Re: lib/55719 (Unwind tables for signal trampoline on amd64 are incorrect)

[Nikhil Benesch <nikhil.benesch%gmail.com@localhost>]

On Sun, Oct 11, 2020 at 10:35 PM Andrew Cagney <andrew.cagney%gmail.com@localhost> wrote:
> So any code wanting the caller's stack pointer will need to parse the
> CFI and then unwind the stack-pointer (CFA is a convenient fiction
> invented by DWARF).
>
> The only real expectation is that the CFA is constant through out the
> lifetime of the frame (one of the things identifying  a single-stepped
> call/return instruction is the changed CFA).

Hmm, ok, so perhaps just picking a different CFA for this frame is
acceptable then. My assertion that the CFA must be equal to RSP on
amd64 is based only on this blog post [0], which is hardly
authoritative.

Kamil's patch as written sets the CFA to both R15 and later (RSP + 8),
but that is easy enough to fix to drop the second .cfi_def_cfa
directive. There is still the issue of generating the FDE one PC
earlier than the beginning of the sigtramp symbol, but maybe I can
work something out.

Also apologies that my last mail was sent in HTML, not plain text.
That should be fixed now.

[0]: https://www.corsix.org/content/cfa-rsp-x86-64