[PATCH v4 2/3] slab: improve KMALLOC_PARTITION_RANDOM randomness

[Marco Elver <elver@google.com>]

When using CONFIG_KMALLOC_PARTITION_RANDOM, _RET_IP_ was previously used
to identify the allocation site. _RET_IP_, however, evaluates to the
caller's parent's instruction pointer rather than the actual allocation
site; this would lead to collisions where a function performs multiple
allocations.

With the generalization to kmalloc_token_t, we now generate the token at
the outermost macro, and using _THIS_IP_ would fix this for all cases.

Unfortunately, the generic implementation of _THIS_IP_ relies on taking
the address of a local label, which is considered broken by both GCC [1]
and Clang [2] because label addresses are only expected to be used with
computed gotos. While the generic version more or less works today, it
is known to be brittle. For example, Clang -O2 always returns 1 when
this function is inlined:

        static inline unsigned long get_ip(void)
        { return ({ __label__ __here; __here: (unsigned long)&&__here; }); }

To provide a reliable unique identifier without breaking architectures
relying on the generic _THIS_IP_, introduce _CODE_LOCATION_: it resolves
to _THIS_IP_ where architectures provide a safe implementation, and
falls back to a zero-cost static marker where _THIS_IP_ is broken.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071 [1]
Link: https://github.com/llvm/llvm-project/issues/138272 [2]
Signed-off-by: Marco Elver <elver@google.com>
---
v4:
* New patch.
---
 include/linux/instruction_pointer.h | 24 ++++++++++++++++++++++++
 include/linux/slab.h                |  2 +-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/include/linux/instruction_pointer.h b/include/linux/instruction_pointer.h
index aa0b3ffea935..ea5bc756bd99 100644
--- a/include/linux/instruction_pointer.h
+++ b/include/linux/instruction_pointer.h
@@ -8,6 +8,30 @@
 
 #ifndef _THIS_IP_
 #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })
+/*
+ * The current generic definition of _THIS_IP_ is considered broken by GCC [1]
+ * and Clang [2]. In particular, the address of a label is only expected to be
+ * used with a computed goto.
+ *
+ *   [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071
+ *   [2] https://github.com/llvm/llvm-project/issues/138272
+ *
+ * Mark it as broken, so that appropriate fallback options can be implemented
+ * for architectures that do not define their own _THIS_IP_.
+ */
+#define HAS_BROKEN_THIS_IP
+#endif
+
+/*
+ * _CODE_LOCATION_ provides a unique identifier for the current code location.
+ * When _THIS_IP_ is broken (generic version), we fall back to a static marker
+ * which guarantees uniqueness and resolves to a constant address at link time,
+ * avoiding runtime overhead and compiler optimizations breaking it.
+ */
+#ifdef HAS_BROKEN_THIS_IP
+#define _CODE_LOCATION_ ({ static const char __here; (unsigned long)&__here; })
+#else
+#define _CODE_LOCATION_ _THIS_IP_
 #endif
 
 #endif /* _LINUX_INSTRUCTION_POINTER_H */
diff --git a/include/linux/slab.h b/include/linux/slab.h
index c232f8a10af6..efab6b2ccf21 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -503,7 +503,7 @@ int kmem_cache_shrink(struct kmem_cache *s);
 typedef struct { unsigned long v; } kmalloc_token_t;
 #ifdef CONFIG_KMALLOC_PARTITION_RANDOM
 extern unsigned long random_kmalloc_seed;
-#define __kmalloc_token(...) ((kmalloc_token_t){ .v = _RET_IP_ })
+#define __kmalloc_token(...) ((kmalloc_token_t){ .v = _CODE_LOCATION_ })
 #elif defined(CONFIG_KMALLOC_PARTITION_TYPED)
 #define __kmalloc_token(...) ((kmalloc_token_t){ .v = __builtin_infer_alloc_token(__VA_ARGS__) })
 #endif
-- 
2.54.0.563.g4f69b47b94-goog