[PATCH v4 5/6] s390/cmpxchg: Fix KASAN stack-out-of-bounds in atomic helpers

[Jan Polensky]

The __arch_cmpxchg1, __arch_cmpxchg2, __arch_xchg1, and __arch_xchg2
functions emulate 1-byte and 2-byte atomic operations using 4-byte
cmpxchg instructions, since s390 lacks native 1/2-byte cmpxchg support.

When KASAN is enabled, the READ_ONCE() operations in these functions
trigger stack-out-of-bounds warnings because they perform 4-byte reads
when only 1 or 2 bytes should be accessed.

Mark these functions as __no_sanitize_or_inline to prevent KASAN
instrumentation while maintaining correct functionality.

This resolves the following KASAN error during rust_atomics KUnit tests:

  BUG: KASAN: stack-out-of-bounds in rust_helper_atomic_i8_xchg+0xb2/0xc0
  Read of size 4 at addr 001bff7ffdbefcf0 by task kunit_try_catch/142

Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx>
Suggested-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Jan Polensky <japo@xxxxxxxxxxxxx>
---
 arch/s390/include/asm/cmpxchg.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/s390/include/asm/cmpxchg.h b/arch/s390/include/asm/cmpxchg.h
index 008357996262..e6ac55cf3c17 100644
--- a/arch/s390/include/asm/cmpxchg.h
+++ b/arch/s390/include/asm/cmpxchg.h
@@ -35,7 +35,7 @@ static __always_inline u64 __csg_asm(u64 ptr, u64 old, u64 new)
 	return old;
 }

-static inline u8 __arch_cmpxchg1(u64 ptr, u8 old, u8 new)
+static __no_sanitize_or_inline u8 __arch_cmpxchg1(u64 ptr, u8 old, u8 new)
 {
 	union {
 		u8 b[4];
@@ -58,7 +58,7 @@ static inline u8 __arch_cmpxchg1(u64 ptr, u8 old, u8 new)
 	return old;
 }

-static inline u16 __arch_cmpxchg2(u64 ptr, u16 old, u16 new)
+static __no_sanitize_or_inline u16 __arch_cmpxchg2(u64 ptr, u16 old, u16 new)
 {
 	union {
 		u16 b[2];
@@ -173,7 +173,7 @@ static __always_inline u64 __arch_cmpxchg(u64 ptr, u64 old, u64 new, int size)

 void __xchg_called_with_bad_pointer(void);

-static inline u8 __arch_xchg1(u64 ptr, u8 x)
+static __no_sanitize_or_inline u8 __arch_xchg1(u64 ptr, u8 x)
 {
 	int shift = (3 ^ (ptr & 3)) << 3;
 	u32 mask, old, new;
@@ -188,7 +188,7 @@ static inline u8 __arch_xchg1(u64 ptr, u8 x)
 	return old >> shift;
 }

-static inline u16 __arch_xchg2(u64 ptr, u16 x)
+static __no_sanitize_or_inline u16 __arch_xchg2(u64 ptr, u16 x)
 {
 	int shift = (2 ^ (ptr & 2)) << 3;
 	u32 mask, old, new;
--
2.53.0