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

[Yonghong Song <yhs@fb.com>]

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>
---
 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