[PATCH 0/4] support for text-relative kallsyms table

From: Ard Biesheuvel
Date: Wed Jan 20 2016 - 04:06:08 EST

This implements text-relative kallsyms address tables. This was developed
as part of my series to implement KASLR/CONFIG_RELOCATABLE for arm64, but
I think it may be beneficial to other architectures as well, so I am
presenting it as a separate series.

The idea is that on 64-bit builds, it is rather wasteful to use absolute
addressing for kernel symbols since they are all within a couple of MBs
of each other. On top of that, the absolute addressing implies that, when
the kernel is relocated at runtime, each address in the table needs to be
fixed up individually.

Since all section-relative addresses are already emitted relative to _text,
it is quite straight-forward to record only the offset, and add the absolute
address of _text at runtime when referring to the address table.

The reduction ranges from around 250 KB uncompressed vmlinux size and 10 KB
compressed size (s390) to 3 MB/500 KB for ppc64 (although, in the latter case,
the reduction in uncompressed size is primarily __init data)

Kees Cook was so kind to test these against x86_64, and confirmed that KASLR
still operates as expected.

Ard Biesheuvel (4):
kallsyms: add support for relative offsets in kallsyms address table
powerpc: enable text relative kallsyms for ppc64
s390: enable text relative kallsyms for 64-bit targets
x86_64: enable text relative kallsyms for 64-bit targets

arch/powerpc/Kconfig | 1 +
arch/s390/Kconfig | 1 +
arch/x86/Kconfig | 1 +
init/Kconfig | 14 ++++++++
kernel/kallsyms.c | 35 +++++++++++++-----
scripts/kallsyms.c | 38 +++++++++++++++++---
scripts/link-vmlinux.sh | 4 +++
scripts/namespace.pl | 1 +
8 files changed, 82 insertions(+), 13 deletions(-)