Messages in this thread | | | From | Ard Biesheuvel <> | Date | Mon, 30 Oct 2017 13:11:23 +0000 | Subject | Re: [PATCH v3] arm64: prevent regressions in compressed kernel image size when upgrading to binutils 2.27 |
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On 30 October 2017 at 13:08, Will Deacon <will.deacon@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?
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