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

[Will Deacon]

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?

Will