kbuild: add an elfnote for whether vmlinux is built with lto

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>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM/Clang v12.0.0-rc4 (x86-64)
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

include/linux/elfnote-lto.h

+#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 */

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;

scripts/mod/modpost.c

@@ -2193,10 +2193,12 @@ 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");