linux-kernel - Re: [PATCH][next] jbd2: Avoid dozens of -Wflex-array-member-not-at-end warnings

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Message-ID: <fe0e9c86-fa44-425e-a955-aa9e401b6334@embeddedor.com>
Date: Thu, 31 Oct 2024 09:54:36 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Jan Kara <jack@...e.cz>, "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
 linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] jbd2: Avoid dozens of
 -Wflex-array-member-not-at-end warnings



On 31/10/24 06:33, Jan Kara wrote:
> On Fri 25-10-24 13:32:58, 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.
>>
>> Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
>> a flexible structure (`struct shash_desc`) where the size of the
>> flexible-array member (`__ctx`) is known at compile-time, and
>> refactor the rest of the code, accordingly.
>>
>> So, with this, fix 77 of the following warnings:
>>
>> include/linux/jbd2.h:1800:35: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>> ---
>>   include/linux/jbd2.h | 13 +++++--------
>>   1 file changed, 5 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
>> index 8aef9bb6ad57..ce4560e62d3b 100644
>> --- a/include/linux/jbd2.h
>> +++ b/include/linux/jbd2.h
>> @@ -1796,22 +1796,19 @@ static inline unsigned long jbd2_log_space_left(journal_t *journal)
>>   static inline u32 jbd2_chksum(journal_t *journal, u32 crc,
>>   			      const void *address, unsigned int length)
>>   {
>> -	struct {
>> -		struct shash_desc shash;
>> -		char ctx[JBD_MAX_CHECKSUM_SIZE];
>> -	} desc;
>> +	DEFINE_RAW_FLEX(struct shash_desc, desc, __ctx, 1);
> 
> Am I missing some magic here or the 1 above should be
> JBD_MAX_CHECKSUM_SIZE?

This seems to be 32-bit code, and the element type of the flex-array
member `__ctx` is `void *`. Therefore, we have:

`sizeof(ctx) == 4` when `char ctx[JBD_MAX_CHECKSUM_SIZE];`

To maintain the same size, we tell `DEFINE_RAW_FLEX()` to allocate `1`
element for the flex array, as in 32-bit `sizeof(void *) == 4`.

--
Gustavo