[PATCH kbuild v3 2/2] kbuild: add an elfnote with type BUILD_COMPILER_LTO_INFO

[Yonghong Song <yhs@xxxxxx>]

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 type BUILD_COMPILER_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/

Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
 include/linux/compiler.h | 8 ++++++++
 include/linux/elfnote.h  | 1 +
 init/version.c           | 2 ++
 scripts/mod/modpost.c    | 1 +
 4 files changed, 12 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index df5b405e6305..b92930877277 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -245,6 +245,14 @@ static inline void *offset_to_ptr(const int *off)
  */
 #define prevent_tail_call_optimization()	mb()
 
+#include <linux/elfnote.h>
+
+#ifdef CONFIG_LTO
+#define BUILD_COMPILER_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_BUILD_LTO, 1)
+#else
+#define BUILD_COMPILER_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_BUILD_LTO, 0)
+#endif
+
 #include <asm/rwonce.h>
 
 #endif /* __LINUX_COMPILER_H */
diff --git a/include/linux/elfnote.h b/include/linux/elfnote.h
index 04af7ac40b1a..f5ec2b50ab7d 100644
--- a/include/linux/elfnote.h
+++ b/include/linux/elfnote.h
@@ -100,5 +100,6 @@
  * The types for "Linux" owned notes.
  */
 #define LINUX_ELFNOTE_BUILD_SALT	0x100
+#define LINUX_ELFNOTE_BUILD_LTO		0x101
 
 #endif /* _LINUX_ELFNOTE_H */
diff --git a/init/version.c b/init/version.c
index 92afc782b043..a4f74b06fe78 100644
--- a/init/version.c
+++ b/init/version.c
@@ -9,6 +9,7 @@
 
 #include <generated/compile.h>
 #include <linux/build-salt.h>
+#include <linux/compiler.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_COMPILER_LTO_INFO;
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 24725e50c7b4..713c0d5d5525 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -2195,6 +2195,7 @@ static void add_header(struct buffer *b, struct module *mod)
 	buf_printf(b, "#include <linux/compiler.h>\n");
 	buf_printf(b, "\n");
 	buf_printf(b, "BUILD_SALT;\n");
+	buf_printf(b, "BUILD_COMPILER_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