Re: RFC: Audit external function called indirectly via GOT

[Florian Weimer <fw@deneb.enyo.de>]

* Carlos O'Donell:

> On 03/21/2018 03:04 PM, Florian Weimer wrote:
>> * H. J. Lu:
>> 
>>>> Could we ship a template for the PLT entries in ld.so instead?  And if
>>>> needed, map it from the file together with an address array, like this?
>>>
>>> This won't work since linker needs to know exactly PLT layout to generate
>>> JUMP_SLOT relocations for LD_AUDIT.
>> 
>> Why would we need JUMP_SLOT relocations?  Couldn't we install suitable
>> interceptors for GLOB_DAT relocations instead, as long as they resolve
>> to external function symbols?
>
> I think your suggestion might work, but why alter the existing
> behaviour which users expect and is documented in countless linker
> text books?

If you have references, please add them to the glibc implementation or
the wiki.  It would certainly help those who are trying to work on the
code.

My understanding is that the whole thing is quite underdocumented.
For LD_AUDIT in particular, we only have the Solaris documentation,
and that's for an independent implementation.

> Existing tooling to process such relocations and entries could
> remain unchanged and we would continue to support LD_AUDIT.

My understanding is that H.J.'s proposal requires changes when running
in non-audit mode.  It certainly requires relinking all binaries,
perhaps even with special flags.

Using ld.so-generated thunks for all GLOB_DAT function symbol
relocations would happen in audit mode only and should work with
existing binaries which were built with -Wl,-z,now.