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

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Message-ID: <b59ed01f-d9d5-4de8-8a12-1e506962b2d9@embeddedor.com>
Date: Fri, 3 Oct 2025 15:51:24 +0100
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: dsterba@...e.cz, "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Chris Mason <clm@...com>, David Sterba <dsterba@...e.com>,
 linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] btrfs: Avoid -Wflex-array-member-not-at-end warning



On 10/3/25 15:35, David Sterba wrote:
> On Fri, Oct 03, 2025 at 03:11:06PM +0100, Gustavo A. R. Silva wrote:
>> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
>> getting ready to enable it, globally.
>>
>> Move the conflicting declaration to the end of the corresponding
>> structure. Notice that `struct fs_path` is a flexible structure,
>> this is a structure that contains a flexible-array member (`char
>> inline_buf[];` in this case).
> 
> It contains a flexible member but also a padding array and a limit
> calculated for the usable space of inline_buf (FS_PATH_INLINE_SIZE),
> it's not the pattern where flexible array is in the middle of a
> structure and could potentially overwrite other members.

Yep, I notice that while reviewing the context in which this is being
used. :)

> 
>> Fix the following warning:
>>
>> fs/btrfs/send.c:181:24: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> Otheriwse OK to fix the warning.

Thanks.

> 
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>> ---
>>   fs/btrfs/send.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
>> index 9230e5066fc6..2b7cf49a35bb 100644
>> --- a/fs/btrfs/send.c
>> +++ b/fs/btrfs/send.c
>> @@ -178,7 +178,6 @@ struct send_ctx {
>>   	u64 cur_inode_rdev;
>>   	u64 cur_inode_last_extent;
>>   	u64 cur_inode_next_write_offset;
>> -	struct fs_path cur_inode_path;
>>   	bool cur_inode_new;
>>   	bool cur_inode_new_gen;
>>   	bool cur_inode_deleted;
>> @@ -305,6 +304,9 @@ struct send_ctx {
>>   
>>   	struct btrfs_lru_cache dir_created_cache;
>>   	struct btrfs_lru_cache dir_utimes_cache;
>> +
>> +	/* Must be last --ends in a flexible-array member. */
>                          ^^
> 
> Is this an en dash?

Not sure what you mean.

-Gustavo

> 
>> +	struct fs_path cur_inode_path;
>>   };
>>   
>>   struct pending_dir_move {
>> -- 
>> 2.43.0
>>