Re: [PATCH] nvmet-fc: Remove __counted_by from nvmet_fc_tgt_queue.fod[]

[Nathan Chancellor <nathan@kernel.org>]

Ping? This is still relevant and I don't think this is a compiler bug
that would justify withholding this change.

On Wed, May 29, 2024 at 02:42:40PM -0700, Nathan Chancellor wrote:
> Work for __counted_by on generic pointers in structures (not just
> flexible array members) has started landing in Clang 19 (current tip of
> tree). During the development of this feature, a restriction was added
> to __counted_by to prevent the flexible array member's element type from
> including a flexible array member itself such as:
> 
>   struct foo {
>     int count;
>     char buf[];
>   };
> 
>   struct bar {
>     int count;
>     struct foo data[] __counted_by(count);
>   };
> 
> because the size of data cannot be calculated with the standard array
> size formula:
> 
>   sizeof(struct foo) * count
> 
> This restriction was downgraded to a warning but due to CONFIG_WERROR,
> it can still break the build. The application of __counted_by on the fod
> member of 'struct nvmet_fc_tgt_queue' triggers this restriction,
> resulting in:
> 
>   drivers/nvme/target/fc.c:151:2: error: 'counted_by' should not be applied to an array with element of unknown size because 'struct nvmet_fc_fcp_iod' is a struct type with a flexible array member. This will be an error in a future compiler version [-Werror,-Wbounds-safety-counted-by-elt-type-unknown-size]
>     151 |         struct nvmet_fc_fcp_iod         fod[] __counted_by(sqsize);
>         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   1 error generated.
> 
> Remove this use of __counted_by to fix the warning/error. However,
> rather than remove it altogether, leave it commented, as it may be
> possible to support this in future compiler releases.
> 
> Cc: stable@vger.kernel.org
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2027
> Fixes: ccd3129aca28 ("nvmet-fc: Annotate struct nvmet_fc_tgt_queue with __counted_by")
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  drivers/nvme/target/fc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/nvme/target/fc.c b/drivers/nvme/target/fc.c
> index 337ee1cb09ae..381b4394731f 100644
> --- a/drivers/nvme/target/fc.c
> +++ b/drivers/nvme/target/fc.c
> @@ -148,7 +148,7 @@ struct nvmet_fc_tgt_queue {
>  	struct workqueue_struct		*work_q;
>  	struct kref			ref;
>  	/* array of fcp_iods */
> -	struct nvmet_fc_fcp_iod		fod[] __counted_by(sqsize);
> +	struct nvmet_fc_fcp_iod		fod[] /* __counted_by(sqsize) */;
>  } __aligned(sizeof(unsigned long long));
>  
>  struct nvmet_fc_hostport {
> 
> ---
> base-commit: c758b77d4a0a0ed3a1292b3fd7a2aeccd1a169a4
> change-id: 20240529-drop-counted-by-fod-nvmet-fc-tgt-queue-50edd2f8d60e
> 
> Best regards,
> -- 
> Nathan Chancellor <nathan@kernel.org>
>