 and -Os, with single evaluation confirmed in the generated IR - patch 2 changes vmlinux text by -2 bytes on x86_64 and -72 bytes on armv5, and arm64 gains 44 symbol bytes v1: https://lore.kernel.org/r/20260808105622.62026-1-kmehltretter@gmail.com RFC: https://lore.kernel.org/r/20260712120728.96628-1-kmehltretter@gmail.com Karl Mehltretter (3): slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN slab: check for ZERO_SIZE_PTR by exact match slab: test zero-size allocations in slub_kunit include/linux/slab.h | 20 +++++++++++++++++--- lib/tests/slub_kunit.c | 43 ++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 60 insertions(+), 3 deletions(-) base-commit: bcc44b6785f216eb939226ade6e3910baa30516b -- 2.53.0