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

[Alexandre Chartre <alexandre.chartre@oracle.com>]

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.