Re: Ensuring symbol resolution order at runtime

[Cary Coutant <ccoutant at google dot com>]

> Scenario 1: An executable is being linked, and the linker is provided the
> names of two static libraries. Both of those libraries export a symbol named
> "foo", and one of the objects being linked into the executable refers to
> that symbol.
>
> In this situation, it is understood that the linker will choose a symbol
> "foo" to satisfy the reference from the *first* library specified in the
> linker's arguments that provides a symbol "foo".

Yes, as long as the reference to "foo" comes from an object that
precedes both libraries in the link order. It's possible to do a link
like this:

  ld does-not-call-foo.o lib1.a calls-foo.o lib2.a

and the "foo" from lib2.a will be chosen.

> Scenario 2: An executable is being linked, and the static linker is provided
> the names of two shared libraries. Both of those libraries export a symbol
> named "foo", and one of the objects being linked into the executable refers
> to that symbol.
>
> In this situation, it is understood that the static linker will locate a
> symbol "foo" to satisfy the reference from the *first* library specified in
> the linker's arguments that provides a symbol "foo". This is fine. However,
> at runtime, when the dynamic linker has the responsibility for loading these
> shared libraries into the process and resolving symbols, is there any
> guarantee that the the *same* "foo" will be chosen by the dynamic linker?

As long as the runtime loader sees the same libraries that the linker
saw, yes, there is a guarantee. However, if lib1.so is replaced with a
library that does not define "foo", then the dynamic loader will bind
"foo" to the definition in lib2.so without any complaint.

This is standard Unix behavior, and is unlike what you would get on
Windows, for example, where the static linker records the binding of
each definition.

> Scenario 3: An executable is being linked, and the static linker is provided
> the name of a shared library. That shared library contains a reference to
> another shared library. Both of those libraries export a symbol named "foo",
> and one of the objects being linked into the executable refers to that
> symbol.
>
> In this situation, the static linker only has the symbol "foo" from the
> first library available to satisfy the reference, because it was not made
> explicitly aware of the second library that also provides "foo". At runtime,
> it seems reasonable to assume that the dynamic linker will load the first
> shared library into the process before the second shared library, and
> *should* resolve symbol references from the executable into that library
> before loading the second shared library into the process.

Yes, subject to the conditions I mentioned above.

> I'm trying to work on a problem related to multiple shared libraries linking
> to libssl.so (from OpenSSL), and need to come up with a way to ensure that
> only *one* library calls the initialization functions from libssl, without
> requiring any cooperating behavior from the other libraries that would
> normally do it as well. My approach so far has been to 'mask' the real
> functions from libssl.so in another shared library that is loaded *first*
> into the process, thereby ensuring that no matter what libraries are loaded
> into the process later, they will not be able to gain access to the
> underlying functions from libssl.so (other than via exploiting dlsym() to do
> so explicitly).
>
> So, is this approach reasonable, and if so, is it backed up by standards or
> other documents that prescribe the linker's behavior for these scenarios?
> I'm really not keen on relying on any assumptions about the dynamic linker's
> behavior (other than that it will follow the relevant standards if it claims
> to do so).
>
> Thanks in advance for any light you can shed on this :-)

I'd think a better approach would be to use a static variable in the
initialization function to protect it from being called twice. Even
better would be to have libssl's initialization functions actually
marked as initialization functions so that they run automatically when
the library is loaded.

-cary