[Mesa-dev] [PATCH 1/3] glsl: on UBO/SSBOs link error, the number of active blocks remains 0

[Timothy Arceri]

On 10/03/17 08:46, Andres Gomez wrote:
> On Fri, 2017-03-10 at 08:32 +1100, Timothy Arceri wrote:
>> On 23/02/17 19:55, Andres Gomez wrote:
>>> Commit f1293b2f9bc3 split apart buffer block arrays but introduced
>>> also the possibility of a recount of active
>>> blocks (NumUniformBlocks/NumShaderStorageBlocks) which would be
>>> incoherent with the actual amount of active
>>> blocks (UniformBlocks/ShaderStorageBlocks) in the program.
>>>
>>> This could cause a segmentation fault if trying to use the index of a
>>> block in a link failed program.
>>
>> Where exactly does this segfault happen?
>> interstage_cross_validate_uniform_blocks() should exit linking because
>> we returned false.
>
> It does exit, the segfault is not happening when running this code but
> when using the link failed program later, as commented.
>
> I caught this by using piglit's shader runner. In the "init_test"
> function the last action is to call "setup_ubos":
> https://cgit.freedesktop.org/piglit/tree/tests/shaders/shader_runner.c#n3704
>
> This is done regardless the link status gotten previously. At
> "setup_ubos" we ask first for the active UBOs:
> https://cgit.freedesktop.org/piglit/tree/tests/shaders/shader_runner.c#n2652
>
> And if there is any, we make use of them:
> https://cgit.freedesktop.org/piglit/tree/tests/shaders/shader_runner.c#n2662
>
> That's when the SIGSEV happens.
>
> Of course, you could argue what are you doing using a link failed
> program ☺ but the fact is that it should not return a value for the
> active UBOs other than 0 and, most importantly, it shouldn't crash.
>
> I hope this clarifies your doubt.
>

I'm not sure this is a bug. The section 7.3. (PROGRAM OBJECTS) of the 
OpenGL 4.5 spec says:

"If a program is  not linked successfully, the link may have failed for 
a number of reasons, including cases where the program required more 
resources than supported by the implementation. Implementations are 
permitted, but not required, to record lists of resources that would
have been considered active had the program linked successfully."