Re: [FFmpeg-devel] CBS

[James Almer <jamrial@gmail.com>]

On 8/6/2024 2:54 PM, Andreas Rheinhardt wrote:
> Michael Niedermayer:
>> On Tue, Aug 06, 2024 at 07:05:38PM +0200, Michael Niedermayer wrote:
>>> Hi
>>>
>>> Did CBS win the obfuscated C contest yet?
>>>
>>> I was just looking at a msan issue and then looked at this:
>>>
>>> CHECK(FUNC_SEI(message_list)(ctx, rw, &current->message_list, 1));
>>>
>>>
>>> #define CHECK(call) do { \
>>>          err = (call); \
>>>          if (err < 0) \
>>>              return err; \
>>>      } while (0)
>>>
>>> #define FUNC_NAME2(rw, codec, name) cbs_ ## codec ## _ ## rw ## _ ## name
>>> #define FUNC_NAME1(rw, codec, name) FUNC_NAME2(rw, codec, name)
>>> #define FUNC_H264(name) FUNC_NAME1(READWRITE, h264, name)
>>> #define FUNC_H265(name) FUNC_NAME1(READWRITE, h265, name)
>>> #define FUNC_H266(name) FUNC_NAME1(READWRITE, h266, name)
>>> #define FUNC_SEI(name)  FUNC_NAME1(READWRITE, sei,  name)
>>>
>>> #define SEI_FUNC(name, args) \
>>> static int FUNC(name) args;  \
>>> static int FUNC(name ## _internal)(CodedBitstreamContext *ctx, \
>>>                                     RWContext *rw, void *cur,   \
>>>                                     SEIMessageState *state)     \
>>> { \
>>>      return FUNC(name)(ctx, rw, cur, state); \
>>> } \
>>> static int FUNC(name) args
>>>
>>>
>>> anyway, can we remove all preprocessor use from cbs ?
> 
> I don't think that this is really obfuscated.
> 
>>
>> the issue iam looking at is due to
>>
>> SEI_FUNC(sei_pic_timing, (CodedBitstreamContext *ctx, RWContext *rw, H264RawSEIPicTiming *current, SEIMessageState *sei))
>>
>> having different active SPS on writing than reading, so the write code
>> has nal_hrd_parameters_present_flag set while the read had that 0
>> so uninitialized data is written
>>
>> I cannot find any match for "cbs" in MAINTAINERS, also there are no copyright
>> with names in the cbs code.
> 
> 1. I just sent a patch that "fixes" this.
> 2. But actually, there is a deeper bug here: We would need to defer
> parsing certain SEI message units to a second pass when the currently
> active SPS is known. This can happen with spec-compliant input (and even

Is this a scenario where a slice that referenced an SPS was parsed, then 
a SEI message, then another slice that references another SPS, and the 
SEI expects the latter to be active despite it being coded before the slice?

> more so with spec-incompliant input, which is presumably what the fuzzer
> produced).
> 
> - Andreas
> 
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