Re: [musl] Shared library loading

[Rich Felker <dalias@libc.org>]

On Mon, May 25, 2020 at 08:26:51PM +0300, Alexander Scherbatiy wrote:
> On 22.05.2020 21:25, Rich Felker wrote:
> 
> >The concept here is that non-pathname library names should be loaded
> >from the library path and not replaced by something (typically a
> >"module" or "plug-in") in a different, explicitly-loaded location that
> >happens to have a colliding base filename.
> >
> >For example suppose your application loads modules from $libdir/myapp/
> >and has a module named "libfoo.so". Unbeknownst to you, there's also a
> >"libfoo.so" in the system paths, and some library you potentially load
> >indirectly (maybe the GPU driver for some video hardware you've never
> >heard of) depends on "libfoo.so".
> >
> >If dlopen("$libdir/myapp/libfoo.so") had put "libfoo.so" in the
> >namespace such that it would satisfy future load requests for the name
> >"libfoo.so", the subsequent load would break due to getting the wrong
> >(unrelated) library.
>   Is it correct that RTLD_GLOBAL flag is in some kind of obsolete in
> musl because calling
>     dlopen("$libdir/myapp/libfoo.so", RTLD_NOW | RTLD_GLOBAL);
>     dlopen("$libdir2/myapp2/libbar.so", RTLD_LAZY);
> 
>    does not allow libbar.so to find library $libdir/myapp/libfoo.so
> which it is depend on?

This is not what RTLD_GLOBAL means. RTLD_GLOBAL is a matter of whether
the library's symbols appear in the global symbol namespace to resolve
relocations in future-loaded libraries and dlsym with RTLD_DEFAULT. It
has nothing to do with whether the library is visible for satisfying
DT_NEEDED. The library is *always* visible for this purpose.

>   What is the suggested way in musl to load $libdir/myapp/libfoo.so
> from a custom directory so libbar.so can find it?

Probably putting the dir in your application's rpath (or in the rpath
of libbar). dlopen is not a tool for (effectively) adding additional
directories to the library search path.

Rich