linux-kernel - [PATCH -mm v2] stackdepot: fix -Wstringop-overflow warning

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Date: Thu,  1 Feb 2024 10:04:30 +0100
From: Marco Elver <elver@...gle.com>
To: elver@...gle.com, Andrew Morton <akpm@...ux-foundation.org>
Cc: Andrey Konovalov <andreyknvl@...il.com>, Alexander Potapenko <glider@...gle.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, kasan-dev@...glegroups.com, 
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: [PATCH -mm v2] stackdepot: fix -Wstringop-overflow warning

Since 113a61863ecb ("Makefile: Enable -Wstringop-overflow globally")
string overflow checking is enabled by default. Within stackdepot, the
compiler (GCC 13.2.0) assumes that a multiplication overflow may be
possible and flex_array_size() can return SIZE_MAX (4294967295 on
32-bit), resulting in this warning:

 In function 'depot_alloc_stack',
     inlined from 'stack_depot_save_flags' at lib/stackdepot.c:688:4:
 arch/x86/include/asm/string_32.h:150:25: error: '__builtin_memcpy' specified bound 4294967295 exceeds maximum object size 2147483647 [-Werror=stringop-overflow=]
   150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
 lib/stackdepot.c:459:9: note: in expansion of macro 'memcpy'
   459 |         memcpy(stack->entries, entries, flex_array_size(stack, entries, nr_entries));
       |         ^~~~~~
 cc1: all warnings being treated as errors

This is due to depot_alloc_stack() accepting an 'int nr_entries' which
could be negative without deeper analysis of callers.

The call to depot_alloc_stack() from stack_depot_save_flags(), however,
only passes in its nr_entries which is unsigned int. Fix the warning by
switching depot_alloc_stack()'s nr_entries to also be unsigned.

Link: https://lore.kernel.org/all/20240201135747.18eca98e@canb.auug.org.au/
Fixes: d869d3fb362c ("stackdepot: use variable size records for non-evictable entries")
Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
Signed-off-by: Marco Elver <elver@...gle.com>
---
v2:
* Just switch 'nr_entries' to unsigned int which is already the case
  elsewhere.
---
 lib/stackdepot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 8f3b2c84ec2d..4a7055a63d9f 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -420,7 +420,7 @@ static inline size_t depot_stack_record_size(struct stack_record *s, unsigned in
 
 /* Allocates a new stack in a stack depot pool. */
 static struct stack_record *
-depot_alloc_stack(unsigned long *entries, int nr_entries, u32 hash, depot_flags_t flags, void **prealloc)
+depot_alloc_stack(unsigned long *entries, unsigned int nr_entries, u32 hash, depot_flags_t flags, void **prealloc)
 {
 	struct stack_record *stack = NULL;
 	size_t record_size;
-- 
2.43.0.429.g432eaa2c6b-goog