linux-kernel - Re: [PATCH v2 4/4] venus: hfi_parser: Add check to keep the number of codecs within range

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Date:   Mon, 14 Aug 2023 12:04:17 +0530
From:   Vikash Garodia <quic_vgarodia@...cinc.com>
To:     Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
        <stanimir.k.varbanov@...il.com>, <agross@...nel.org>,
        <andersson@...nel.org>, <konrad.dybcio@...aro.org>,
        <mchehab@...nel.org>, <hans.verkuil@...co.com>,
        <tfiga@...omium.org>
CC:     <linux-media@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] venus: hfi_parser: Add check to keep the number of
 codecs within range


On 8/12/2023 12:21 AM, Bryan O'Donoghue wrote:
> On 11/08/2023 17:02, Vikash Garodia wrote:
>>
>>
>> On 8/11/2023 4:11 PM, Bryan O'Donoghue wrote:
>>> On 11/08/2023 09:49, Vikash Garodia wrote:
>>>>
>>>> On 8/11/2023 2:12 PM, Bryan O'Donoghue wrote:
>>>>> On 11/08/2023 07:04, Vikash Garodia wrote:
>>>>>>
>>>>>> On 8/10/2023 5:03 PM, Bryan O'Donoghue wrote:
>>>>>>> On 10/08/2023 03:25, Vikash Garodia wrote:
>>>>>>>> +    if (hweight_long(core->dec_codecs) + hweight_long(core->enc_codecs) >
>>>>>>>> MAX_CODEC_NUM)
>>>>>>>> +        return;
>>>>>>>> +
>>>>>>>
>>>>>>> Shouldn't this be >= ?
>>>>>> Not needed. Lets take a hypothetical case when core->dec_codecs has
>>>>>> initial 16
>>>>>> (0-15) bits set and core->enc_codecs has next 16 bits (16-31) set. The bit
>>>>>> count
>>>>>> would be 32. The codec loop after this check would run on caps array index
>>>>>> 0-31.
>>>>>> I do not see a possibility for OOB access in this case.
>>>>>>
>>>>>>>
>>>>>>> struct hfi_plat_caps caps[MAX_CODEC_NUM];
>>>>>>>
>>>>>>> ---
>>>>>>> bod
>>>>>>>
>>>>>
>>>>> Are you not doing a general defensive coding pass in this series ie
>>>>>
>>>>> "[PATCH v2 2/4] venus: hfi: fix the check to handle session buffer
>>>>> requirement"
>>>>
>>>> In "PATCH v2 2/4", there is a possibility if the check does not consider "=".
>>>> Here in this patch, I do not see a possibility.
>>>>
>>>>>
>>>>> ---
>>>>> bod
>>>
>>> But surely hweight_long(core->dec_codecs) + hweight_long(core->enc_codecs) ==
>>> MAX_CODEC_NUM is an invalid offset ?
>>
>> No, it isn't. Please run through the loop with the bitmasks added upto 32 and
>> see if there is a possibility of OOB.
> 
> IDK Vikash, the logic here seems suspect.
> 
> We have two loops that check for up to 32 indexes per loop. Why not have a
> capabilities index that can accommodate all 64 bits ?
Max codecs supported can be 32, which is also a very high number. At max the
hardware supports 5-6 codecs, including both decoder and encoder. 64 indices is
would not be needed.

> Why is it valid to have 16 encoder bits and 16 decoder bits but invalid to have
> 16 encoder bits with 17 decoder bits ? While at the same time valid to have 0
> encoder bits but 17 decoder bits ?
The addition of the encoder and decoder should be 32. Any combination which adds
to it, would go through. For ex, (17 dec + 15 enc) OR (32 dec + 0 enc) OR (0 dec
+ 32 enc) etc are valid combination theoretically, though there are only few
decoders and encoders actually supported by hardware.

Regards,
Vikash