Re: [PATCH v4 00/28] objtool: Function validation tracing

[Alexandre Chartre]

On 11/17/25 10:42, David Laight wrote:
> On Mon, 17 Nov 2025 08:50:45 +0100
> Alexandre Chartre <alexandre.chartre@xxxxxxxxxx> wrote:
>
> > On 11/14/25 22:34, Josh Poimboeuf wrote:
> ...
> > David raises the issue that a side-by-side display requires a large window.
> >
> > The compact display could be like this:
> >
> > Alternative with single instruction:
> >
> >     bb8:  do_one_initcall+0x1a8    <alternative.bb8>
> >                                    = callq  *0x0(%rip)        # 0xbbe <pv_ops+0xf8>    (if default)
> >                                    = sti                                               (if !X86_FEATURE_XENPV)
> >                                    = callq  BUG_func                                   (if +X86_FEATURE_ALWAYS)
> >
> > Alternative with multiple instructions:
> >
> >     82e7:  __switch_to_asm+0x27    <alternative.82e7>
> >                                    = DEFAULT
> >     82e7:  __switch_to_asm+0x27    | jmp    0x8312 <__switch_to_asm+0x52>
> >                                    |
> >                                    = !X86_FEATURE_ALWAYS
> >     82e7:  __switch_to_asm+0x27    | NOP1
> >     82e8:  __switch_to_asm+0x28    | NOP1
> >     82e9:  __switch_to_asm+0x29    | callq  0x82ef <__switch_to_asm+0x2f>
> >     82ee:  __switch_to_asm+0x2e    | int3
> >     82ef:  __switch_to_asm+0x2f    | add    $0x8,%rsp
> >     82f3:  __switch_to_asm+0x33    | lfence
> >                                    |
> >                                    = X86_FEATURE_RSB_CTXSW
> >     82e7:  __switch_to_asm+0x27    | mov    $0x10,%r12
> >     82ee:  __switch_to_asm+0x2e    | callq  0x82f4 <__switch_to_asm+0x34>
> >     82f3:  __switch_to_asm+0x33    | int3
> >     82f4:  __switch_to_asm+0x34    | callq  0x82fa <__switch_to_asm+0x3a>
> >     82f9:  __switch_to_asm+0x39    | int3
> >     82fa:  __switch_to_asm+0x3a    | add    $0x10,%rsp
> >     82fe:  __switch_to_asm+0x3e    | dec    %r12
> >     8301:  __switch_to_asm+0x41    | jne    0x82ee <__switch_to_asm+0x2e>
> >     8303:  __switch_to_asm+0x43    | lfence
> >     8306:  __switch_to_asm+0x46    | movq   $0xffffffffffffffff,%gs:0x0(%rip)        # 0x20b <__x86_call_depth>
>
> That does looks better.
> Although I think there ought to be some indication of the 31 NOP bytes
> at the end of the middle alternative.

I am now compacting the code by removing all trailing NOPs. I should probably
improve that with printing the actual number of NOPs, for example NOP31 (or nop31)

> I'd also decode those callq as 'callq .+6' - not sure what other people think?
> It is rather specific to that code.

This is done by libopcodes. I will need to check if there is an option to display
the branch distance instead of the branch target.

Thanks,

alex.