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