[PATCH v3] mmc: mmc_test: Fix __counted_by handling after kzalloc_flex() conversion

[Prabhakar]

From: Lad Prabhakar <prabhakar.mahadev-lad...@bp.renesas.com>

Fix logic issues introduced by the kzalloc_flex() conversion in
mmc_test_alloc_mem() due to interaction with the __counted_by
annotation on the flexible array.

Bounds-checking sanitizers rely on the counter field reflecting the
allocated array size before any array access occurs. However, use
mem->cnt both as the allocation size and as the runtime insertion
index, causing incorrect indexing and potentially invalid bounds
tracking.

Initialize mem->cnt to the maximum allocated number of segments
immediately after kzalloc_flex(), then use a separate local index
variable to track successfully allocated entries. Update mem->cnt to
the actual number of initialized elements before returning or entering
the cleanup path.

Also rewrite mmc_test_free_mem() to use a forward for-loop, improving
readability and ensuring only initialized entries are freed.

Fixes: c3126dccfd7b ("mmc: mmc_test: use kzalloc_flex")
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad...@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+rene...@glider.be>
---
v2->v3:
- Switched back to v1 version of the patch.
- Addressed review comments from Geert.
- Add RB tag from Geert which was received on v1.

v1->v2:
- Started with cnt = 0 and incremented before assignment to ensure
  accurate tracking of initialized entries in mmc_test_alloc_mem().
- In mmc_test_free_mem(), replaced the while loop with a forward for-loop to
  safely iterate over initialized entries without risking underflow.
- Updated commit message to clarify the issue and the fix.

v2: 
https://lore.kernel.org/all/20260519133025.618255-1-prabhakar.mahadev-lad...@bp.renesas.com/
v1: 
https://lore.kernel.org/all/20260513201315.3186621-1-prabhakar.mahadev-lad...@bp.renesas.com/
---
 drivers/mmc/core/mmc_test.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/drivers/mmc/core/mmc_test.c b/drivers/mmc/core/mmc_test.c
index ab38e4c45a8d..4dc16649e61d 100644
--- a/drivers/mmc/core/mmc_test.c
+++ b/drivers/mmc/core/mmc_test.c
@@ -318,9 +318,9 @@ static void mmc_test_free_mem(struct mmc_test_mem *mem)
 {
        if (!mem)
                return;
-       while (mem->cnt--)
-               __free_pages(mem->arr[mem->cnt].page,
-                            mem->arr[mem->cnt].order);
+       for (unsigned int i = 0; i < mem->cnt; i++)
+               __free_pages(mem->arr[i].page,
+                            mem->arr[i].order);
        kfree(mem);
 }
 
@@ -341,6 +341,7 @@ static struct mmc_test_mem *mmc_test_alloc_mem(unsigned 
long min_sz,
        unsigned long page_cnt = 0;
        unsigned long limit = nr_free_buffer_pages() >> 4;
        struct mmc_test_mem *mem;
+       unsigned int idx = 0;
 
        if (max_page_cnt > limit)
                max_page_cnt = limit;
@@ -375,23 +376,26 @@ static struct mmc_test_mem *mmc_test_alloc_mem(unsigned 
long min_sz,
                                goto out_free;
                        break;
                }
-               mem->arr[mem->cnt].page = page;
-               mem->arr[mem->cnt].order = order;
-               mem->cnt += 1;
+               mem->arr[idx].page = page;
+               mem->arr[idx].order = order;
+               idx += 1;
                if (max_page_cnt <= (1UL << order))
                        break;
                max_page_cnt -= 1UL << order;
                page_cnt += 1UL << order;
-               if (mem->cnt >= max_segs) {
+               if (idx >= mem->cnt) {
                        if (page_cnt < min_page_cnt)
                                goto out_free;
                        break;
                }
        }
 
+       mem->cnt = idx;
+
        return mem;
 
 out_free:
+       mem->cnt = idx;
        mmc_test_free_mem(mem);
        return NULL;
 }
-- 
2.54.0