[PATCH][next] crypto: drbg - Avoid -Wflex-array-member-not-at-end warning

[PATCH][next] crypto: drbg - Avoid -Wflex-array-member-not-at-end warning

[Gustavo A. R. Silva]

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the new TRAILING_OVERLAP() helper to fix the following warning:

crypto/drbg.c:1445:27: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

This helper creates a union between a flexible-array member (FAM) and a
set of MEMBERS that would otherwise follow it.

This overlays the trailing MEMBER char ctx[]; onto the FAM struct
shash_desc::__ctx,[] while keeping the FAM and the start of MEMBER
aligned.

The static_assert() ensures this alignment remains, and it's
intentionally placed inmediately after the corresponding structures --no
blank line in between.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 crypto/drbg.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/crypto/drbg.c b/crypto/drbg.c
index 511a27c91813..e9f9775c237f 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1442,9 +1442,12 @@ static void drbg_kcapi_set_entropy(struct crypto_rng *tfm,
 
 #if defined(CONFIG_CRYPTO_DRBG_HASH) || defined(CONFIG_CRYPTO_DRBG_HMAC)
 struct sdesc {
-	struct shash_desc shash;
-	char ctx[];
+	/* Must be last as it ends in a flexible-array member. */
+	TRAILING_OVERLAP(struct shash_desc, shash, __ctx,
+		char ctx[];
+	);
 };
+static_assert(offsetof(struct sdesc, shash.__ctx) == offsetof(struct sdesc, ctx));
 
 static int drbg_init_hash_kernel(struct drbg_state *drbg)
 {
-- 
2.43.0

[v2 PATCH] crypto: drbg - Delete unused ctx from struct sdesc

[Herbert Xu]

On Mon, Nov 10, 2025 at 07:44:56PM +0900, Gustavo A. R. Silva wrote:
>
> diff --git a/crypto/drbg.c b/crypto/drbg.c
> index 511a27c91813..e9f9775c237f 100644
> --- a/crypto/drbg.c
> +++ b/crypto/drbg.c
> @@ -1442,9 +1442,12 @@ static void drbg_kcapi_set_entropy(struct crypto_rng *tfm,
>  
>  #if defined(CONFIG_CRYPTO_DRBG_HASH) || defined(CONFIG_CRYPTO_DRBG_HMAC)
>  struct sdesc {
> -	struct shash_desc shash;
> -	char ctx[];
> +	/* Must be last as it ends in a flexible-array member. */
> +	TRAILING_OVERLAP(struct shash_desc, shash, __ctx,
> +		char ctx[];
> +	);
>  };
> +static_assert(offsetof(struct sdesc, shash.__ctx) == offsetof(struct sdesc, ctx));

This isn't even used.  Just delete it:

---8<---
The ctx array in struct sdesc is never used.  Delete it as it's
bogus since the previous member ends with a flexible array.

Reported-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/crypto/drbg.c b/crypto/drbg.c
index 511a27c91813..1d433dae9955 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1443,7 +1443,6 @@ static void drbg_kcapi_set_entropy(struct crypto_rng *tfm,
 #if defined(CONFIG_CRYPTO_DRBG_HASH) || defined(CONFIG_CRYPTO_DRBG_HMAC)
 struct sdesc {
 	struct shash_desc shash;
-	char ctx[];
 };
 
 static int drbg_init_hash_kernel(struct drbg_state *drbg)
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [v2 PATCH] crypto: drbg - Delete unused ctx from struct sdesc

[Gustavo A. R. Silva]

On 11/19/25 13:38, Herbert Xu wrote:
> On Mon, Nov 10, 2025 at 07:44:56PM +0900, Gustavo A. R. Silva wrote:
>>
>> diff --git a/crypto/drbg.c b/crypto/drbg.c
>> index 511a27c91813..e9f9775c237f 100644
>> --- a/crypto/drbg.c
>> +++ b/crypto/drbg.c
>> @@ -1442,9 +1442,12 @@ static void drbg_kcapi_set_entropy(struct crypto_rng *tfm,
>>   
>>   #if defined(CONFIG_CRYPTO_DRBG_HASH) || defined(CONFIG_CRYPTO_DRBG_HMAC)
>>   struct sdesc {
>> -	struct shash_desc shash;
>> -	char ctx[];
>> +	/* Must be last as it ends in a flexible-array member. */
>> +	TRAILING_OVERLAP(struct shash_desc, shash, __ctx,
>> +		char ctx[];
>> +	);
>>   };
>> +static_assert(offsetof(struct sdesc, shash.__ctx) == offsetof(struct sdesc, ctx));
> 
> This isn't even used.  Just delete it:

Awesome. :)

Thanks!
-Gustavo

> 
> ---8<---
> The ctx array in struct sdesc is never used.  Delete it as it's
> bogus since the previous member ends with a flexible array.
> 
> Reported-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
> 
> diff --git a/crypto/drbg.c b/crypto/drbg.c
> index 511a27c91813..1d433dae9955 100644
> --- a/crypto/drbg.c
> +++ b/crypto/drbg.c
> @@ -1443,7 +1443,6 @@ static void drbg_kcapi_set_entropy(struct crypto_rng *tfm,
>   #if defined(CONFIG_CRYPTO_DRBG_HASH) || defined(CONFIG_CRYPTO_DRBG_HMAC)
>   struct sdesc {
>   	struct shash_desc shash;
> -	char ctx[];
>   };
>   
>   static int drbg_init_hash_kernel(struct drbg_state *drbg)