path: root/init
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authorYonghong Song <yhs@fb.com>2021-04-01 16:27:23 -0700
committerMasahiro Yamada <masahiroy@kernel.org>2021-04-25 05:25:42 +0900
commit1fdd7433a98a2f5511f49ad3f3b82bdd6f77265c (patch)
tree40e453e9523913b43cba5d62f288c5810dfa37cf /init
parent6e74bc4c84546ddbf67ed0f4d45284c9bb153846 (diff)
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>
Diffstat (limited to 'init')
1 files changed, 2 insertions, 0 deletions
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";

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