Re: [RFC PATCH 1/2] x86: Fix rbp saving in pt_regs on irq entry

Jan Beulich
2011-01-07T07:45:58Z

>>> On 06.01.11 at 18:12, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Thu, Jan 06, 2011 at 04:58:54PM +0000, Jan Beulich wrote:
>> >>> On 06.01.11 at 17:54, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>> > On Thu, Jan 06, 2011 at 04:39:39PM +0000, Jan Beulich wrote:
>> >> >>> On 06.01.11 at 17:22, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>> >> > On Thu, Jan 06, 2011 at 04:10:55PM +0000, Jan Beulich wrote:
>> >> >> >>> On 06.01.11 at 16:45, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>> >> >> > Before we had:
>> >> >> >
>> >> >> >
>> >> >> > leaveq
>> >> >> >
>> >> >> > CFI_RESTORE rbp
>> >> >> > CFI_DEF_CFA_REGISTER rsp
>> >> >> > CFI_ADJUST_CFA_OFFSET -8
>> >> >> >
>> >> >> > So CFI_RESTORE means rbp has now the value of the base frame of
>> >> >> > the calling frame (the base frame pointer of the interrupted proc) ?
>> >> >>
>> >> >> No - all it means is that %rbp now has its original (caller or
>> >> >> interrupted procedure) value again (i.e. an unwinder should not
>> >> >> try to read it from the stack [or other previously recorded
>> >> >> location] anymore).
>> >> >>
>> >> >> > And what follows means that rsp-8 points to the return address?
>> >> >>
>> >> >> No - .cfi_def_cfa_register says which register serves as the frame
>> >> >> pointer, and .cfi_adjust_cfa_offset says to adjust the offset from
>> >> >> the frame pointer to the top [or bottom] of frame. At any time
>> >> >>
>> >> >> CFA = cfa_register + cfa_offset
>> >> >>
>> >> >> with CFA being what all locations on the stack are expressed
>> >> >> relative to.
>> >> >
>> >> > Ok.
>> >> >
>> >> > So here rsp points to pt_regs::r11
>> >> >
>> >> > I don't understand why locations relative to the stack must be
>> >> > expressed here by taking rsp - 8 as a base.
>> >>
>> >> Nothing says rsp-8. The annotations merely say to set the base
>> >> register to rsp and to *adjust* the offset by -8 (after all, that's
>> >> what the leaveq instruction does).
>> >
>> > Ah! So CFA acts like a virtual frame base pointer right?
>>
>> Correct.
>
> Ah great. I was starting to prepare for the case you come to stab me :)
>
> So what do you think about that:
>
> leaveq
>
> CFI_RESTORE rbp
> CFI_DEF_CFA_REGISTER rsp
> CFI_ADJUST_CFA_OFFSET -8
>
> /* we did not save rbx, restore only from ARGOFFSET */
> addq $8, %rsp
> CFI_ADJUST_CFA_OFFSET -16
>
>
> Does that look correct to you? We increased rsp to start recovering
> the regs from the right place, but the frame pointer of the current
> proc must stay what it was.

As you hinted in your subsequent reply - it's -8 here (that's
why the directive is named *adjust*; there are other
directives allowing to *set* an offset).

> Now I don't understand how this is all useful as this is not a normal
> proc but an interruption. We can't get back the return address from
> the CFA. Or am I missing something?

Unwind annotations, when written correctly, allow unwinding
through all kinds of execution flows, including interrupts or
exceptions as well as including stack switches.

Jan