Need to load debugging symbols for shared library in GDB

Question

I am using GDB to debug a program that uses libpthread. There is an error happening in pthread_create and need to step into that function. Unfortunately when I am debugging my program, it does not load the shared library symbols properly so I can't step over the source code and examine program behaviour meaningfully.. This is the output as soon as I start gdb.

Remote debugging using 127.0.0.1:21293
warning: limiting remote suggested packet size (206696 bytes) to 16384
Failed to read a valid object file image from memory.

So I believe the last message is related to the failure to read debugging symbols. This is despite having the libc6-dbg package installed. This is the truncated output of "where" at a point just before a SIGSEGV is encountered (in pthread_create, the function I want to examine in the debugger)

#0  0x68200ce2 in ?? ()
#1  0x68403cbf in ?? ()
#2  0x687571b0 in ?? ()
#3  0x6874c638 in ?? ()
#4  0x68867a72 in ?? ()
....

The process' /proc/.../maps shows where libpthread is mapped into memory.

683f8000-68410000 r-xp 00000000 08:01 3017052                            /lib/i386-linux-gnu/i686/cmov/libpthread-2.19.so
68410000-68411000 r--p 00017000 08:01 3017052                            /lib/i386-linux-gnu/i686/cmov/libpthread-2.19.so
68411000-68412000 rw-p 00018000 08:01 3017052                            /lib/i386-linux-gnu/i686/cmov/libpthread-2.19.so

I believe that if I could only manually load the debugging symbols into gdb, then I will be able to step over the source code and find the source of my memory error. However I am unsure about how to do this.

I am debugging a 32 bit program on x86_64 Debian. What should I do to load libpthread symbols into GDB so that I can debug it meaningfully?

Answer 1

Loading debug symbols for a shared library

If the shared library is stripped, and the debug symbols are provided as a separate file, you need to load them after the shared library was loaded by the linker. The symbols should be loaded on the memory address where the shared library is loaded.

Following is an example of loading the symbols:

1. start gdb

   ~$ gdb a.out
   GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
   Copyright (C) 2020 Free Software Foundation, Inc.
   (gdb) info sharedlibrary
   No shared libraries loaded at this time.
   

2. Create break point (on main or any other place) and start the debug

   (gdb) b 35
   Breakpoint 1 at 0xd7c: file test.c, line 35.
   (gdb) r
   Starting program: /root/testing/lib-test/a.out
   

3. Find the memory location where the shared library that you want to debug was loaded (in this example the library is libtest.so.1

   (gdb) info sharedlibrary
   From                To                  Syms Read   Shared Object Library
   0x0000fffff7fcd0c0  0x0000fffff7fe5468  Yes (*)     /lib/ld-linux-aarch64.so.1
   0x0000fffff7f9f890  0x0000fffff7fb65c0  Yes (*)     /usr/local/lib/libtest.so.1
   0x0000fffff7e4bbc0  0x0000fffff7f3b190  Yes         /lib/aarch64-linux-gnu/libc.so.6
   0x0000fffff7dfea50  0x0000fffff7e0ddec  Yes         /lib/aarch64-linux-gnu/libpthread.so.0
   

   so, the library is loaded starting from memory address 0x0000fffff7f9f890

4. Load the symbol file with the address from the share library

   (gdb) add-symbol-file ./libsrc/libtest.dbg 0x0000fffff7f9f890
   add symbol table from file "./libsrc/libtest.dbg" at
    .text_addr = 0xfffff7f9f890
   (y or n) y
   Reading symbols from ./libsrc/libtest.dbg...
   

After this, you can trace the execution flow inside the library, list the lines of the source code, inspect variables by name, etc.