Enable preloading in dlopen-ed shared libraries?

[Fengkai Sun]

Hi list,

As I dive a little deeper into the source code, I found that
`_dl_map_object_deps' is called in both rtld.c and dl-open.c.

The semantics of two invocations are both clear: rtld.c makes a call like
this:
_dl_map_object_deps (main_map, preloads, npreloads, mode == trace, 0);
because ld.so needs to preload the libraries specified by LD_PRELOAD into
the global scope for interposing the symbols.

dl-open.c makes a call like this:
_dl_map_object_deps (new, NULL, 0, 0,
      mode & (__RTLD_DLOPEN | RTLD_DEEPBIND | __RTLD_AUDIT));
because no dlopen-ed library needs to preload anything.

However, I think it might be useful to allow users to preload some
libraries in the local scope after the map of the dlopen-ed library, just
like how ld.so treats preloaded libs in global scope.

By doing so, the user can easily provide a different definition of a symbol
from the one of the main executable, by enabling RTLD_DEEPBIND.
This is useful under some circumstances. For example, a dlopen-ed library
may want to use a separate heap from the main heap, and the user can
provide another malloc implementation for that library.

The auditing interface can do the similar thing, but after doing some
experiments, I found that `la_symbind64' cannot catch the bindings of
global variables, and it cannot hook all of the function bindings.

Would it be a good idea to add an interface to enable preloading in the
local scope of dlopen-ed shared libraries?

Thank you in advance.

--
Best,
Fengkai