linux-kernel - Re: [PATCH 2/4] drm/msm: rework GEM

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Date:   Fri, 30 Nov 2018 10:28:30 -0500
From:   Rob Clark <robdclark@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] drm/msm: rework GEM_INFO ioctl

On Fri, Nov 30, 2018 at 10:14 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Fri, Nov 30, 2018 at 4:02 PM Rob Clark <robdclark@...il.com> wrote:
> >
>
> > -
> > -#define MSM_INFO_FLAGS (MSM_INFO_IOVA)
> > +/* Get or set GEM buffer info.  The requested value can be passed
> > + * directly in 'value', or for data larger than 64b 'value' is a
> > + * pointer to userspace buffer, with 'len' specifying the number of
> > + * bytes copied into that buffer.  For info returned by pointer,
> > + * calling the GEM_INFO ioctl with null 'value' will return the
> > + * required buffer size in 'len'
> > + */
> > +#define MSM_INFO_GET_OFFSET    0x00   /* get mmap() offset, returned by value */
> > +#define MSM_INFO_GET_IOVA      0x01   /* get iova, returned by value */
> >
> >  struct drm_msm_gem_info {
> >         __u32 handle;         /* in */
> > -       __u32 flags;          /* in - combination of MSM_INFO_* flags */
> > -       __u64 offset;         /* out, mmap() offset or iova */
> > +       __u32 info;           /* in - one of MSM_INFO_* */
> > +       __u64 value;          /* in or out */
> > +       __u32 len;            /* in or out */
> >  };
>
> As structure with implicit padding has the problem of possibly leaking
> kernel stack data. It's better to make the padding explicit here so you
> can zero it from the kernel. Also, as I mentioned in the other patch,
> you probably need a new data structure and ioctl command number
> to keep compatiblity with the old interface.

hmm, right, pad field is a good idea.  As far as compat, drm_ioctl()
handles zero-padding so adding new ioctl struct members at the end is
safe (as long as a zero value somehow results in previous behavior)

BR,
-R

>
>      Arnd