dynamic linker / rpath question

[Filippo Arcidiacono]

On 12/13/2013 7:39 PM, Steve Ellcey wrote:
> I have a question about the uclibc dynamic linker vs. the glibc dynamic
> linker.  I have built a GCC cross-toolchain running on an x86 linux
> system and targeting the MIPS CPU.  If I build with the glibc system
> library and dynamic linker everything works fine.  If I build with
> uclibc instead and compile a C++ program and run it under the qemu
> simulator I get:
>
> ./x: can't load library 'libgcc_s.so.1'
>
> In both cases, I use -L and -rpath arguments when linking the executable
> to let the program know where the shared libraries are.
>
> After some digging I found I could 'fix' this problem by using a -rpath
> argument when building libstdc++ so that it knew where to find libgcc_s.so.1.
> I do not have to do this for a toolchain using glibc.
>
> So I think that when running an executable linked against a shared library
> (libstdc++.so) that in turn is linked against a second shared library
> (libgcc_s.so) the uclibc dynamic linker is looking for rpath's in the
> first shared library (libstdc++.so) to find libgcc_s.so but the glibc
> dynamic linker is using the rpath's from the executable to find libgcc_s.so
This is very strange. The libstdc++.so doesn't have any RPATH set. 
Morover the libgcc_s.so
should be found in the standard search path, then the dynamic linker 
doesn't need any info
to find it. Could you build the shared library loader with debugging 
support (SUPPORT_LD_DEBUG=y)
to see where the dynamic linker search the libraries.
In any case the dynamic tag RPATH as searching path are handled 
differently by the glibc and uclibc
dynamic linker.
In particular the latter consider the executable's RPATH as searching 
path to look for immediate
dependencies, while the glibc one the executable's RPATH is used as 
searching path for all
dependencies chain.
Looking the ABI documentation the uclibc dynamic linker does the right 
think.
http://www.sco.com/developers/gabi/latest/ch5.dynamic.html#shobj_dependencies
"The set of directories specified by a given |DT_RUNPATH| entry is used 
to find only the immediate dependencies of the executable or shared 
object containing the |DT_RUNPATH| entry. That is, it is used only for 
those dependencies contained in the |DT_NEEDED| entries of the dynamic 
structure containing the |DT_RUNPATH| entry, itself. One object's 
|DT_RUNPATH| entry does not affect the search for any other object's 
dependencies. "

Some time ago I raised a similar issue, see
"Use executable's RPATH in looking up library searching path." thread in
http://lists.uclibc.org/pipermail/uclibc/2011-September/thread.html

Hope this help you.
>
> Can anyone tell me if I am right about this?  Are there options to
> change the behaviour of the uclibc dynamic linker or options to change
> the GCC libstdc++ build to deal with this issue?
>
> Steve Ellcey
> sellcey at mips.com
Regards,
Filippo
>
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