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

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

On 6/9/25 20:31, 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.
>>
>> - 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.
> 
> What do I need for this to build?  It wasn't compiling due to missing
> bfd.h, so I installed binutils-devel (Fedora) and now I get:

That's because of the more recent binutils versions, while I have been using an old
one (2.30) and some functions have changed. But tools/dis-asm-compat.h handles that
when the appropriate #define is set.

Below is a quick fix (tested with binutils 2.41), and I will work on a proper fix
(by using the tools/ features to check the disassembler version).

diff --git a/tools/objtool/Makefile b/tools/objtool/Makefile
index 00350fc7c662..300ea727e454 100644
--- a/tools/objtool/Makefile
+++ b/tools/objtool/Makefile
@@ -36,6 +36,8 @@ WARNINGS := $(EXTRA_WARNINGS) -Wno-switch-default -Wno-switch-enum -Wno-packed -
  OBJTOOL_CFLAGS := -Werror $(WARNINGS) $(KBUILD_HOSTCFLAGS) -g $(INCLUDES) $(LIBELF_FLAGS)
  OBJTOOL_LDFLAGS := $(LIBELF_LIBS) $(LIBSUBCMD) $(KBUILD_HOSTLDFLAGS) -lopcodes
  
+OBJTOOL_CFLAGS += -DDISASM_INIT_STYLED
+
  # Allow old libelf to be used:
  elfshdr := $(shell echo '$(pound)include <libelf.h>' | $(HOSTCC) $(OBJTOOL_CFLAGS) -x c -E - 2>/dev/null | grep elf_getshdr)
  OBJTOOL_CFLAGS += $(if $(elfshdr),,-DLIBELF_USE_DEPRECATED)


alex.


> In file included from disas.c:12:
> /home/jpoimboe/git/linux/tools/include/tools/dis-asm-compat.h:10:6: error: redeclaration of ‘enum disassembler_style’
>     10 | enum disassembler_style {DISASSEMBLER_STYLE_NOT_EMPTY};
>        |      ^~~~~~~~~~~~~~~~~~
> In file included from /home/jpoimboe/git/linux/tools/objtool/include/objtool/arch.h:10,
>                   from disas.c:6:
> /usr/include/dis-asm.h:53:6: note: originally defined here
>     53 | enum disassembler_style
>        |      ^~~~~~~~~~~~~~~~~~
> /home/jpoimboe/git/linux/tools/include/tools/dis-asm-compat.h: In function ‘init_disassemble_info_compat’:
> /home/jpoimboe/git/linux/tools/include/tools/dis-asm-compat.h:50:9: error: too few arguments to function ‘init_disassemble_info’
>     50 |         init_disassemble_info(info, stream,
>        |         ^~~~~~~~~~~~~~~~~~~~~
> /usr/include/dis-asm.h:480:13: note: declared here
>    480 | extern void init_disassemble_info (struct disassemble_info *dinfo, void *stream,
>        |             ^~~~~~~~~~~~~~~~~~~~~
> make[4]: *** [/home/jpoimboe/git/linux/tools/build/Makefile.build:86: /home/jpoimboe/git/linux/tools/objtool/disas.o] Error 1
> make[4]: *** Waiting for unfinished jobs....
> make[3]: *** [Makefile:65: /home/jpoimboe/git/linux/tools/objtool/objtool-in.o] Error 2
> make[2]: *** [Makefile:73: objtool] Error 2
> make[1]: *** [/home/jpoimboe/git/linux/Makefile:1448: tools/objtool] Error 2
> make: *** [Makefile:248: __sub-make] Error 2
>