linux-kernel - Re: [PATCH] drm/vc4: Add a pad field to align drm_vc4_submit

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Date:   Tue, 1 May 2018 02:11:37 +0200
From:   Stefan Schake <stschake@...il.com>
To:     Eric Anholt <eric@...olt.net>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/vc4: Add a pad field to align drm_vc4_submit_cl to 64 bits.

On Tue, May 1, 2018 at 1:59 AM, Eric Anholt <eric@...olt.net> wrote:
> I had originally asked Stefan Schake to drop the pad field from the
> syncobj changes that just landed, because I couldn't come up with a
> reason to align to 64 bits.
>
> Talking with Dave Airlie about the new v3d driver's submit ioctl, we
> came up with a reason: sizeof() on 64-bit platforms may align to 64
> bits, in which case the userspace will be submitting the aligned size
> and the final 32 bits won't be zero-padded by the kernel.  If
> userspace doesn't zero-fill, then a future ABI change adding a 32-bit
> field at the end could potentially cause the kernel to read undefined
> data from old userspace (our userspace happens to use structure
> initialization that zero-fills, but as a general rule we try not to
> rely on that in the kernel).
>

Did a quick sizeof check on arm64 and that indeed came to 176 mod 8 = 0,
suggesting we need this additional pad. So fwiw

Reviewed-by: Stefan Schake <stschake@...il.com>

Thanks,
Stefan