linux-kernel - Re: [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <da074ecc-e688-493e-bd63-76cd88e1ff0f@embeddedor.com>
Date: Mon, 24 Nov 2025 17:34:18 +0900
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Coly Li <colyli@...as.com>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-bcache@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end
 warning

On 11/13/25 13:24, Gustavo A. R. Silva wrote:
> 
>> I see. I take this patch, with the above complain...
> 
> Thanks, Coly.

BTW, I'm currently looking into the following struct, and I wonder if
it's okay to move struct bkey end; to the end.

drivers/md/bcache/bset.h:157:struct bset_tree {
drivers/md/bcache/bset.h-158-   /*
drivers/md/bcache/bset.h-159-    * We construct a binary tree in an array as if the array
drivers/md/bcache/bset.h-160-    * started at 1, so that things line up on the same cachelines
drivers/md/bcache/bset.h-161-    * better: see comments in bset.c at cacheline_to_bkey() for
drivers/md/bcache/bset.h-162-    * details
drivers/md/bcache/bset.h-163-    */
drivers/md/bcache/bset.h-164-
drivers/md/bcache/bset.h-165-   /* size of the binary tree and prev array */
drivers/md/bcache/bset.h-166-   unsigned int            size;
drivers/md/bcache/bset.h-167-
drivers/md/bcache/bset.h-168-   /* function of size - precalculated for to_inorder() */
drivers/md/bcache/bset.h-169-   unsigned int            extra;
drivers/md/bcache/bset.h-170-
drivers/md/bcache/bset.h-171-   /* copy of the last key in the set */
drivers/md/bcache/bset.h-172-   struct bkey             end;
drivers/md/bcache/bset.h-173-   struct bkey_float       *tree;
drivers/md/bcache/bset.h-174-
drivers/md/bcache/bset.h-175-   /*
drivers/md/bcache/bset.h-176-    * The nodes in the bset tree point to specific keys - this
drivers/md/bcache/bset.h-177-    * array holds the sizes of the previous key.
drivers/md/bcache/bset.h-178-    *
drivers/md/bcache/bset.h-179-    * Conceptually it's a member of struct bkey_float, but we want
drivers/md/bcache/bset.h-180-    * to keep bkey_float to 4 bytes and prev isn't used in the fast
drivers/md/bcache/bset.h-181-    * path.
drivers/md/bcache/bset.h-182-    */
drivers/md/bcache/bset.h-183-   uint8_t                 *prev;
drivers/md/bcache/bset.h-184-
drivers/md/bcache/bset.h-185-   /* The actual btree node, with pointers to each sorted set */
drivers/md/bcache/bset.h-186-   struct bset             *data;
drivers/md/bcache/bset.h-187-};

Something like this:

diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h
index 6ee2c6a506a2..e6daeb0edc02 100644
--- a/drivers/md/bcache/bset.h
+++ b/drivers/md/bcache/bset.h
@@ -169,7 +169,6 @@ struct bset_tree {
         unsigned int            extra;

         /* copy of the last key in the set */
-       struct bkey             end;
         struct bkey_float       *tree;

         /*
@@ -184,6 +183,9 @@ struct bset_tree {

         /* The actual btree node, with pointers to each sorted set */
         struct bset             *data;
+
+       /* Must be last as it ends in a flexible-array member. */
+       struct bkey             end;
  };

This would fix 15 more -Wflex-array-member-not-at-end warnings.

Thanks
-Gustavo