say what bytes in heap blocks are uninitialized about kmsan

[eugenis]

It's possible to generate unique origin ids for parts of a heap allocation with 4-byte granularity.

[dvyukov]

You mean generate via stackdepot?
I was thinking that maybe we can eat some bits from origin and put offset there, so it's still only 1 stackdepot id. We would need to propagate the offset during chaining and print in report.

[eugenis]

There is a trade off. You can eat through origin id values faster (I think we've seen a case when we ran out of them, but it's probably less of an issue for the kernel), or you can consume a bit more memory per allocation.

These "sub-origins" can be represented with a node in ChainedOriginDepot. This way the stack trace would only be stored once.

In any case, this can result in an explosion of chained origins. Imagine any kind of N*M algoritm, like matrix multiplication. Userspace msan has limits to deal with this.

[dvyukov]

Why will offsets in origin ids lead to explosion of chained origins? We strip offset, chain, attach offset again. Stackdepot always sees ids without offsets.

[eugenis]

You are right. There is no explosion if we only track offsets in the original allocation, and not in subsequent stores.