Re: How to detect symbol interposition?

[Florian Weimer <fweimer at redhat dot com>]

On 06/28/2016 07:00 PM, Carlos O'Donell wrote:
> On 06/28/2016 10:12 AM, Florian Weimer wrote:
> > I need a way to detect symbol interposition from within libc.
>
> I assume you mean you will be writing code that lives in libc and needs
> to determine that ld.so has interposed a libc symbol with a symbol from
> another library?

Yes.

> > It seems that this is not directly distinguishable from interposition
> > based on addresses alone:
>
> [snip]
>
> > If I have a hidden alias of malloc within libc (say, __libc_malloc),
> > so that I can get the original address, it will be not equal to
> > malloc because malloc in the entire process will point to the PLT
> > stub.  The PLT stub will call malloc, of course, but you cannot tell
> > this from its address.
>
> And this changes depending on the architecture details.

Are their architectures where the interposition is made explicit and 
shows up in .dynsym?

> The only solution that I know of is to use the LD_AUDIT mechanisms to
> catch the PLT resolution and observe the final address of the resolution.

The logic has to go into libc.so proper, so LD_AUDIT is not an option.

Unless there are architectures where the static linker constructs PLT 
stubs with special relocations (RTLD_NEXT-style) and then interposes the 
PLT stub via .dynsym, surely ld.so has access to this information, at 
least at a conceptual level?

Thanks,
Florian