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

From: Ard Biesheuvel
Date: Thu Jan 21 2016 - 01:45:57 EST

On 21 January 2016 at 06:10, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
> Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> writes:
>> 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.
> Nice work!


> AFAICT this should work for every arch, as long as they start with _text
> (esp: data and init must be > _text). In addition, it's not harmful on
> 32 bit archs.
> IOW, I'd like to turn it on for everyone and discard some code. But
> it's easier to roll in like you've done first.
> Should we enable it by default for every arch for now, and see what
> happens?

As you say, this only works if every symbol >= _text, which is
obviously not the case per the conditional in scripts/kallsyms.c,
which emits _text + n or _text - n depending on whether the symbol
precedes or follows _text. The git log tells me for which arch this
was originally implemented, but it does not tell me which other archs
have come to rely on it in the meantime.

On top of that, ia64 fails to build with this option, since it has
some whitelisted absolute symbols that look suspiciously like they
could be emitted as _text relative (and it does not even matter in the
absence of CONFIG_RELOCATABLE on ia64, afaict) but I don't know
whether we can just override their types as T, since it would also
change the type in the contents of /proc/kallsyms. So some guidance
would be appreciated here.

So I agree that it would be preferred to have a single code path, but
I would need some help validating it on architectures I don't have
access to.


>> 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/ | 4 +++
>> scripts/ | 1 +
>> 8 files changed, 82 insertions(+), 13 deletions(-)
>> --
>> 2.5.0