Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable

Zachary Amsden
2007-03-20T01:23:33Z

Jeremy Fitzhardinge wrote:
> For example, say we wanted to put a general call for sti into entry.S,
> where its expected it won't touch any registers. In that case, we'd
> have a sequence like:
>
> push %eax
> push %ecx
> push %edx
> call paravirt_cli
> pop %edx
> pop %ecx
> pop %eax
>
>
> If we parse the relocs, then we'd find the reference to paravirt_cli.
> If we look at the byte before and see 0xe8, then we can see if its a
> call. If we then work out in each direction and see matched push/pops,
> then we know what registers can be trashed in the call. This also
> allows us to determine the callsite size, and therefore how much space
> we need for inlining.
>

No, that is a very dangerous suggestion. You absolutely *cannot* do
this safely without explicitly marking the start EIP of this code. You
*must* use metadata to do that. It is never safe to disassemble
backwards or "rewind" EIP for x86 code.

Zach