Re: [PATCH 09/13] kbuild: do not create built-in.a.symversions or lib.a.symversions

Masahiro Yamada
2021-08-28T11:58:55Z

On Thu, Aug 19, 2021 at 3:41 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Aug 19, 2021 at 09:57:40AM +0900, Masahiro Yamada wrote:
> > Merge all *.o.symversions in scripts/link-vmlinux.sh instead of
> > merging them in the unit of built-in.a or lib.a.
> >
> > This is a preparation for further code cleanups.
>
> Looks good, though I wonder about this becoming serialized during the
> link phase rather than doing the work per-target. I mean, it always had
> to collect them all during the link phase (with "cat"), but before it
> wasn't running $(AR) serially to do it.
>
> I'll ponder how this might be made a little more parallel. But for now:
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> -Kees
>

I measured the cost of merging all the *.symversions.

For a typical use-case
(x86_64 defconfig + CONFIG_LTO_CLANG_THIN + CONFIG_MODVERSIONS),
my shell script took about 0.40 msec
for merging all the individual *.symversions files.

Most of the cost of 0.40 msec came from the 'cat' command.
The 'cat' command is kind of slow when you concatenate
a large number of files.

I implemented the equivalent functionality with a perl script,
which worked in only 0.04 msec.

I think 0.04 msec should be acceptable cost because
this commit eliminates all the intermediate built-in.a.symversions
and lib.a.symversions, saving disk space.

I also tried allyesconfig + CONFIG_LTO_CLANG_THIN + CONFIG_MODVERSIONS
(the heaviest load), but the result is similar.

This is because most of EXPORT_SYMBOL's come from the core part of
the kernel, and enabling drivers as built-in does not give much impact, I think.

So, I will plan to submit v2 with perl implementation.

The detailed test code is as follows:

masahiro@oscar:~/workspace/linux-kbuild$ cat scripts/merge-symvers.sh
#!/bin/sh

append_symversion()
{
if [ -f ${1}.symversions ]; then
cat ${1}.symversions >> .tmp_symversions.lds
fi
}

# If CONFIG_LTO_CLANG is selected, collect generated symbol versions into
# .tmp_symversions.lds
gen_symversions()
{
rm -f .tmp_symversions.lds

for a in ${KBUILD_VMLINUX_OBJS} ${KBUILD_VMLINUX_LIBS}; do
case $a in
*.a)
for o in $(${AR} t ${a}); do
append_symversion ${o}
done
;;
*)
append_symversion ${a}
;;
esac
done
}

gen_symversions

masahiro@oscar:~/workspace/linux-kbuild$ cat scripts/merge-symvers.pl
#!/usr/bin/env perl
# SPDX-License-Identifier: GPL-2.0-only

use autodie;
use strict;
use warnings;
use Getopt::Long 'GetOptions';

my $ar;
my $output;

GetOptions(
'a|ar=s' => \$ar,
'o|output=s' => \$output,
);

# Collect all objects
my @objects;

foreach (@ARGV) {
if (/\.o$/) {
# Some objects (head-y) are linked to vmlinux directly.
push(@objects, $_);
} elsif (/\.a$/) {
# Most of built-in objects are contained in built-in.a or lib.a.
# Use 'ar -t' to get the list of the contained objects.
$_ = `$ar -t $_`;
push(@objects, split(/\n/));
} else {
die "$_: unknown file type\n";
}
}

open(my $out_fh, '>', "$output");

foreach (@objects) {
# The symbol CRCs for foo/bar/baz.o is output to
foo/bar/baz.o.symversions
s/(.*)/$1.symversions/;

if (! -e $_) {
# .symversions does not exist if the object does not contain
# EXPORT_SYMBOL at all. Skip it.
next;
}

open(my $in_fh, '<', "$_");
# Concatenate all the existing *.symversions files.
print $out_fh do { local $/; <$in_fh> };
close $in_fh;
}

close $out_fh;

