linux-stable/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 */
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> 2021-04-01 23:27:23 +00:00			`#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 */`