Re: [PATCH] i386/audit: stop scribbling on the stack frame

Richard Guy Briggs
2014-10-27T02:06:46Z

On 14/10/24, Andy Lutomirski wrote:
> On Fri, Oct 24, 2014 at 1:19 PM, H. Peter Anvin <hpa@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.

So you could just as easily clobber %eax since that'll be restored from
PT_EAX(%esp) anyways in the following step...

Or instead of popping these two values, could ajust the stack with
addl $8,%esp
CFI_ADJUST_CFA_OFFSET -8
since we don't need either value popped off the stack?

> --Andy
>
> > -hpa

- RGB

--
Richard Guy Briggs <rbriggs@redhat.com>
Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
Remote, Ottawa, Canada
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