Re: [RFC 00/13] objtool: Function validation tracing

[Alexandre Chartre]

On 6/6/25 17:58, Josh Poimboeuf wrote:
> On Fri, Jun 06, 2025 at 05:34:27PM +0200, Alexandre Chartre wrote:
> > Hi,
> >
> > This RFC provides two changes to objtool.
> >
> > - Disassemble code with libopcodes instead of running objdump
> >
> >    objtool executes the objdump command to disassemble code. In particular,
> >    if objtool fails to validate a function then it will use objdump to
> >    disassemble the entire file which is not very helpful when processing
> >    a large file (like vmlinux.o).
> >
> >    Using libopcodes provides more control about the disassembly scope and
> >    output, and it is possible to disassemble a single instruction or
> >    a single function. Now when objtool fails to validate a function it
> >    will disassemble that single function instead of disassembling the
> >    entire file.
>
> Ah, nice to get rid of that awful objdump hack.
>
> > - Add the --trace <function> option to trace function validation
> >
> >    Figuring out why a function validation has failed can be difficult because
> >    objtool checks all code flows (including alternatives) and maintains
> >    instructions states (in particular call frame information).
> >
> >    The trace option allows to follow the function validation done by objtool
> >    instruction per instruction, see what objtool is doing and get function
> >    validation information. An output example is shown below.
>
> This is pretty freaking awesome!
>
> I assume we could eventually build on this work to have an "objtool
> disas" subcommand, which would basically be an improved "objdump -d"
> which annotates alternatives and other runtime patching.

Yes, good idea. I will look at it.

alex.