linux-hardening - [PATCH 1/3][next] lib/stackdepot.c: Replace one-element array with flexible-array member

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Date:   Thu, 1 Oct 2020 10:25:08 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-hardening@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>
Subject: [PATCH 1/3][next] lib/stackdepot.c: Replace one-element array with
 flexible-array member

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in
struct stack_record, instead of a one-element array, and use the
struct_size() helper to calculate the size for the allocation.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays

Build-tested-by: kernel test robot <lkp@...el.com>
Link: https://lore.kernel.org/lkml/5f75876b.x9zdN10esiC0qLHV%25lkp@intel.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 lib/stackdepot.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 2caffc64e4c8..c6106cfb7950 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -62,7 +62,7 @@ struct stack_record {
 	u32 hash;			/* Hash in the hastable */
 	u32 size;			/* Number of frames in the stack */
 	union handle_parts handle;
-	unsigned long entries[1];	/* Variable-sized array of entries. */
+	unsigned long entries[];	/* Variable-sized array of entries. */
 };
 
 static void *stack_slabs[STACK_ALLOC_MAX_SLABS];
@@ -104,9 +104,8 @@ static bool init_stack_slab(void **prealloc)
 static struct stack_record *depot_alloc_stack(unsigned long *entries, int size,
 		u32 hash, void **prealloc, gfp_t alloc_flags)
 {
-	int required_size = offsetof(struct stack_record, entries) +
-		sizeof(unsigned long) * size;
 	struct stack_record *stack;
+	size_t required_size = struct_size(stack, entries, size);
 
 	required_size = ALIGN(required_size, 1 << STACK_ALLOC_ALIGN);
 
-- 
2.27.0