Re: [PATCH 3/3] objtool: Support stack layout changes in alternatives

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Mon, Jan 04, 2021 at 09:16:33AM -0600, Josh Poimboeuf wrote:

> > There's another fun scenario:
> > 
> >   0x00      CALL *pv_ops.save_fl            PUSHF
> >   0x01                                      NOP2
> >   ..
> >   0x03                                      NOP5
> >   ..
> >   0x07      NOP2
> >   0x08                                      POP %RAX
> >   0x09      <insn>
> > 
> > No conflicting boundary at 0x07, but still buggered.
> > 
> > Let me go read the actual patch to see if this is handled.
> 
> That scenario looks good, see ORC below:
> 
> .diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index cad08703c4ad..4079a430ab3f 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1483,3 +1483,8 @@ SYM_CODE_START(rewind_stack_do_exit)
>       call    do_exit
>  SYM_CODE_END(rewind_stack_do_exit)
>  .popsection
> +
> +SYM_FUNC_START(peter)
> +     ALTERNATIVE "call *pv_ops+288(%rip); .byte 0x66,0x90", "pushf; .byte 
> 0x66,0x90; .byte 0x66,0x66,0x66,0x90; popq %rax", X86_FEATURE_ALWAYS
> +     ret
> +SYM_FUNC_END(peter)
> 
> 
> 00000000000014e0 <peter>:
>     14e0:       ff 15 00 00 00 00       callq  *0x0(%rip)        # 14e6 
> <peter+0x6>
>                         14e2: R_X86_64_PC32     pv_ops+0x11c
>     14e6:       66 90                   xchg   %ax,%ax
>     14e8:       c3                      retq
> 
> alt replacement:
>   cf:   9c                      pushfq
>   d0:   66 90                   xchg   %ax,%ax
>   d2:   66 66 66 90             data16 data16 xchg %ax,%ax
>   d6:   58                      pop    %rax
> 
> 
> 
> ORC:
> 
> .entry.text+14e0: sp:sp+8 bp:(und) type:call end:0
> .entry.text+14e1: sp:sp+16 bp:(und) type:call end:0
> .entry.text+14e6: sp:sp+8 bp:(und) type:call end:0
> .entry.text+14e7: sp:sp+16 bp:(und) type:call end:0
> .entry.text+14e8: sp:sp+8 bp:(und) type:call end:0
> .entry.text+14e9: sp:(und) bp:(und) type:call end:0

Aaah, I was thinking the (LHS) NOP2 lookup would find the (RHS) PUSHF
and fail, but yes, it will emit it's own +8 and find that ofcourse!

So then yes, we only need to concern outselves with same offset
conflicts, and that does indeed simplify things considerably.