[PATCH v2] arm64: prevent regressions in compressed kernel image size when upgrading to binutils 2.27

[Nick Desaulniers]

Upon upgrading to binutils 2.27, we found that our lz4 compressed kernel
images were significantly larger, resulting is 10ms boot time regressions.

As noted by Rahul:
"aarch64 binaries uses RELA relocations, where each relocation entry
includes an addend value. This is similar to x86_64.  On x86_64, the
addend values are also stored at the relocation offset for relative
relocations. This is an optimization: in the case where code does not
need to be relocated, the loader can simply skip processing relative
relocations.  In binutils-2.25, both bfd and gold linkers did this for
x86_64, but only the gold linker did this for aarch64.  The kernel build
here is using the bfd linker, which stored zeroes at the relocation
offsets for relative relocations.  Since a set of zeroes compresses
better than a set of non-zero addend values, this behavior was resulting
in much better lz4 compression.

The bfd linker in binutils-2.27 is now storing the actual addend values
at the relocation offsets. The behavior is now consistent with what it
does for x86_64 and what gold linker does for both architectures.  The
change happened in this upstream commit:
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=1f56df9d0d5ad89806c24e71f296576d82344613
Since a bunch of zeroes got replaced by non-zero addend values, we see
the side effect of lz4 compressed image being a bit bigger.

To get the old behavior from the bfd linker, "--no-apply-dynamic-relocs"
flag can be used:
$ LDFLAGS="--no-apply-dynamic-relocs" ./build/build.sh
With this flag, the compressed image size is back to what it was with
binutils-2.25.

If the kernel is using ASLR, there aren't additional runtime costs to
--no-apply-dynamic-relocs, as the relocations will need to be applied
again anyway after the kernel is relocated to a random address.

If the kernel is not using ASLR, then presumably the current default
behavior of the linker is better. Since the static linker performed the
dynamic relocs, and the kernel is not moved to a different address at
load time, it can skip applying the relocations all over again."

Some measurements:

$ ld -v
GNU ld (binutils-2.25-f3d35cf6) 2.25.51.20141117
                    ^
$ ls -l vmlinux
-rwxr-x--- 1 ndesaulniers eng 300652760 Oct 26 11:57 vmlinux
$ ls -l Image.lz4-dtb
-rw-r----- 1 ndesaulniers eng 16932627 Oct 26 11:57 Image.lz4-dtb

$ ld -v
GNU ld (binutils-2.27-53dd00a1) 2.27.0.20170315
                    ^
pre patch:
$ ls -l vmlinux
-rwxr-x--- 1 ndesaulniers eng 300376208 Oct 26 11:43 vmlinux
$ ls -l Image.lz4-dtb
-rw-r----- 1 ndesaulniers eng 18159474 Oct 26 11:43 Image.lz4-dtb

post patch:
$ ls -l vmlinux
-rwxr-x--- 1 ndesaulniers eng 300376208 Oct 26 12:06 vmlinux
$ ls -l Image.lz4-dtb
-rw-r----- 1 ndesaulniers eng 16932466 Oct 26 12:06 Image.lz4-dtb

We've also verified gzip improves by 0.69 MB.

Any compression scheme should be able to get better results from the
longer runs of zeros, not just GZIP and LZ4.

10ms boot time savings isn't anything to get excited about, but users of
arm64+compression+bfd-2.27 should not have to pay a boot time penalty for no
runtime improvement.

Reported-by: Gopinath Elanchezhian <gelanchezhian@xxxxxxxxxx>
Reported-by: Sindhuri Pentyala <spentyala@xxxxxxxxxx>
Reported-by: Wei Wang <wvw@xxxxxxxxxx>
Suggested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Suggested-by: Rahul Chaudhry <rahulchaudhry@xxxxxxxxxx>
Suggested-by: Siqi Lin <siqilin@xxxxxxxxxx>
Suggested-by: Stephen Hines <srhines@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Changes since v1:
* dropped LZ4 only outer conditional, per Ard.
* changed inner conditional for all RELOCATABLE, not just
  RANDOMIZE_BASE, per Ard.
* updated commit message with findings for gzip.
* added Ard to suggested by line in commit message.

 arch/arm64/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 939b310913cf..9f47d4276a21 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -18,6 +18,10 @@ ifneq ($(CONFIG_RELOCATABLE),)
 LDFLAGS_vmlinux		+= -pie -shared -Bsymbolic
 endif
 
+ifeq ($(CONFIG_RELOCATABLE), y)
+LDFLAGS_vmlinux		+= $(call ld-option, --no-apply-dynamic-relocs)
+endif
+
 ifeq ($(CONFIG_ARM64_ERRATUM_843419),y)
   ifeq ($(call ld-option, --fix-cortex-a53-843419),)
 $(warning ld does not support --fix-cortex-a53-843419; kernel may be susceptible to erratum)
-- 
2.15.0.rc2.357.g7e34df9404-goog