Re: RFC: Audit external function called indirectly via GOT

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Mar 27, 2018 at 6:39 PM, Cary Coutant <ccoutant@gmail.com> wrote:
> I haven't seen a response yet to the primary point I was trying to make:
>
>> My suggestion was that the GOT entry could be statically initialized
>> by the linker to point to the provisional PLT entry, rather than
>> forcing the dynamic loader to go through all this messy computation.
>> If auditing is not enabled, it would process the GLOB_DAT relocation
>> normally, and set the GOT entry to point to the actual function,
>> bypassing the provisional PLT and PLTGOT entries completely. If
>> auditing is enabled, it could simply ignore the GLOB_DAT relocation
>> (or, if the binary is PIE, it could process it as a RELATIVE
>> relocation), and the -fno-plt calls will end up jumping to the
>> provisional PLT entry.
>>
>> (This is already how we handle the PLTGOT entries: the linker
>> statically initializes the entries to point to part (b)* of the PLT
>> entry, while putting JUMP_SLOT relocations for those entries into the
>> JMPREL table.)
>>
>> I think if you do that, none of these extra dynamic table entries will
>> be needed, ...

Your scheme is very similar to mine.   Both generate one GLOB_DAT
and one JUMP_SLOT relocation for the same function symbol.  But
only one of them should be used at run-time.  Your scheme may be
simpler when LD_AUDIT is used since you don't need to update GOT
slot.  But you still need to decide if a GLOB_DAT relocation should be
skipped for LD_AUDIT.

> ... or this secondary point:
>
>> ... except for the IGNORE_JMPREL flag that indicates there are
>> no JMPREL slots other than those for the provisional PLT entries. How
>> useful is that flag? If the final program has even one external call
>> that was *not* compiled with -fno-plt, you won't be able to set it.
>> Would it be better to partition the JMPREL and PLT tables into
>> "regular" and "provisional" entries? That would take just a single new
>> DT_PROVISIONAL_JMPREL entry to tell the dynamic loader where the
>> JMPREL entries for the provisional PLT entries begin; it can ignore
>> everything past that point when auditing is turned off.
>

This sounds a good idea..  I will take a look.

-- 
H.J.