slab: Remove __malloc attribute from realloc functions

The __malloc attribute should not be applied to "realloc" functions, as
the returned pointer may alias the storage of the prior pointer. Instead
of splitting __malloc from __alloc_size, which would be a huge amount of
churn, just create __realloc_size for the few cases where it is needed.

Thanks to Geert Uytterhoeven <geert@linux-m68k.org> for reporting build
failures with gcc-8 in earlier version which tried to remove the #ifdef.
While the "alloc_size" attribute is available on all GCC versions, I
forgot that it gets disabled explicitly by the kernel in GCC < 9.1 due
to misbehaviors. Add a note to the compiler_attributes.h entry for it.

Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Marco Elver <elver@google.com>
Cc: linux-mm@kvack.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

include/linux/compiler_attributes.h

@@ -35,7 +35,8 @@
 
 /*
  * Note: do not use this directly. Instead, use __alloc_size() since it is conditionally
- * available and includes other attributes.
+ * available and includes other attributes. For GCC < 9.1, __alloc_size__ gets undefined
+ * in compiler-gcc.h, due to misbehaviors.
  *
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
  * clang: https://clang.llvm.org/docs/AttributeReference.html#alloc-size

include/linux/compiler_types.h

@@ -271,14 +271,16 @@ struct ftrace_likely_data {
 
 /*
  * Any place that could be marked with the "alloc_size" attribute is also
- * a place to be marked with the "malloc" attribute. Do this as part of the
- * __alloc_size macro to avoid redundant attributes and to avoid missing a
- * __malloc marking.
+ * a place to be marked with the "malloc" attribute, except those that may
+ * be performing a _reallocation_, as that may alias the existing pointer.
+ * For these, use __realloc_size().
  */
 #ifdef __alloc_size__
 # define __alloc_size(x, ...)	__alloc_size__(x, ## __VA_ARGS__) __malloc
+# define __realloc_size(x, ...)	__alloc_size__(x, ## __VA_ARGS__)
 #else
 # define __alloc_size(x, ...)	__malloc
+# define __realloc_size(x, ...)
 #endif
 
 #ifndef asm_volatile_goto

include/linux/slab.h

@@ -184,7 +184,7 @@ int kmem_cache_shrink(struct kmem_cache *s);
 /*
  * Common kmalloc functions provided by all allocators
  */
-void * __must_check krealloc(const void *objp, size_t new_size, gfp_t flags) __alloc_size(2);
+void * __must_check krealloc(const void *objp, size_t new_size, gfp_t flags) __realloc_size(2);
 void kfree(const void *objp);
 void kfree_sensitive(const void *objp);
 size_t __ksize(const void *objp);
@@ -647,10 +647,10 @@ static inline __alloc_size(1, 2) void *kmalloc_array(size_t n, size_t size, gfp_
  * @new_size: new size of a single member of the array
  * @flags: the type of memory to allocate (see kmalloc)
  */
-static inline __alloc_size(2, 3) void * __must_check krealloc_array(void *p,
-								    size_t new_n,
-								    size_t new_size,
-								    gfp_t flags)
+static inline __realloc_size(2, 3) void * __must_check krealloc_array(void *p,
+								      size_t new_n,
+								      size_t new_size,
+								      gfp_t flags)
 {
 	size_t bytes;
 
@@ -774,7 +774,7 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
 }
 
 extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
-		      __alloc_size(3);
+		      __realloc_size(3);
 extern void kvfree(const void *addr);
 extern void kvfree_sensitive(const void *addr, size_t len);
 

mm/slab_common.c

@@ -1150,8 +1150,8 @@ module_init(slab_proc_init);
 
 #endif /* CONFIG_SLAB || CONFIG_SLUB_DEBUG */
 
-static __always_inline void *__do_krealloc(const void *p, size_t new_size,
-					   gfp_t flags)
+static __always_inline __realloc_size(2) void *
+__do_krealloc(const void *p, size_t new_size, gfp_t flags)
 {
 	void *ret;
 	size_t ks;