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

[David Laight <david.laight.linux@gmail.com>]

On Thu, 13 Nov 2025 17:48:49 +0100
Alexandre Chartre <alexandre.chartre@oracle.com> wrote:

> Hi,
> 
> These patches change objtool to disassemble code with libopcodes instead
> of running objdump. You will find below:
> 
> - Changes: list of changes made in this version
> - Overview: overview of the changes
> - Notes: description of some particular behavior
> - Examples: output examples
...
> Example 3 (--disas option): Alternatives with multiple instructions
> -------------------------------------------------------------------
> Alternatives with multiple instructions are displayed side-by-side, with
> an header describing the alternative. The code in the first column is the
> default code of the alternative.
> 
> 
> $ ./tools/objtool/objtool --disas=__switch_to_asm --link vmlinux.o
> __switch_to_asm:
>   82c0:  __switch_to_asm+0x0      push   %rbp                                               
>   82c1:  __switch_to_asm+0x1	  push   %rbx                                               
>   82c2:  __switch_to_asm+0x2	  push   %r12                                               
>   82c4:  __switch_to_asm+0x4	  push   %r13                                               
>   82c6:  __switch_to_asm+0x6	  push   %r14                                               
>   82c8:  __switch_to_asm+0x8	  push   %r15                                               
>   82ca:  __switch_to_asm+0xa	  mov    %rsp,0x1670(%rdi)                                  
>   82d1:  __switch_to_asm+0x11	  mov    0x1670(%rsi),%rsp                                  
>   82d8:  __switch_to_asm+0x18	  mov    0xad8(%rsi),%rbx                                   
>   82df:  __switch_to_asm+0x1f	  mov    %rbx,%gs:0x0(%rip)        # 0x82e7 <__stack_chk_guard>
>   82e7:  __switch_to_asm+0x27	  | <alternative.82e7>                   | !X86_FEATURE_ALWAYS                  | X86_FEATURE_RSB_CTXSW
>   82e7:  __switch_to_asm+0x27	  | jmp    0x8312 <__switch_to_asm+0x52> | NOP1                                 | mov    $0x10,%r12
>   82e8:  __switch_to_asm+0x28	  |                                      | NOP1                                 |
>   82e9:  __switch_to_asm+0x29	  | NOP1                                 | callq  0x82ef <__switch_to_asm+0x2f> |
>   82ea:  __switch_to_asm+0x2a	  | NOP1                                 |                                      |
>   82eb:  __switch_to_asm+0x2b	  | NOP1                                 |                                      |
>   82ec:  __switch_to_asm+0x2c	  | NOP1                                 |                                      |
>   82ed:  __switch_to_asm+0x2d	  | NOP1                                 |                                      |
>   82ee:  __switch_to_asm+0x2e	  | NOP1                                 | int3                                 | callq  0x82f4 <__switch_to_asm+0x34>
>   82ef:  __switch_to_asm+0x2f	  | NOP1                                 | add    $0x8,%rsp                     |
>   82f0:  __switch_to_asm+0x30	  | NOP1                                 |                                      |
>   82f1:  __switch_to_asm+0x31	  | NOP1                                 |                                      |
>   82f2:  __switch_to_asm+0x32	  | NOP1                                 |                                      |
>   82f3:  __switch_to_asm+0x33	  | NOP1                                 | lfence                               | int3
>   82f4:  __switch_to_asm+0x34	  | NOP1                                 |                                      | callq  0x82fa <__switch_to_asm+0x3a>
>   82f5:  __switch_to_asm+0x35	  | NOP1                                 |                                      |
>   82f6:  __switch_to_asm+0x36	  | NOP1                                 |                                      |
>   82f7:  __switch_to_asm+0x37	  | NOP1                                 |                                      |
>   82f8:  __switch_to_asm+0x38	  | NOP1                                 |                                      |
>   82f9:  __switch_to_asm+0x39	  | NOP1                                 |                                      | int3
>   82fa:  __switch_to_asm+0x3a	  | NOP1                                 |                                      | add    $0x10,%rsp
>   82fb:  __switch_to_asm+0x3b	  | NOP1                                 |                                      |
>   82fc:  __switch_to_asm+0x3c	  | NOP1                                 |                                      |
>   82fd:  __switch_to_asm+0x3d	  | NOP1                                 |                                      |
>   82fe:  __switch_to_asm+0x3e	  | NOP1                                 |                                      | dec    %r12
>   82ff:  __switch_to_asm+0x3f	  | NOP1                                 |                                      |
>   8300:  __switch_to_asm+0x40	  | NOP1                                 |                                      |
>   8301:  __switch_to_asm+0x41	  | NOP1                                 |                                      | jne    0x82ee <__switch_to_asm+0x2e>
>   8302:  __switch_to_asm+0x42	  | NOP1                                 |                                      |
>   8303:  __switch_to_asm+0x43	  | NOP1                                 |                                      | lfence
>   8304:  __switch_to_asm+0x44	  | NOP1                                 |                                      |
>   8305:  __switch_to_asm+0x45	  | NOP1                                 |                                      |
>   8306:  __switch_to_asm+0x46	  | NOP1                                 |                                      | movq   $0xffffffffffffffff,%gs:0x0(%rip)        # 0x20b <__x86_call_depth>
>   8307:  __switch_to_asm+0x47	  | NOP1                                 |                                      |
>   8308:  __switch_to_asm+0x48	  | NOP1                                 |                                      |
>   8309:  __switch_to_asm+0x49	  | NOP1                                 |                                      |
>   830a:  __switch_to_asm+0x4a	  | NOP1                                 |                                      |
>   830b:  __switch_to_asm+0x4b	  | NOP1                                 |                                      |
>   830c:  __switch_to_asm+0x4c	  | NOP1                                 |                                      |
>   830d:  __switch_to_asm+0x4d	  | NOP1                                 |                                      |
>   830e:  __switch_to_asm+0x4e	  | NOP1                                 |                                      |
>   830f:  __switch_to_asm+0x4f	  | NOP1                                 |                                      |
>   8310:  __switch_to_asm+0x50	  | NOP1                                 |                                      |
>   8311:  __switch_to_asm+0x51	  | NOP1                                 |                                      |
>   8312:  __switch_to_asm+0x52	    pop    %r15                                               
>   8314:  __switch_to_asm+0x54	    pop    %r14                                               
>   8316:  __switch_to_asm+0x56	    pop    %r13                                               
>   8318:  __switch_to_asm+0x58	    pop    %r12                                               
>   831a:  __switch_to_asm+0x5a	    pop    %rbx                                               
>   831b:  __switch_to_asm+0x5b	    pop    %rbp                                               
>   831c:  __switch_to_asm+0x5c	    jmpq   0x8321 <__switch_to>  

That might be rather easier to read if the alternatives followed each other.
Not all of us want to use a very wide window to look at object files.
(I didn't see any other example like that either.)

Similarly in Ex 5:
 332d4:  early_ioremap_pmd+0x4	    callq  *0x0(%rip)        # 0x332da <pv_ops+0x150> | mov    %cr3,%rax  (!X86_FEATURE_XENPV) | callq  xen_read_cr3  (+X86_FEATURE_ALWAYS)   # <alternative.332d4>
might be more readable flipped to something like:
 332d4:  early_ioremap_pmd+0x4	    callq  *0x0(%rip)        # 0x332da <pv_ops+0x150>
	   !X86_FEATURE_XENPV:          mov    %cr3,%rax
	   +X86_FEATURE_ALWAYS:         callq  xen_read_cr3

	David