linux-kernel - Re: [PATCH][next] RDMA/uverbs: Avoid -Wflex-array-member-not-at-end warnings

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Date: Fri, 1 Mar 2024 16:14:46 -0800
From: Kees Cook <keescook@...omium.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] RDMA/uverbs: Avoid -Wflex-array-member-not-at-end
 warnings

On Fri, Mar 01, 2024 at 12:37:45PM -0600, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
> ready to enable it globally.
> 
> There are currently a couple of objects (`alloc_head` and `bundle`) in
> `struct bundle_priv` that contain a couple of flexible structures:
> 
> struct bundle_priv {
>         /* Must be first */
>         struct bundle_alloc_head alloc_head;
> 
> 	...
> 
>         /*
>          * Must be last. bundle ends in a flex array which overlaps
>          * internal_buffer.
>          */
>         struct uverbs_attr_bundle bundle;
>         u64 internal_buffer[32];
> };
> 
> So, in order to avoid ending up with a couple of flexible-array members
> in the middle of a struct, we use the `struct_group_tagged()` helper to
> separate the flexible array from the rest of the members in the flexible
> structures:
> 
> struct uverbs_attr_bundle {
>         struct_group_tagged(uverbs_attr_bundle_hdr, hdr,
> 		... the rest of the members
>         );
>         struct uverbs_attr attrs[];
> };
> 
> With the change described above, we now declare objects of the type of
> the tagged struct without embedding flexible arrays in the middle of
> another struct:
> 
> struct bundle_priv {
>         /* Must be first */
>         struct bundle_alloc_head_hdr alloc_head;
> 
>         ...
> 
>         struct uverbs_attr_bundle_hdr bundle;
>         u64 internal_buffer[32];
> };
> 
> We also use `container_of()` whenever we need to retrieve a pointer
> to the flexible structures.
> 
> Notice that the `bundle_size` computed in `uapi_compute_bundle_size()`
> remains the same.
> 
> So, with these changes, fix the following warnings:
> 
> drivers/infiniband/core/uverbs_ioctl.c:45:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>    45 |         struct bundle_alloc_head alloc_head;
>       |                                  ^~~~~~~~~~
> drivers/infiniband/core/uverbs_ioctl.c:67:35: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>    67 |         struct uverbs_attr_bundle bundle;
>       |                                   ^~~~~~
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

This looks complex, but I think it's simpler that other changes that
would have much more collateral impact. Thanks for figuring out a
workable solution!

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook