Is there a way to unhide hidden-visibility symbols with GNU binutils?

Question

I'm working on a script to make uClibc usable on an existing glibc-targetted gcc/binutils toolchain, and the one problem I'm left with is that pthread_cancel needs to dlopen libgcc_s.so.1. The version supplied with the host gcc is linked to depend on glibc, so I'm instead using ld's -u option to pull in the needed symbols (and their dependencies) from libgcc_eh.a to make a replacement libgcc_s.so.1:

gcc -specs uclibc.specs -Wl,-u,_Unwind_Resume -Wl,-u,__gcc_personality_v0 \
    -Wl,-u,_Unwind_ForcedUnwind -Wl,-u,_Unwind_GetCFA -shared -o libgcc_s.so.1

In principle I would be done, but all the symbols in libgcc_eh.a have their visibility set to hidden, so in the output .so file, they all become local and don't get added to the .dynsym symbol table.

I'm looking for a way to use binutils (perhaps objcopy? or a linker script?) on either the .so file or the original .o files in libgcc_eh.a to un-hide these symbols. Is this possible?

Answer 1

objcopy doesn't seem to have this feature, but you can do it with the ELFkickers rebind tool:

rebind --visibility default file.o SYMBOLS...

This must be done on the original .o files. If you try to do it on the .so it'll be too late, because the hidden symbols will have been omitted from the .dynsym section.

Answer 2

I think you should be able to use --globalize-symbol in objcopy.

e.g.

$ nm /usr/lib/gcc/i686-redhat-linux/4.6.3/libgcc_eh.a | grep emutls_alloc

00000000 t emutls_alloc
$ objcopy  --globalize-symbol=emutls_alloc /usr/lib/gcc/i686-redhat-linux/4.6.3/libgcc_eh.a /tmp/libgcc_eh.a
$ nm /tmp/libgcc_eh.a |grep emutls_alloc
00000000 T emutls_alloc

You can provide --globalize-symbol several times to objcopy, but you'll need to explicitly mention the full symbol name of all the symbols you want to globalize.

Though I'm not sure what kind of breakage could occur turning libgcc_eh.a into a shared object, as libgcc_eh.a is presumably compiled without -fpic/-fPIC. Turns out libgcc_eh.a is compiled as position independent code.