Re: [PATCH] i386/audit: stop scribbling on the stack frame
On Fri, Oct 24, 2014 at 1:19 PM, H. Peter Anvin <hpa(a)zytor.com> wrote:
On 10/23/2014 12:38 PM, Eric Paris wrote:
>>
>> After the call __audit_syscall_entry aren't they already polluted?
>> Isn't that the reason we need to reload EAX?
>
> Well, I guess EAX is special...
>
Because system calls are "asmlinkage", all the parameters are on the
stack, but %eax is used as the index into the system call table. This
should thus be fine until we get rid of regparm(0) entirely, if that
ever happens.
...and because __audit_syscall_entry *isn't* asmlinkage, it uses the
other convention, which is where the confusion comes from. And, by
the time you get to sysenter_do_call, nothing cares about ecx, so you
can freely clobber it while popping from the stack. I get it now.
--Andy
-hpa
--
Andy Lutomirski
AMA Capital Management, LLC