[PATCH v3 1/5] slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN

[Karl Mehltretter <kmehltretter@gmail.com>]

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.

This can mislead compiler optimizations. 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: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 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 cda126def67a..563dadc16d82 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -262,13 +262,16 @@ 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)
@@ -625,6 +628,13 @@ 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