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Subject: Re: [PATCH v8 3/7] drm: lock drm_global_mutex earlier in the ioctl
 handler
To:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie, sumit.semwal@linaro.org,
        christian.koenig@amd.com, jani.nikula@linux.intel.com,
        joonas.lahtinen@linux.intel.com, rodrigo.vivi@intel.com,
        chris@chris-wilson.co.uk, ville.syrjala@linux.intel.com,
        matthew.auld@intel.com, dan.carpenter@oracle.com,
        tvrtko.ursulin@intel.com, matthew.d.roper@intel.com,
        lucas.demarchi@intel.com, karthik.b.s@intel.com,
        jose.souza@intel.com, manasi.d.navare@intel.com,
        airlied@redhat.com, aditya.swarup@intel.com, andrescj@chromium.org,
        linux-graphics-maintainer@vmware.com, zackr@vmware.com,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        intel-gfx@lists.freedesktop.org, linux-media@vger.kernel.org,
        linaro-mm-sig@lists.linaro.org, skhan@linuxfoundation.org,
        gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Daniel Vetter <daniel@ffwll.ch>
References: <20210826020122.1488002-1-desmondcheongzx@gmail.com>
 <20210826020122.1488002-4-desmondcheongzx@gmail.com>
 <YSdltHVQnIr+vkTn@phenom.ffwll.local>
From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Message-ID: <c9e08f10-035f-63dd-698c-faa93c935cd6@gmail.com>
Date:   Tue, 31 Aug 2021 05:04:04 +0800
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On 26/8/21 5:58 pm, Daniel Vetter wrote:
> On Thu, Aug 26, 2021 at 10:01:18AM +0800, Desmond Cheong Zhi Xi wrote:
>> In a future patch, a read lock on drm_device.master_rwsem is
>> held in the ioctl handler before the check for ioctl
>> permissions. However, this inverts the lock hierarchy of
>> drm_global_mutex --> master_rwsem.
>>
>> To avoid this, we do some prep work to grab the drm_global_mutex
>> before checking for ioctl permissions.
>>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>> ---
>>   drivers/gpu/drm/drm_ioctl.c | 18 +++++++++---------
>>   1 file changed, 9 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
>> index d25713b09b80..158629d88319 100644
>> --- a/drivers/gpu/drm/drm_ioctl.c
>> +++ b/drivers/gpu/drm/drm_ioctl.c
>> @@ -772,19 +772,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
>>   	if (drm_dev_is_unplugged(dev))
>>   		return -ENODEV;
>>   
>> +	/* Enforce sane locking for modern driver ioctls. */
>> +	if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
> 
> Maybe have a local bool locked_ioctl for this so it's extremely clear it's
> the same condition in both?
> 
> Either way: Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 

Thanks for the suggestion and review. Sounds good, I'll update and send 
out a new version.

(Sorry for delays, been busy with moving)

>> +		mutex_lock(&drm_global_mutex);
>> +
>>   	retcode = drm_ioctl_permit(flags, file_priv);
>>   	if (unlikely(retcode))
>> -		return retcode;
>> +		goto out;
>>   
>> -	/* Enforce sane locking for modern driver ioctls. */
>> -	if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) ||
>> -	    (flags & DRM_UNLOCKED))
>> -		retcode = func(dev, kdata, file_priv);
>> -	else {
>> -		mutex_lock(&drm_global_mutex);
>> -		retcode = func(dev, kdata, file_priv);
>> +	retcode = func(dev, kdata, file_priv);
>> +
>> +out:
>> +	if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
>>   		mutex_unlock(&drm_global_mutex);
>> -	}
>>   	return retcode;
>>   }
>>   EXPORT_SYMBOL(drm_ioctl_kernel);
>> -- 
>> 2.25.1
>>
>