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

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On Fri, Apr 2, 2021 at 8:07 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song <yhs@xxxxxx> 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@xxxxxx/
> >   [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@xxxxxx/
> >
> > Suggested-by: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
> > Signed-off-by: Yonghong Song <yhs@xxxxxx>
>
> LGTM thanks Yonghong!
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>

Thanks for the patch.

Feel free to add:

Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx> # 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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