patches/old/lib-stackdepot-annotate-racy-slab_index-accesses.patch

From: Andrey Konovalov <andreyknvl@google.com>
Subject: lib/stackdepot: annotate racy slab_index accesses
Date: Mon, 30 Jan 2023 21:49:40 +0100

Accesses to slab_index are protected by slab_lock everywhere except in a
sanity check in stack_depot_fetch.  The read access there can race with
the write access in depot_alloc_stack.

Use WRITE/READ_ONCE() to annotate the racy accesses.

As the sanity check is only used to print a warning in case of a violation
of the stack depot interface usage, it does not make a lot of sense to use
proper synchronization.

Link: https://lkml.kernel.org/r/19512bb03eed27ced5abeb5bd03f9a8381742cb1.1675111415.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


--- a/lib/stackdepot.c~lib-stackdepot-annotate-racy-slab_index-accesses
+++ a/lib/stackdepot.c
@@ -269,8 +269,11 @@ depot_alloc_stack(unsigned long *entries
 			return NULL;
 		}
 
-		/* Move on to the next slab. */
-		slab_index++;
+		/*
+		 * Move on to the next slab.
+		 * WRITE_ONCE annotates a race with stack_depot_fetch.
+		 */
+		WRITE_ONCE(slab_index, slab_index + 1);
 		slab_offset = 0;
 		/*
 		 * smp_store_release() here pairs with smp_load_acquire() in
@@ -492,6 +495,8 @@ unsigned int stack_depot_fetch(depot_sta
 			       unsigned long **entries)
 {
 	union handle_parts parts = { .handle = handle };
+	/* READ_ONCE annotates a race with depot_alloc_stack. */
+	int slab_index_cached = READ_ONCE(slab_index);
 	void *slab;
 	size_t offset = parts.offset << DEPOT_STACK_ALIGN;
 	struct stack_record *stack;
@@ -500,9 +505,9 @@ unsigned int stack_depot_fetch(depot_sta
 	if (!handle)
 		return 0;
 
-	if (parts.slab_index > slab_index) {
+	if (parts.slab_index > slab_index_cached) {
 		WARN(1, "slab index %d out of bounds (%d) for stack id %08x\n",
-			parts.slab_index, slab_index, handle);
+			parts.slab_index, slab_index_cached, handle);
 		return 0;
 	}
 	slab = stack_slabs[parts.slab_index];
_