On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song <
y...@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...@fb.com/
> [2]
https://lore.kernel.org/bpf/20210331001623...@fb.com/
>
> Suggested-by: Arnaldo Carvalho de Melo <
arnald...@gmail.com>
> Signed-off-by: Yonghong Song <
y...@fb.com>
LGTM thanks Yonghong!
Reviewed-by: Nick Desaulniers <
ndesau...@google.com>
> ---
> 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