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