linux-kernel - Re: [PATCH] doc: botching-up-ioctls: Make it clearer why structs must be padded

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Message-ID: <87muxi5739.fsf@anholt.net>
Date:   Wed, 02 May 2018 09:56:58 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Daniel Vetter <daniel.vetter@...ll.ch>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        linux-doc@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH] doc: botching-up-ioctls: Make it clearer why structs must be padded

Daniel Vetter <daniel.vetter@...ll.ch> writes:

> This came up in discussions when reviewing drm patches.
>
> Cc: Eric Anholt <eric@...olt.net>
> Cc: linux-doc@...r.kernel.org
> Cc: Jonathan Corbet <corbet@....net>
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
>
> --
>
> Aside: I wonder whether we shouldn't move this to some other place and
> rst-ify it? Any good suggestions?
> -Daniel
> ---
>  Documentation/ioctl/botching-up-ioctls.txt | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/ioctl/botching-up-ioctls.txt b/Documentation/ioctl/botching-up-ioctls.txt
> index d02cfb48901c..883fb034bd04 100644
> --- a/Documentation/ioctl/botching-up-ioctls.txt
> +++ b/Documentation/ioctl/botching-up-ioctls.txt
> @@ -73,7 +73,9 @@ will have a second iteration or at least an extension for any given interface.
>     future extensions is going right down the gutters since someone will submit
>     an ioctl struct with random stack garbage in the yet unused parts. Which
>     then bakes in the ABI that those fields can never be used for anything else
> -   but garbage.
> +   but garbage. This is also the reason why you must explicitly pad all
> +   structures, even if you never use them in an array - the padding the compiler
> +   might insert could contain garbage.

I hadn't realized that we had this document in git, or I probably would
have written this patch.  I think this makes it clear enough how I got
vc4 and v3d wrong.  Thanks!

Reviewed-by: Eric Anholt <eric@...olt.net>

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