Re: [PATCH 0/5] x86: CVE-2017-5715, aka Spectre

[Uros Bizjak <ubizjak at gmail dot com>]

On Wed, Jan 17, 2018 at 7:29 PM, Woodhouse, David <dwmw@amazon.co.uk> wrote:
> I'm not sure I understand the concern. When compiling a large project for
> -m32 vs. -m64, there must be a million times the compiler has to decide
> whether to emit "r" or "e" before a register name. HJ's patch already does
> this for the thunk symbol. What is the future requirement that I am not
> understanding, and that is so hard?

No, the concern is not with one extra fputc in the compiler.

IIRC, these thunks are also intended to be called from c code. So,
when one compiles this code on 64bit target, the thunk has different
name than when mentioned code is compiled on 32bit target. This puts
an extra burden on the developer, which has to use correct thunk name
in their code. Sure, this can be solved trivially with #ifdef
__x86_64__, so the issue is minor, but I thought it has to be
mentioned before the name is set in stone.

BTW: The names of the registers are ax, bx, di, si, bp, ... and this
is reflected in 32bit PIC thunk names. "e" prefix stands for
"extended", and "r" was added to be consistent with r8 ... r15. The
added pack of registers on 64bit target has different naming rules for
sub-word access, e.g. r8b, r10w, r12d.

Uros.