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

[Will Deacon]

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?

Will