RFC: Audit external function called indirectly via GOT

[Florian Weimer]

On 03/28/2018 08:41 PM, H.J. Lu wrote:

>> I don't see why it would need JUMP_SLOT relocations if it simply
>> auto-generates PLT stub equivalents and installs them in GLOB_DAT
>> relocations.
> 
> My understanding is that LD_AUDIT is based on JUMP_SLOT relocations.

The current implementation on x86, yes, to avoid the need for run-time 
code  generation.  But that doesn't mean it's the best way forward. 
Certainly not if the toolchain no longer generates JUMP_SLOT relocations 
(like it did at some point in the past).

>> Anyway, going back to the larger question what we need here.
>>
>> I used  this as a test case for audit support with BIND_NOW:
>>
>> latrace /bin/true --help
>>
>> Most of Fedora is compiled with BIND_NOW.  Fedora 26 does not print latrace
>> messages (the problem I mentioned earlier), Fedora 27 works (yay), Fedora 28
>> crashes (meh).
>>
>> So depending on which side Fedora 28+ falls, I think your approach might be
>> viable.  I expect that a future binutils version would do this by default,
>> and beyond the additional dynamic section tags, new PLT stubs would only be
>> created for no-plt functions because current binutils is supposed to
>> generate PLT entries again (after they went missing for -z now binaries for
>> some time).

> -fno-plt is a compiler option, not a linker option.  Linker generates PLT for
> PLT32 relocations to external functions.

That doesn't change the point—if future binutils versions elide 
JUMP_SLOT relocations, then your proposal is not going to solve our 
issue.  As I wrote, I cannot verify the current state because the 
toolchain regressed again.

Thanks,
Florian