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Subject: Re: [PATCH] drm: assure aux_dev is nonzero before using it
To:     =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>
Cc:     Jani Nikula <jani.nikula@linux.intel.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        airlied@linux.ie, dkwon@redhat.com,
        Joe Donahue <jdonohue@redhat.com>,
        John Feeney <jfeeney@redhat.com>
References: <20190523110905.22445-1-tcamuso@redhat.com>
 <87v9y0mept.fsf@intel.com> <5111581c-9d73-530d-d3ff-4f6950bf3f8c@redhat.com>
 <20190710135617.GE5942@intel.com>
 <374b7e4e-40a2-f3c0-ae14-c533bd42243f@redhat.com>
 <20190712170657.GL5942@intel.com>
From:   Tony Camuso <tcamuso@redhat.com>
Message-ID: <841db615-a353-2174-6fd6-a3302f352b08@redhat.com>
Date:   Mon, 23 Sep 2019 11:03:35 -0400
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On 7/12/19 1:06 PM, Ville Syrjälä wrote:
> On Fri, Jul 12, 2019 at 12:07:46PM -0400, Tony Camuso wrote:
>> On 7/10/19 9:56 AM, Ville Syrjälä wrote:
>>> On Wed, Jul 10, 2019 at 09:47:11AM -0400, Tony Camuso wrote:
>>>> On 5/24/19 4:36 AM, Jani Nikula wrote:
>>>>> On Thu, 23 May 2019, tcamuso <tcamuso@redhat.com> wrote:
>>>>>>    From Daniel Kwon <dkwon@redhat.com>
>>>>>>
>>>>>> The system was crashed due to invalid memory access while trying to access
>>>>>> auxiliary device.
>>>>>>
>>>>>> crash> bt
>>>>>> PID: 9863   TASK: ffff89d1bdf11040  CPU: 1   COMMAND: "ipmitool"
>>>>>>     #0 [ffff89cedd7f3868] machine_kexec at ffffffffb0663674
>>>>>>     #1 [ffff89cedd7f38c8] __crash_kexec at ffffffffb071cf62
>>>>>>     #2 [ffff89cedd7f3998] crash_kexec at ffffffffb071d050
>>>>>>     #3 [ffff89cedd7f39b0] oops_end at ffffffffb0d6d758
>>>>>>     #4 [ffff89cedd7f39d8] no_context at ffffffffb0d5bcde
>>>>>>     #5 [ffff89cedd7f3a28] __bad_area_nosemaphore at ffffffffb0d5bd75
>>>>>>     #6 [ffff89cedd7f3a78] bad_area at ffffffffb0d5c085
>>>>>>     #7 [ffff89cedd7f3aa0] __do_page_fault at ffffffffb0d7080c
>>>>>>     #8 [ffff89cedd7f3b10] do_page_fault at ffffffffb0d70905
>>>>>>     #9 [ffff89cedd7f3b40] page_fault at ffffffffb0d6c758
>>>>>>        [exception RIP: drm_dp_aux_dev_get_by_minor+0x3d]
>>>>>>        RIP: ffffffffc0a589bd  RSP: ffff89cedd7f3bf0  RFLAGS: 00010246
>>>>>>        RAX: 0000000000000000  RBX: 0000000000000000  RCX: ffff89cedd7f3fd8
>>>>>>        RDX: 0000000000000000  RSI: 0000000000000000  RDI: ffffffffc0a613e0
>>>>>>        RBP: ffff89cedd7f3bf8   R8: ffff89f1bcbabbd0   R9: 0000000000000000
>>>>>>        R10: ffff89f1be7a1cc0  R11: 0000000000000000  R12: 0000000000000000
>>>>>>        R13: ffff89f1b32a2830  R14: ffff89d18fadfa00  R15: 0000000000000000
>>>>>>        ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
>>>>>>        RIP: 00002b45f0d80d30  RSP: 00007ffc416066a0  RFLAGS: 00010246
>>>>>>        RAX: 0000000000000002  RBX: 000056062e212d80  RCX: 00007ffc41606810
>>>>>>        RDX: 0000000000000000  RSI: 0000000000000002  RDI: 00007ffc41606ec0
>>>>>>        RBP: 0000000000000000   R8: 000056062dfed229   R9: 00002b45f0cdf14d
>>>>>>        R10: 0000000000000002  R11: 0000000000000246  R12: 00007ffc41606ec0
>>>>>>        R13: 00007ffc41606ed0  R14: 00007ffc41606ee0  R15: 0000000000000000
>>>>>>        ORIG_RAX: 0000000000000002  CS: 0033  SS: 002b
>>>>>>
>>>>>> ----------------------------------------------------------------------------
>>>>>>
>>>>>> It was trying to open '/dev/ipmi0', but as no entry in aux_dir, it returned
>>>>>> NULL from 'idr_find()'. This drm_dp_aux_dev_get_by_minor() should have done a
>>>>>> check on this, but had failed to do it.
>>>>>
>>>>> I think the better question is, *why* does the idr_find() return NULL? I
>>>>> don't think it should, under any circumstances. I fear adding the check
>>>>> here papers over some other problem, taking us further away from the
>>>>> root cause.
>>>>>
>>>>> Also, can you reproduce this on a recent upstream kernel? The aux device
>>>>> nodes were introduced in kernel v4.6. Whatever you reproduced on v3.10
>>>>> is pretty much irrelevant for upstream.
>>>>>
>>>>>
>>>>> BR,
>>>>> Jani.
>>>>
>>>> I have not been able to reproduce this problem.
>>>
>>> mknod /dev/foo c <drm_dp_aux major> 255
>>> cat /dev/foo
>>>
>>> should do it.
>>
>> How do I determine <drm_dp_aux major>?
> 
> ls,file,stat. Take your pick.
> 

Problem here is I can't ls,file,stat /dev/foo until after it's created,
but I need to know the drm_dp_aux major number befroe I can use mknod.

What am I missing here?