Re: [PATCH kbuild v4] kbuild: add an elfnote for whether vmlinux is built with lto

[Yonghong Song <yhs@fb.com>]

Masahiro and Michal,

Friendly ping. Any comments on this patch?

The addition LTO .notes information emitted by kernel is used by pahole
in the following patch:
    https://lore.kernel.org/bpf/20210401025825.2254746-1-yhs@fb.com/
    (dwarf_loader: check .notes section for lto build info)

Thanks,

Yonghong

On 4/6/21 12:05 AM, Sedat Dilek wrote:
> On Fri, Apr 2, 2021 at 8:07 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@googlegroups.com> wrote:
>>
>> On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song <yhs@fb.com> wrote:
>>>
>>> Currently, clang LTO built vmlinux won't work with pahole.
>>> LTO introduced cross-cu dwarf tag references and broke
>>> current pahole model which handles one cu as a time.
>>> The solution is to merge all cu's as one pahole cu as in [1].
>>> We would like to do this merging only if cross-cu dwarf
>>> references happens. The LTO build mode is a pretty good
>>> indication for that.
>>>
>>> In earlier version of this patch ([2]), clang flag
>>> -grecord-gcc-switches is proposed to add to compilation flags
>>> so pahole could detect "-flto" and then merging cu's.
>>> This will increate the binary size of 1% without LTO though.
>>>
>>> Arnaldo suggested to use a note to indicate the vmlinux
>>> is built with LTO. Such a cheap way to get whether the vmlinux
>>> is built with LTO or not helps pahole but is also useful
>>> for tracing as LTO may inline/delete/demote global functions,
>>> promote static functions, etc.
>>>
>>> So this patch added an elfnote with a new type LINUX_ELFNOTE_LTO_INFO.
>>> The owner of the note is "Linux".
>>>
>>> With gcc 8.4.1 and clang trunk, without LTO, I got
>>>    $ readelf -n vmlinux
>>>    Displaying notes found in: .notes
>>>      Owner                Data size        Description
>>>    ...
>>>      Linux                0x00000004       func
>>>       description data: 00 00 00 00
>>>    ...
>>> With "readelf -x ".notes" vmlinux", I can verify the above "func"
>>> with type code 0x101.
>>>
>>> With clang thin-LTO, I got the same as above except the following:
>>>       description data: 01 00 00 00
>>> which indicates the vmlinux is built with LTO.
>>>
>>>    [1] https://lore.kernel.org/bpf/20210325065316.3121287-1-yhs@fb.com/
>>>    [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@fb.com/
>>>
>>> Suggested-by: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>
>> LGTM thanks Yonghong!
>> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
>>
> 
> Thanks for the patch.
> 
> Feel free to add:
> 
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM/Clang v12.0.0-rc4 (x86-64)
> 
> As a note for the pahole side:
> Recent patches require an adaptation of the define and its comment.
> 
> 1. LINUX_ELFNOTE_BUILD_LTO -> LINUX_ELFNOTE_LTO_INFO
> 2. include/linux/elfnote.h -> include/linux/elfnote-lto.h
> 
> - Sedat -
> 
>>> ---
>>>   include/linux/elfnote-lto.h | 14 ++++++++++++++
>>>   init/version.c              |  2 ++
>>>   scripts/mod/modpost.c       |  2 ++
>>>   3 files changed, 18 insertions(+)
>>>   create mode 100644 include/linux/elfnote-lto.h
>>>
>>> Changelogs:
>>>    v3 -> v4:
>>>      . put new lto note in its own header file similar to
>>>        build-salt.h. (Nick)
>>>    v2 -> v3:
>>>      . abandoned the approach of adding -grecord-gcc-switches,
>>>        instead create a note to indicate whether it is a lto build
>>>        or not. The note definition is in compiler.h. (Arnaldo)
>>>    v1 -> v2:
>>>      . limited to add -grecord-gcc-switches for LTO_CLANG
>>>        instead of all clang build
>>>
>>> diff --git a/include/linux/elfnote-lto.h b/include/linux/elfnote-lto.h
>>> new file mode 100644
>>> index 000000000000..d4635a3ecc4f
>>> --- /dev/null
>>> +++ b/include/linux/elfnote-lto.h
>>> @@ -0,0 +1,14 @@
>>> +#ifndef __ELFNOTE_LTO_H
>>> +#define __ELFNOTE_LTO_H
>>> +
>>> +#include <linux/elfnote.h>
>>> +
>>> +#define LINUX_ELFNOTE_LTO_INFO 0x101
>>> +
>>> +#ifdef CONFIG_LTO
>>> +#define BUILD_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_LTO_INFO, 1)
>>> +#else
>>> +#define BUILD_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_LTO_INFO, 0)
>>> +#endif
>>> +
>>> +#endif /* __ELFNOTE_LTO_H */
>>> diff --git a/init/version.c b/init/version.c
>>> index 92afc782b043..1a356f5493e8 100644
>>> --- a/init/version.c
>>> +++ b/init/version.c
>>> @@ -9,6 +9,7 @@
>>>
>>>   #include <generated/compile.h>
>>>   #include <linux/build-salt.h>
>>> +#include <linux/elfnote-lto.h>
>>>   #include <linux/export.h>
>>>   #include <linux/uts.h>
>>>   #include <linux/utsname.h>
>>> @@ -45,3 +46,4 @@ const char linux_proc_banner[] =
>>>          " (" LINUX_COMPILER ") %s\n";
>>>
>>>   BUILD_SALT;
>>> +BUILD_LTO_INFO;
>>> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
>>> index 24725e50c7b4..98fb2bb024db 100644
>>> --- a/scripts/mod/modpost.c
>>> +++ b/scripts/mod/modpost.c
>>> @@ -2191,10 +2191,12 @@ static void add_header(struct buffer *b, struct module *mod)
>>>           */
>>>          buf_printf(b, "#define INCLUDE_VERMAGIC\n");
>>>          buf_printf(b, "#include <linux/build-salt.h>\n");
>>> +       buf_printf(b, "#include <linux/elfnote-lto.h>\n");
>>>          buf_printf(b, "#include <linux/vermagic.h>\n");
>>>          buf_printf(b, "#include <linux/compiler.h>\n");
>>>          buf_printf(b, "\n");
>>>          buf_printf(b, "BUILD_SALT;\n");
>>> +       buf_printf(b, "BUILD_LTO_INFO;\n");
>>>          buf_printf(b, "\n");
>>>          buf_printf(b, "MODULE_INFO(vermagic, VERMAGIC_STRING);\n");
>>>          buf_printf(b, "MODULE_INFO(name, KBUILD_MODNAME);\n");
>>> --
>>> 2.30.2
>>>
>>
>>
>> --
>> Thanks,
>> ~Nick Desaulniers
>>
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