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slab: improve KMALLOC_PARTITION_RANDOM randomness

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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>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260511200136.3201646-2-elver@google.com
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
This commit is contained in:
Marco Elver 2026-05-11 22:00:49 +02:00 committed by Vlastimil Babka (SUSE)
parent feb662d916
commit 0fc1bd38a1
2 changed files with 25 additions and 1 deletions
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24
include/linux/instruction_pointer.h
View File

@ -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 */

2
include/linux/slab.h
View File

@ -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