How to map function address to function in *.so files

Try giving the offset to addr2line, along with the section name. Like this:

addr2line -j .text -e libtst.so 0x26887

Edit: By the way, if it wasn’t clear, the 0x26887 comes from what you provided:

0xb77dc887(fun2 addr+offset)-0xb77b6000 (lib starting addr) = 0x26887 (result)