Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

[Jani Nikula]

On Wed, 23 Nov 2016, Daniel Vetter <daniel@xxxxxxxx> wrote:
> On Wed, Nov 23, 2016 at 11:23:23AM +0000, Liviu Dudau wrote:
>> On Wed, Nov 23, 2016 at 01:00:07PM +0200, Jani Nikula wrote:
>> > On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@xxxxxxx> 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@xxxxxxxxxxxx>
>> > > Cc: Rob Clark <robdclark@xxxxxxxxx>
>> > > Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
>> > > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
>> > >
>> > > Signed-off-by: Liviu Dudau <Liviu.Dudau@xxxxxxx>
>> > > ---
>> > > 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().
>> 
>> Agree. That is why I prefer v1 where I return immediately on NULL pointers.
>
> The question for v1 is why did you hit that? "broken driver code" isn't
> really a good reason, au contraire it's a reason to not merge your patch:
> We do not want to hide driver bugs silently.

Moreover, v1 puts the burden back on the *caller* of the function to
check for NULL return, while it previously could not even return NULL.

The function is fine. It isn't broken. Don't try to fix it.

BR,
Jani.



>
> There's definitely cases where handling NULL automatically is reasonable,
> e.g. kfree(). But a NULL drm_format_name_buf sounds like, at least a quick
> grep shows that all callers just put this struct onto the stack.
> -Daniel

-- 
Jani Nikula, Intel Open Source Technology Center