How do I find a line of code with pyelftools/libdwarf

Question

I have a function name and an offset from the top of that function. I know I can find the line of code from looking at the assembly listing file and compute the offset for the line of code and get the line number that way.

What I'm trying to do is use the .o file to get that same information. I can see the DWARF information for the ELF file and can find the DIE for function in the DWARF data, but how do I actually see the info for the instructions of that function and map that to a line of code. I've been using pyelftools so I would hopefully like to be able to use that but I am open to other options if I can't use pyelftools.

Answer 1

There's a sample in pyelftools that does that: https://github.com/eliben/pyelftools/blob/master/examples/dwarf_decode_address.py

Specifically, finding the line for the address goes like this:

def decode_file_line(dwarfinfo, address):
    # Go over all the line programs in the DWARF information, looking for
    # one that describes the given address.
    for CU in dwarfinfo.iter_CUs():
        # First, look at line programs to find the file/line for the address
        lineprog = dwarfinfo.line_program_for_CU(CU)
        prevstate = None
        for entry in lineprog.get_entries():
            # We're interested in those entries where a new state is assigned
            if entry.state is None:
                continue
            if entry.state.end_sequence:
                # if the line number sequence ends, clear prevstate.
                prevstate = None
                continue
            # Looking for a range of addresses in two consecutive states that
            # contain the required address.
            if prevstate and prevstate.address <= address < entry.state.address:
                filename = lineprog['file_entry'][prevstate.file - 1].name
                line = prevstate.line
                return filename, line
            prevstate = entry.state
    return None, None