linux-kernel - Re: [PATCH][next] drm: i915_drm.h: Replace zero-length array with flexible-array member

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Message-ID: <87h7z4pj0g.fsf@intel.com>
Date:   Wed, 04 Mar 2020 11:27:27 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH][next] drm: i915_drm.h: Replace zero-length array with flexible-array member

On Tue, 03 Mar 2020, "Gustavo A. R. Silva" <gustavo@...eddedor.com> wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:

My earlier question about making the change in uapi was really about
potentially bumping up userspace compiler requirements, though I did not
actually say so. :)

I guess effectively uapi already requires C99 to build? And we
(i915_drm.h) have both [0] and []. So go for it.

What's your baseline? I think you've missed one instance of struct
i915_engine_class_instance engines[0];

BR,
Jani.


>
> struct foo {
>         int stuff;
>         struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>  include/uapi/drm/i915_drm.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 2813e579b480..413d923b332a 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -1729,7 +1729,7 @@ struct i915_context_engines_load_balance {
>  
>  	__u64 mbz64; /* reserved for future use; must be zero */
>  
> -	struct i915_engine_class_instance engines[0];
> +	struct i915_engine_class_instance engines[];
>  } __attribute__((packed));
>  
>  #define I915_DEFINE_CONTEXT_ENGINES_LOAD_BALANCE(name__, N__) struct { \
> @@ -1767,7 +1767,7 @@ struct i915_context_engines_bond {
>  	__u64 flags; /* all undefined flags must be zero */
>  	__u64 mbz64[4]; /* reserved for future use; must be zero */
>  
> -	struct i915_engine_class_instance engines[0];
> +	struct i915_engine_class_instance engines[];
>  } __attribute__((packed));
>  
>  #define I915_DEFINE_CONTEXT_ENGINES_BOND(name__, N__) struct { \

-- 
Jani Nikula, Intel Open Source Graphics Center