Stream: git-wasmtime

Topic: wasmtime / issue #9264 Question regarding AArch64 Dwarf C...

Wasmtime GitHub notifications bot (Sep 17 2024 at 13:05):

matlo607 opened issue #9264:

Question goal

Clarification about the current usage of the DWARF CFI directive DW_CFA_val_expression by the project for the purpose of unwinding on AArch64 with Pointer Authentication enabled (i.e. -march=<arch> -mbranch-protection=pac-ret).

References

• #3183 at the origin of the GCC ticket.
• Bug 104689 opened by @akirilov-arm - aarch64: libgcc: DW_CFA_val_expression is not supported for RA_SIGN_SATE register
• #4195 implemented by @akirilov-arm using .cfi_window_save (a.k.a. .cfi_negate_ra_state) instead of DW_CFA_val_expression to set the RA state.

Details

Today, DW_CFA_val_expression is supposedly only used for backtracing in the Cranelift project.
At the end in #4195, @akirilov-arm didn't use DW_CFA_val_expression to set the RA state register.
The code seems to have evolved since then and moved to https://github.com/bytecodealliance/wasmtime/blob/main/crates/fiber/src/unix/aarch64.rs.
This code uses .cfi_window_save, so no DW_CFA_val_expression are emitted.

Question

As a conclusion, the project doesn't seem to use the exotic setting of the RA state register that was requested in Bug 104689.
Is my understanding correct ?
Is it an issue for you if GCC's AArch64 backend removes the support for setting the RA state register with DW_CFA_val_expression ?

Wasmtime GitHub notifications bot (Sep 18 2024 at 18:29):

akirilov-arm commented on issue #9264:

It has been a while since I have worked on Cranelift, but as far as I can tell, the compiler still uses the same DWARF CFI directive when generating function prologues.

The fiber implementation that you have looked at is something completely separate.

Last updated: Dec 23 2024 at 12:05 UTC