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

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

On 6/10/25 09:07, Alexandre Chartre wrote:
> 
> 
> 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).
> 

Here is the patch to handle both old and new binutils versions:

8<------------------------------------------------------------------->8
diff --git a/tools/objtool/Makefile b/tools/objtool/Makefile
index 00350fc7c662..91a2858fea14 100644
--- a/tools/objtool/Makefile
+++ b/tools/objtool/Makefile
@@ -7,6 +7,11 @@ srctree := $(patsubst %/,%,$(dir $(CURDIR)))
  srctree := $(patsubst %/,%,$(dir $(srctree)))
  endif
  
+FEATURE_USER = .objtool
+FEATURE_TESTS = disassembler-init-styled
+FEATURE_DISPLAY = disassembler-init-styled
+include $(srctree)/tools/build/Makefile.feature
+
  LIBSUBCMD_DIR = $(srctree)/tools/lib/subcmd/
  ifneq ($(OUTPUT),)
    LIBSUBCMD_OUTPUT = $(abspath $(OUTPUT))/libsubcmd
@@ -40,6 +45,10 @@ OBJTOOL_LDFLAGS := $(LIBELF_LIBS) $(LIBSUBCMD) $(KBUILD_HOSTLDFLAGS) -lopcodes
  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)
  
+ifeq ($(feature-disassembler-init-styled), 1)
+OBJTOOL_CFLAGS += -DDISASM_INIT_STYLED
+endif
+
  # Always want host compilation.
  HOST_OVERRIDES := CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)"
  
8<------------------------------------------------------------------->8

alex.


> 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
>>
>