Ard Biesheuvel [off-list ref] writes:

On 21 January 2016 at 07:45, Ard Biesheuvel [off-list ref] wrote:

quoted

On 21 January 2016 at 06:10, Rusty Russell [off-list ref] wrote:

quoted

Ard Biesheuvel [off-list ref] writes:

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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

quoted

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

How annoying.  OK, until ia64 is removed, we'll leave the option.

Thanks,
Rusty.