linux-kernel - Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format

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Message-ID: <87vavewjew.fsf@intel.com>
Date:   Wed, 23 Nov 2016 13:00:07 +0200
From:   Jani Nikula <jani.nikula@...el.com>
To:     Liviu Dudau <Liviu.Dudau@....com>,
        Eric Engestrom <eric@...estrom.ch>
Cc:     David Airlie <airlied@...ux.ie>,
        DRI devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rob Clark <robdclark@...il.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@....com> wrote:
> drm_get_format_name() de-references the buf parameter without checking
> if the pointer was not NULL. Given that the function is EXPORT-ed, lets
> sanitise the parameters before proceeding.
>
> v2: Use BUG_ON() to annoy users that did not pass valid parameters to function.
>
> Fixes: b3c11ac267d461d3d5 ("drm: move allocation out of drm_get_format_name())
> Cc: Eric Engestrom <eric@...estrom.ch>
> Cc: Rob Clark <robdclark@...il.com>
> Cc: Jani Nikula <jani.nikula@...el.com>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
>
> Signed-off-by: Liviu Dudau <Liviu.Dudau@....com>
> ---
> I still think sanity checking the parameters of an exported function is worth
> doing, even if the way one triggers the NULL pointer crash is priviledged. Not
> a big fan of the verbosity of BUG_ON() and would rather silently reject NULL buf
> pointer, but that is a matter of taste.

There really is no meaningful difference between doing BUG_ON(!bug)
vs. just letting buf->str oops. The kernel is full of functions that
expect sensible pointers, and I don't see why this one in particular
should be so special to warrant a BUG_ON().

BR,
Jani.

>
>
>  drivers/gpu/drm/drm_fourcc.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> index 90d2cc8..6d80239 100644
> --- a/drivers/gpu/drm/drm_fourcc.c
> +++ b/drivers/gpu/drm/drm_fourcc.c
> @@ -85,6 +85,8 @@ EXPORT_SYMBOL(drm_mode_legacy_fb_format);
>   */
>  const char *drm_get_format_name(uint32_t format, struct drm_format_name_buf *buf)
>  {
> +	BUG_ON(!buf);
> +
>  	snprintf(buf->str, sizeof(buf->str),
>  		 "%c%c%c%c %s-endian (0x%08x)",
>  		 printable_char(format & 0xff),

-- 
Jani Nikula, Intel Open Source Technology Center