masahiro@oscar:~/workspace/linux-kbuild$ git diff
diff --git a/Makefile b/Makefile
index 3ef3685b7e4a..5b8fe617769a 100644
--- a/Makefile
+++ b/Makefile
@@ -1175,6 +1175,14 @@ vmlinux: scripts/link-vmlinux.sh
autoksyms_recursive $(vmlinux-deps) FORCE

targets := vmlinux

+PHONY += merge-symvers-by-shell merge-symvers-by-perl
+
+merge-symvers-by-shell:
+ time sh scripts/merge-symvers.sh
+
+merge-symvers-by-perl:
+ time perl scripts/merge-symvers.pl -a $(AR) -o
.tmp_symversions.lds $(KBUILD_VMLINUX_OBJS) $(KBUILD_VMLINUX_LIBS)
+
# The actual objects are generated when descending,
# make sure no implicit rule kicks in
$(sort $(vmlinux-deps) $(subdir-modorder)): descend ;

masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 defconfig
*** Default configuration is based on 'x86_64_defconfig'
#
# configuration written to .config
#
masahiro@oscar:~/workspace/linux-kbuild$ ./scripts/config -d
LTO_NONE -e LTO_CLANG_THIN -e MODVERSIONS
masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 -s -j24
arch/x86/entry/vdso/Makefile:135: FORCE prerequisite is missing
arch/x86/entry/vdso/Makefile:135: FORCE prerequisite is missing

masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 merge-symvers-by-shell
time sh scripts/merge-symvers.sh
0.40user 0.08system 0:00.47elapsed 101%CPU (0avgtext+0avgdata 7156maxresident)k
0inputs+896outputs (0major+90678minor)pagefaults 0swaps

masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 merge-symvers-by-perl
time perl scripts/merge-symvers.pl -a llvm-ar -o .tmp_symversions.lds
arch/x86/kernel/head_64.o arch/x86/kernel/head64.o
arch/x86/kernel/ebda.o arch/x86/kernel/platform-quirks.o
init/built-in.a usr/built-in.a arch/x86/built-in.a kernel/built-in.a
certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a
security/built-in.a crypto/built-in.a block/built-in.a lib/built-in.a
arch/x86/lib/built-in.a lib/lib.a arch/x86/lib/lib.a
drivers/built-in.a sound/built-in.a net/built-in.a virt/built-in.a
arch/x86/pci/built-in.a arch/x86/power/built-in.a
0.04user 0.02system 0:00.06elapsed 101%CPU (0avgtext+0avgdata 10100maxresident)k
0inputs+896outputs (0major+8590minor)pagefaults 0swaps

masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 allyesconfig
#
# configuration written to .config
#
masahiro@oscar:~/workspace/linux-kbuild$ ./scripts/config -d
LTO_NONE -e LTO_CLANG_THIN
masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 -s -j24
[ snip ]

masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 merge-symvers-by-shell
time sh scripts/merge-symvers.sh
0.41user 0.09system 0:00.50elapsed 101%CPU (0avgtext+0avgdata 7172maxresident)k
0inputs+896outputs (0major+91425minor)pagefaults 0swaps

masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 merge-symvers-by-perl
time perl scripts/merge-symvers.pl -a llvm-ar -o .tmp_symversions.lds
arch/x86/kernel/head_64.o arch/x86/kernel/head64.o
arch/x86/kernel/ebda.o arch/x86/kernel/platform-quirks.o
init/built-in.a usr/built-in.a arch/x86/built-in.a kernel/built-in.a
certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a
security/built-in.a crypto/built-in.a block/built-in.a lib/built-in.a
arch/x86/lib/built-in.a lib/lib.a arch/x86/lib/lib.a
drivers/built-in.a sound/built-in.a samples/built-in.a net/built-in.a
virt/built-in.a arch/x86/pci/built-in.a arch/x86/power/built-in.a
arch/x86/video/built-in.a
0.08user 0.02system 0:00.11elapsed 100%CPU (0avgtext+0avgdata 15984maxresident)k
0inputs+896outputs (0major+11506minor)pagefaults 0swaps

--
Best Regards
Masahiro Yamada