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

[Ard Biesheuvel]

On 21 January 2016 at 07:45, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> 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!
>>
>
> Thanks
>
>> 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.
>

Digging a little deeper, it appears that it would be non-trivial to
port this to ia64:

...
a000000000040720 A __kernel_syscall_via_break
a000000000040740 A __kernel_sigtramp
a000000000040a00 A __kernel_syscall_via_epc
a000000100000000 T ia64_ivt
a000000100000000 T __start_ivt_text
a000000100000000 T _stext
a000000100000000 T _text
...

The top three symbols are the absolute symbols that are explicitly
whitelisted by scripts/kallsyms.c, and they are too far from 0 and too
far from _text to be representable in 32 bits



> 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.
>
> Thanks,
> Ard.
>
>
>>> 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(-)
>>>
>>> --
>>> 2.5.0