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

Message ID 20161123105213.27674-1-Liviu.Dudau@arm.com
State New
Headers show

Commit Message

Liviu Dudau Nov. 23, 2016, 10:52 a.m.
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@engestrom.ch>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.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.


 drivers/gpu/drm/drm_fourcc.c | 2 ++
 1 file changed, 2 insertions(+)

Comments

Jani Nikula Nov. 23, 2016, 11 a.m. | #1
On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@arm.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@engestrom.ch>
> Cc: Rob Clark <robdclark@gmail.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>
> Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.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),
Liviu Dudau Nov. 23, 2016, 11:23 a.m. | #2
On Wed, Nov 23, 2016 at 01:00:07PM +0200, Jani Nikula wrote:
> On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@arm.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@engestrom.ch>
> > Cc: Rob Clark <robdclark@gmail.com>
> > Cc: Jani Nikula <jani.nikula@intel.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> >
> > Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.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().

Agree. That is why I prefer v1 where I return immediately on NULL pointers.

Best regards,
Liviu

> 
> 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
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel
Jani Nikula Nov. 23, 2016, 12:47 p.m. | #3
On Wed, 23 Nov 2016, Daniel Vetter <daniel@ffwll.ch> 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@arm.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@engestrom.ch>
>> > > Cc: Rob Clark <robdclark@gmail.com>
>> > > Cc: Jani Nikula <jani.nikula@intel.com>
>> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>> > >
>> > > Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.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().
>> 
>> 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
Liviu Dudau Nov. 23, 2016, 1:38 p.m. | #4
On Wed, Nov 23, 2016 at 02:47:53PM +0200, Jani Nikula wrote:
> On Wed, 23 Nov 2016, Daniel Vetter <daniel@ffwll.ch> 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@arm.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@engestrom.ch>
> >> > > Cc: Rob Clark <robdclark@gmail.com>
> >> > > Cc: Jani Nikula <jani.nikula@intel.com>
> >> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> >> > >
> >> > > Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.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().
> >> 
> >> 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.

I was updating a stashed series and discovered that signature of the function has changed.
When I looked at how it changed and I got past the "you pass as a parameter a pointer 
to a struct that is used as a buffer and then that buffer is returned by function" weirdness,
I thought that at least checking for bad parameters should be done.

> 
> 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.

OK. I just like defensive programming, that's all. :)

Best regards,
Liviu

> 
> 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

Patch

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),