[PATCH v3] arm64: prevent regressions in compressed kernel image size when upgrading to binutils 2.27

[Ard Biesheuvel]

On 30 October 2017 at 13:12, Will Deacon <will.deacon at arm.com> wrote:
> On Mon, Oct 30, 2017 at 01:11:23PM +0000, Ard Biesheuvel wrote:
>> On 30 October 2017 at 13:08, Will Deacon <will.deacon at arm.com> wrote:
>> > On Fri, Oct 27, 2017 at 09:33:41AM -0700, Nick Desaulniers wrote:
>> >> Upon upgrading to binutils 2.27, we found that our lz4 and gzip
>> >> 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" make
>> >> 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."
>> >
>> > Do you have any numbers booting an uncompressed kernel Image without ASLR
>> > to see if skipping the relocs makes a measurable difference there?
>> >
>>
>> Do you mean built with ASLR support but executing at the offset it was
>> linked at?
>
> Yeah, sorry for being vague. Basically, the case where the relocs have all
> been resolved statically. In other words: what do we lose by disabling this
> optimisation?
>

The code does not deal with that at all, currently: given that this is
new behavior in 2.27, the relocs are processed unconditionally,
regardless of whether the image is loaded at its default base or not.