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SubjectRe: [PATCH 2/3 v3] x86: entry_64.S: always allocate complete "struct pt_regs"
On Feb 26, 2015 1:55 AM, "Denys Vlasenko" <vda.linux@googlemail.com> wrote:
>
> On Wed, Feb 25, 2015 at 10:59 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Wed, Feb 25, 2015 at 1:28 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> >> On 02/25/2015 09:10 PM, Andy Lutomirski wrote:
> >> This part?
> >>
> >> .macro FORK_LIKE func
> >> ENTRY(stub_\func)
> >> CFI_STARTPROC
> >> - popq %r11 /* save return address */
> >> - PARTIAL_FRAME 0
> >> - SAVE_REST
> >> - pushq %r11 /* put it back on stack */
> >> + DEFAULT_FRAME 0, 8 /* offset 8: return address */
> >> + SAVE_EXTRA_REGS 8
> >> FIXUP_TOP_OF_STACK %r11, 8
> >> - DEFAULT_FRAME 0 8 /* offset 8: return address */
> >> call sys_\func
> >> RESTORE_TOP_OF_STACK %r11, 8
> >> - ret $REST_SKIP /* pop extended registers */
> >> + ret
> >> CFI_ENDPROC
> >> END(stub_\func)
> >> .endm
> >>
> >> FORK_LIKE clone
> >> FORK_LIKE fork
> >> FORK_LIKE vfork
> >>
> >> But the old code (SAVE_REST thing) was also saving registers here.
> >> It had to jump through hoops (pop return address, SAVE_REST,
> >> push return address) to do that.
> >> After the patch, "SAVE_EXTRA_REGS 8" does the same, just without
> >> pop/push pair.
> >>
> >> I just don't see what's wrong with it. Can you elaborate?
> >
> > SAVE_REST pushed the regs onto the stack, whereas SAVE_EXTRA_REGS just
> > writes them in place. It's possible for this to be called when the
> > regs have already been saved.
>
> If that would be the case - that is, if SAVE_REST was saving extra copy
> of registers on stack, then FIXUP_TOP_OF_STACK %r11, 8 would be working
> on wrong locations. The "8" there says "we have full pt_regs on stack,
> plus extra 8 bytes (the return address)". Your conjecture would mean
> that in fact there would be more bytes on stack, and FIXUP_TOP_OF_STACK
> would corrupt iret stack. Evidently, since old code was not crashing,
> this wasn't happening. SAVE_REST was really creating the "tail" of pt_regs

Ugh, you're right.

The FIXUP_TOP_OF_STACK indeed looks duplicated, bit t that's less
harmful and was already the case.

--Andy
.
>
> In addition to my previous tests, I ran my home machine with
> patched kernel. Unfortunately, it works for me :(
>
> Will try on yet another machine.
>
> --
> vda


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