Re: [PATCH] drm/msm: UAPI error reporting

[Konrad Dybcio]

On 23.11.2024 3:41 AM, Rob Clark wrote:
> On Fri, Nov 22, 2024 at 4:19 PM Konrad Dybcio
> <konrad.dybcio@xxxxxxxxxxxxxxxx> wrote:
>>
>> On 22.11.2024 4:51 PM, Rob Clark wrote:
>>> On Fri, Nov 22, 2024 at 4:21 AM Konrad Dybcio
>>> <konrad.dybcio@xxxxxxxxxxxxxxxx> wrote:
>>>>
>>>> On 21.11.2024 5:48 PM, Rob Clark wrote:
>>>>> From: Rob Clark <robdclark@xxxxxxxxxxxx>
>>>>>
>>>>> Debugging incorrect UAPI usage tends to be a bit painful, so add a
>>>>> helper macro to make it easier to add debug logging which can be enabled
>>>>> at runtime via drm.debug.
>>>>>
>>>>> Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
>>>>> ---
>>>>
>>>> [...]
>>>>
>>>>> +/* Helper for returning a UABI error with optional logging which can make
>>>>> + * it easier for userspace to understand what it is doing wrong.
>>>>> + */
>>>>> +#define UERR(err, drm, fmt, ...) \
>>>>> +     ({ DRM_DEV_DEBUG_DRIVER((drm)->dev, fmt, ##__VA_ARGS__); -(err); })
>>>>> +
>>>>>  #define DBG(fmt, ...) DRM_DEBUG_DRIVER(fmt"\n", ##__VA_ARGS__)
>>>>>  #define VERB(fmt, ...) if (0) DRM_DEBUG_DRIVER(fmt"\n", ##__VA_ARGS__)
>>>>
>>>> I'm generally not a fan of adding driver-specific debug prints..
>>>>
>>>> Maybe that's something that could be pushed to the drm-common layer
>>>> or even deeper down the stack?
>>>
>>> Even if we had something like DRM_DBG_UABI_ERROR() I'd probably still
>>> just #define UERR() to be a wrapper for it, since line length/wrapping
>>> tends to be a bit of a challenge.  And I have a fairly substantial
>>> patch stack on top of this adding sparse/vm_bind support.  (Debugging
>>> that was actually the motivation for this patch.)
>>
>> Alright, let's not get in the way then
>>
>>> I noticed that xe has something similar, but slightly different shape,
>>> in the form of XE_IOCTL_DBG().. but that kinda just moves the line
>>> length problem into the if() conditional.  (And doesn't provide the
>>> benefit of being able to display the incorrect param.)
>>
>> Maybe rust comes one day and the lines will start growing vertically ;)
> 
> Rust for the userspace facing rendernode side of the driver, in
> particular, would be interesting for me, tbh.  Especially if handle
> related rust<->c layers are designed properly.  I've lost track of how
> many handle lifetime race condition UAF's I've seen ;-)
> 
> Re-writing entire drivers is a big lift, especially when there is so
> much hw+features to enable.  KMS is limited to drm master (generally a
> somewhat privileged process), so less of a concern from a security
> standpoint.  Much of the GPU side of things is "boring" power related
> stuff (suspend/resume/devfreq).  But the rendernode ioctls are open to
> any process that can use the GPU in a typical setup.

The boring part would benefit greatly from automatic scope exit
cleanup.. We've had lots of issues in the past with that (that are
hopefully? sorted out now, or should I say, for now)

Konrad