On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song <yhs@xxxxxx> wrote: > > 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@xxxxxx/ > [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@xxxxxx/ > > Suggested-by: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx> > Signed-off-by: Yonghong Song <yhs@xxxxxx> LGTM thanks Yonghong! Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > --- > 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 > -- Thanks, ~Nick Desaulniers
