[openssl-dev] [PATCH] Insert CFI directives in x86_64 SHA1 implementation to enable backtracing

[Wim Lewis]

On Mar 25, 2015, at 2:42 PM, Matt Cross <matt.cross at gmail.com> wrote:
> This is done to align %rsp to a 64 byte boundary, and the original %rsp is stored on the stack; so the only way to get the actual frame pointer is to read 64(%rsp) and add an offset to that.  I managed to do that by inserting a raw DWARF expression.  It's not clear to me that the perlasm preprocessor could (or should) do this; but perhaps it makes sense to add some directives for the perlasm preprocessor and let it generate the raw DWARF expression.

I think the cfi_cfa_indirect pseudo-directive handles that case (it’s defined in x86gas.pl and used in, e.g., aesni-x86, which does a similar stack alignment). It uses a minuscule “assembler” for DWARF location opcode expressions included in the 3-cfi patch, which makes it easier to handle cases where the assembly is doing something clever.

The SHA256 and SHA512 implementations on x86_32 did something trickier: each round adjusted the stack frame forwards by some number of bytes, so that it could read the previous round and write the new data from fixed offsets from $esp (presumably this saves a register). I didn’t try writing DWARF info to unwind that. I don’t think the x86_64 code does that, though, since registers aren’t as scarce.


> If I understand correctly, your patch was not folded in.  Do you recall why?

IIRC, the maintainer said it would be too much trouble to integrate.