Fix a crash trying to save an empty AudioStream

[hpvb]

My friends, gather around as I learned something about the C standard that is horrifying and may keep you, dear reader, up at night.

My journey began trying to fix something entirely unrelated and not wanting to wait for ubsan builds when changing a testcase. So me, in my infinite naivete just built the engine with tests=yes, but optimizations turned on.

This resulted in a segfault on "[Audio][AudioStreamWAV] Save empty file".

Well, then, I thought. Lets built with asan then and find out where this happens! Would it surprise you, my fellow traveler, that the results were that no such crash occurred?

Thus, to the debugger I go! Fearless, with great optimism. Where I find that through many an indirection the crash came because, somehow, CowData::_unref() was getting called with a _ptr set to 0x1.

This can of course only end in tears. Or segmentation faults as we try to read an atomic variable at the somewhat inconveniently situated address at 0xfffffffffffffff0.

So I went and looked at the likely culprit, blaming many an innocent recent change along the way. I shall spare you the falsly accused. But if for some reason you slept poorly last night, I can assure you that the voodoo dolls have been put away and will not be harmed further.

So in AudioStreamWAV::get_data() we go, where we find a perfectly reasonable function! It checks to see whether or not its data is empty, and if it is not it will resize a temporary Vector to have data_bytes of space, after which it will do a prefectly pedestrian memcpy() and all is well in the world.

Or so it seems! After many an hour of despair and disassembly I, at last, decided to look at where the data gets set! A breakthrough! Because of the padding data is never empty! So the code always runs!

Eureka! One would think. But then, foolishly, I looked into the get_data() function one more. My mortal enemy was staring me in the face, laughing. Because it did not care about this. Sure, the check was worthless but still... What are we left with.

At this point I could feel the method mocking me.

"I resize the vector to 0, I then memcpy zero bytes into it." It said, DARING me to object to this state of affairs.

And yet, if I changed the function to check for "data_bytes" rather than data.is_empty() no crashes.

Was this a compiler bug? Am I losing my mind? But then... I remembered the mantra of the wise compiler druids... "It Is Not A Compiler Bug".

But what then! The bug does not happen when memory is being watched! Valgrind agreed that while accessing the SafeRefCount at 0xfffffffffffffff0 was incredibly rude, it did not inform me of anything else untoward happening. So I read the memcpy() manpage... nothing... I read the the memcpy() posix spec... nothing.

Finally, in despair and because I had nothing left to lose... The ISO C language specification. As I was reading, I could hear AudioStreamWAV::get_data() cackling, knowing that its time was up, but proud of the madness it caused in my soul. Knowing I would never be the same.

The behavior is undefined if either dest or src is an invalid or null pointer.

So... Here I stand before you, a broken person. But one richer in knowledge.

I write you this from the depths of madness in the hopes that you, dear reader, can be spared this ordeal.

May god have mercy on our souls.

We trigger the following sequence of events:

• memcpy(null, null, 0) is UB, thus dest and src cannot be null
• we inline the calls to the ctor and dtor
• now we have a function that does something that "proves" dest cannot be null
• we inline cowdata::_unref() which does a null check, on something that the compiler just convinced itself cannot be null
• the compiler removes the dead code branch where _ptr == nullptr
• we start to do pointer arithmetic on a nullptr and get send to uninitialized memory.

Co-Authored-By: Jason Beckmann [email protected]

[AThousandShips]

Duplicate of:

• Fix undefined memcpy in AudioStreamWAV::get_data() #97790

[AThousandShips]

Also I think we should avoid this long commit messages as engaging as it is, it makes a mess of the log print

[hpvb]

For posterity I want to explain why the fact that memcpy() with either dest or src is undefined behavior matters:

We trigger the following sequence of events:

1. memcpy(null, null, 0) is UB, thus dest and src cannot be null
2. we inline the calls to the ctor and dtor
3. now we have a function that does something that "proves" dest cannot be null
4. we inline cowdata::_unref() which does a null check, on something that the compiler just convinced itself cannot be null
5. the compiler removes the dead code branch where _ptr == nullptr
6. we start to do pointer arithmetic on a nullptr and get send to uninitialized memory.

[akien-mga]

Thanks!