LD_PRELOAD doesn't work with dynamic object built with `-static-pie`

LD_PRELOAD doesn't work with dynamic object built with `-static-pie`

[Baojun Wang]

Hi libc,

I tried to build a shared object with `-static-pie`, and then used it as
`LD_PRELOAD`, but it didn't work, do you know why it failed? I put more
details in a gist:
https://gist.github.com/wangbj/3fb7356286ee38755005cbde26ca84ae

The reason I'm using this is to hope to create a shared object (.so)
without dependencies on any other libraries, but the shared object itself
can use library such as glibc.

Thanks
Baojun

Re: LD_PRELOAD doesn't work with dynamic object built with `-static-pie`

[Florian Weimer]

* Baojun Wang:

> I tried to build a shared object with `-static-pie`, and then used it as
> `LD_PRELOAD`, but it didn't work, do you know why it failed?

This will not work for multiple reasons.  The static PIE object will not
have a dynamic symbol table, so no interposition takes place.  It also
uses relocations which are not supported in ELF shared objects.

glibc 2.30 will provide a clearer diagnostic for this, rejecting loading
the object outright.

Thanks,
Florian

Re: LD_PRELOAD doesn't work with dynamic object built with `-static-pie`

[Baojun Wang]

Thank you for the clarification. I did turn on `LD_DEBUG` and was surprised
it didn't work.

On Tue, May 21, 2019 at 2:02 AM Florian Weimer <fweimer@redhat.com> wrote:

> * Baojun Wang:
>
> > I tried to build a shared object with `-static-pie`, and then used it as
> > `LD_PRELOAD`, but it didn't work, do you know why it failed?
>
> This will not work for multiple reasons.  The static PIE object will not
> have a dynamic symbol table, so no interposition takes place.  It also
> uses relocations which are not supported in ELF shared objects.
>
> glibc 2.30 will provide a clearer diagnostic for this, rejecting loading
> the object outright.
>
> Thanks,
> Florian
>