[llvm-dev] MTE -- discussion on Exception unwinding ABI

[Matthew Malcomson via llvm-dev]

On 05/05/2020 01:32, Evgenii Stepanov wrote:
> 
> 
> 
> Approach 1 sounds perfect to me. Conveniently, both unwinders ignore 
> unrecognized characters in the augmentation string.
> 
> In our experience with ASan, errors caused by failing to unpoison/untag 
> a stack frame are cryptic, and pretty hard to debug. But they can be 
> caused by a number of other things, not just the unwinder - vfork, 
> longjmp and friends in libc, custom stack manipulation anywhere (ex. ART 
> has longjmp implemented in assembly). We could implement a "verified" 
> mode to catch these cases - a compilation flag that checks that the 
> entire frame is SP-accessible at function entry.
> 
> We could use the letter "G" as that's what stands for "tag" in the 
> instruction mnemonics (STG, IRG).

Awesome -- and the "G" letter seems like a good idea to me.

I'll wait until the end of the week to give others a chance to comment, 
but if no-one objects by then I think we can go ahead with Approach 1 
using the letter "G".

> 
> I had a thought about extending Dwarf with a way to specify a range of 
> offsets to be untagged within the frame (and default to the entire frame 
> if not specified). But it feels like the performance savings would not 
> be worth the extra complexity.
>