linux-kernel - Re: [PATCH][next] drm/nouveau/outp: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <ce82e21a-b21e-4f9b-928b-84168d4f5bbb@embeddedor.com>
Date: Fri, 28 Mar 2025 09:44:11 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Danilo Krummrich <dakr@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Lyude Paul <lyude@...hat.com>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, dri-devel@...ts.freedesktop.org,
 nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] drm/nouveau/outp: Avoid
 -Wflex-array-member-not-at-end warning



On 28/03/25 09:05, Danilo Krummrich wrote:
> On Fri, Mar 28, 2025 at 08:45:32AM -0600, 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 where the size of the flexible-array member
>> is known at compile-time, and refactor the rest of the code,
>> accordingly.
>>
>> So, with these changes, fix the following warning:
>>
>> drivers/gpu/drm/nouveau/nvif/outp.c:199:45: 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>
>> ---
>>   drivers/gpu/drm/nouveau/nvif/outp.c | 16 +++++++---------
>>   1 file changed, 7 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/nouveau/nvif/outp.c b/drivers/gpu/drm/nouveau/nvif/outp.c
>> index 6daeb7f0b09b..403cf16d5e84 100644
>> --- a/drivers/gpu/drm/nouveau/nvif/outp.c
>> +++ b/drivers/gpu/drm/nouveau/nvif/outp.c
>> @@ -195,20 +195,18 @@ nvif_outp_dp_aux_pwr(struct nvif_outp *outp, bool enable)
>>   int
>>   nvif_outp_hda_eld(struct nvif_outp *outp, int head, void *data, u32 size)
>>   {
>> -	struct {
>> -		struct nvif_outp_hda_eld_v0 mthd;
>> -		u8 data[128];
>> -	} args;
>> +	DEFINE_RAW_FLEX(struct nvif_outp_hda_eld_v0, mthd, data, 128);
>>   	int ret;
>>   
>> -	if (WARN_ON(size > ARRAY_SIZE(args.data)))
>> +	if (WARN_ON(size > 128))
> 
> Seems a bit unfortunate that the size is duplicated here.

For now, I think we can do something like this instead:

-       if (WARN_ON(size > ARRAY_SIZE(args.data)))
+       if (WARN_ON(size > (__struct_size(mthd) - sizeof(*mthd))))

Thanks
-Gustavo

> 
> Can we have an accessor macro to derive the size for us?
> 
> 	union {
> 		u8 bytes[struct_size_t(type, member, count)];
> 		type obj;
> 	} name##_u initializer;
> 
> Maybe a macro that returns the size difference between bytes and obj?
> 
>>   		return -EINVAL;
>>   
>> -	args.mthd.version = 0;
>> -	args.mthd.head = head;
>> +	mthd->version = 0;
>> +	mthd->head = head;
>>   
>> -	memcpy(args.data, data, size);
>> -	ret = nvif_mthd(&outp->object, NVIF_OUTP_V0_HDA_ELD, &args, sizeof(args.mthd) + size);
>> +	memcpy(mthd->data, data, size);
>> +	ret = nvif_mthd(&outp->object, NVIF_OUTP_V0_HDA_ELD, mthd,
>> +			__struct_size(mthd) + size);
>>   	NVIF_ERRON(ret, &outp->object, "[HDA_ELD head:%d size:%d]", head, size);
>>   	return ret;
>>   }
>> -- 
>> 2.43.0
>>
>