codetag: debug: skip objext checking when it's for objext itself

objext objects are created with __GFP_NO_OBJ_EXT flag and therefore have
no corresponding objext themselves (otherwise we would get an infinite
recursion). When freeing these objects their codetag will be empty and
when CONFIG_MEM_ALLOC_PROFILING_DEBUG is enabled this will lead to false
warnings. Introduce CODETAG_EMPTY special codetag value to mark
allocations which intentionally lack codetag to avoid these warnings.
Set objext codetags to CODETAG_EMPTY before freeing to indicate that
the codetag is expected to be empty.

Link: https://lkml.kernel.org/r/20240321163705.3067592-34-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Tested-by: Kees Cook <keescook@chromium.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alex Gaynor <alex.gaynor@gmail.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Andreas Hindborg <a.hindborg@samsung.com>
Cc: Benno Lossin <benno.lossin@proton.me>
Cc: "Björn Roy Baron" <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Gary Guo <gary@garyguo.net>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: surenbaghdasaryan

include/linux/alloc_tag.h

@@ -29,6 +29,27 @@ struct alloc_tag {
 	struct alloc_tag_counters __percpu	*counters;
 } __aligned(8);
 
+#ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG
+
+#define CODETAG_EMPTY	((void *)1)
+
+static inline bool is_codetag_empty(union codetag_ref *ref)
+{
+	return ref->ct == CODETAG_EMPTY;
+}
+
+static inline void set_codetag_empty(union codetag_ref *ref)
+{
+	if (ref)
+		ref->ct = CODETAG_EMPTY;
+}
+
+#else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
+
+static inline bool is_codetag_empty(union codetag_ref *ref) { return false; }
+
+#endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
+
 #ifdef CONFIG_MEM_ALLOC_PROFILING
 
 struct codetag_bytes {
@@ -151,6 +172,11 @@ static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes)
 	if (!ref || !ref->ct)
 		return;
 
+	if (is_codetag_empty(ref)) {
+		ref->ct = NULL;
+		return;
+	}
+
 	tag = ct_to_alloc_tag(ref->ct);
 
 	this_cpu_sub(tag->counters->bytes, bytes);

mm/slub.c

@@ -1873,6 +1873,30 @@ static inline enum node_stat_item cache_vmstat_idx(struct kmem_cache *s)
 
 #ifdef CONFIG_SLAB_OBJ_EXT
 
+#ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG
+
+static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
+{
+	struct slabobj_ext *slab_exts;
+	struct slab *obj_exts_slab;
+
+	obj_exts_slab = virt_to_slab(obj_exts);
+	slab_exts = slab_obj_exts(obj_exts_slab);
+	if (slab_exts) {
+		unsigned int offs = obj_to_index(obj_exts_slab->slab_cache,
+						 obj_exts_slab, obj_exts);
+		/* codetag should be NULL */
+		WARN_ON(slab_exts[offs].ref.ct);
+		set_codetag_empty(&slab_exts[offs].ref);
+	}
+}
+
+#else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
+
+static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
+
+#endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
+
 /*
  * The allocated objcg pointers array is not accounted directly.
  * Moreover, it should not come from DMA buffer and is not readily
@@ -1913,6 +1937,7 @@ static int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 		 * assign slabobj_exts in parallel. In this case the existing
 		 * objcg vector should be reused.
 		 */
+		mark_objexts_empty(vec);
 		kfree(vec);
 		return 0;
 	}
@@ -1929,6 +1954,14 @@ static inline void free_slab_obj_exts(struct slab *slab)
 	if (!obj_exts)
 		return;
 
+	/*
+	 * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
+	 * corresponding extension will be NULL. alloc_tag_sub() will throw a
+	 * warning if slab has extensions but the extension of an object is
+	 * NULL, therefore replace NULL with CODETAG_EMPTY to indicate that
+	 * the extension for obj_exts is expected to be NULL.
+	 */
+	mark_objexts_empty(obj_exts);
 	kfree(obj_exts);
 	slab->obj_exts = 0;
 }