Re: aarch64 prelink issue

[Marcus Shawcroft <marcus dot shawcroft at linaro dot org>]

On 26 June 2013 13:42, Mark Salter <msalter@redhat.com> wrote:
> On Mon, 2013-06-24 at 10:18 -0700, Richard Henderson wrote:
>> Try the x86_64 solution:
>>
>>   /* This produces an IP-relative reloc which is resolved at link time. */
>>   extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
>>   return _GLOBAL_OFFSET_TABLE_[0];
>>
>> with the extra hidden attribute, this should result in an ADR(P)
>> reference to the _G_O_T_.
>>
>
> This isn't working for aarch64. It doesn't use a PC-relative reference
> and it tries reading from the GOT at the link time absolute address.


It looks to me that Richard's proposal gives the correct sequence in
elf_machine_dynamic():

        adrp    x0, _GLOBAL_OFFSET_TABLE_
        ldr     x0, [x0,#:lo12:_GLOBAL_OFFSET_TABLE_]
        ret

... but, this exposes an issue in the static linker:

Currently the static linker produces:

.got
<-_GLOBAL_OFFSET_TABLE_
...
.gotplt[0] &_DYNAMIC                             <--DT_PLTGOT
.gotplt[1] reserved for ld.so (&linkmap)
.gotplt[2] reserved fpr ld.so (resolver)
...

This layout is broken wrt the placement of &_DYNAMIC.  The correct
layout should be:

.got[0] &_DYNAMIC                                <-_GLOBAL_OFFSET_TABLE_
...
.gotplt[0] reserved for ld.so (&linkmap)       <--DT_PLTGOT
.gotplt[1] reserved fpr ld.so (resolver)
...

Hence when we use Richard's suggestion we end up picking junk out of .got[0]

I'm still looking at this, but at the moment I think we can fix the
static linker to emit:

.got[0] &_DYNAMIC                                <-_GLOBAL_OFFSET_TABLE_
...
.gotplt[0] reserved for ld.so (&linkmap)       <--DT_PLTGOT
.gotplt[1] reserved for ld.so (resolver)
.gotplt[2] reserved

With this change, I believe that we do not introduce an
incompatibility between existing binaries / .so's and the modified
ld.so because the code within elf_machine_dynamic() is only
instantiated within ld.so and is only used to inspect ld.so's own
image.

The .gotplt[2] entry remains reserved in the new layout to ensure that
an unmodified ld.so can load a binary built using a modified static ld
without clobbering  the first real PLT entry.

I'd really appreciate some feedback on the sanity of this proposal.

Thanks
/Marcus