[TBD]Useful gdb tips/cheatsheat

Set Shared library path

(gdb) set solib-search-path <my directories having so files separated by :>

(gdb) show solib-search-path

Useful link

http://visualgdb.com/gdbreference/commands/set_solib-search-path

Setting environment variable from within GDB

You can use the set environment command in gdb to change the environment gdb uses to launch the program you want to debug. Extract from the gdb help:

(gdb) help set environment

Set environment variable value to give the program.

Arguments are VAR VALUE where VAR is variable name and VALUE is value.

VALUES of environment variables are uninterpreted strings.

This does not affect the program until the next "run" command.

Example with LD_LIBRARY_PATH:

(gdb) set environment LD_LIBRARY_PATH /home/paceholder/projects/geo/lib/debug

Debugging child

If you want to follow the child process instead of the parent process, use the command set follow-fork-mode.

set follow-fork-mode mode

Set the debugger response to a program call of fork or vfork. A call to fork or vfork creates a new process. The mode argument can be:

parent

The original process is debugged after a fork. The child process runs unimpeded. This is the default.

child

The new process is debugged after a fork. The parent process runs unimpeded.

How to use GDB script

Write function in .gdbinit

.gdbinit example

root@ns# cat ~/.gdbinit

define myloop_print

set $total = $arg0

set $i = 1

while ($i < 2)

if $total == 2

set $i = $i + $total

end

set $i = $i + 1

end

print $i

end

Load GDB and run the script

Example for running gdbscript

root@ns# gdb

GNU gdb 6.1.1 [FreeBSD]

GDB is free software, covered by the GNU General Public License, and you are

welcome to change it and/or distribute copies of it under certain conditions.

Type "show copying" to see the conditions.

There is absolutely no warranty for GDB. Type "show warranty" for details.

This GDB was configured as "amd64-marcel-freebsd".

(gdb) myloop_print 2

$1 = 4

(gdb) myloop_print 1

$2 = 2

Useful link: https://stackoverflow.com/questions/20481815/how-to-loop-in-a-gdb-script-till-program-is-finished

How to generate core-dump of a running process without killing it

Use below command after attaching process with gdb

(gdb) generate-core-file

Useful link:https://stackoverflow.com/questions/4610005/gdb-creating-a-core-file

Useful gdb command

x/100x <address> -> print the memory value for address

x/100x <address> -100 -> print the memory value for address starting from -100 position

info register -> prints all registers

p $rsp -> print register rsp

set $rsp=<address> -> modify value of register rsp

disassemble -> shows disassembly code

x/I <function name>-> prints function

attach <pid> -> attach process

watch <memory address> -> breaks to the code which changes the address

layout asm -> opens an UI with assembly code.. useful for walkthrough of assembly code

layout src -> useful for walkthrough of source code. Source code path should be already set.

dir <source code path> -> add the source code search path in the gdb list

source script -> loads gdb script

Method to notice who is changing the memory address

use watch

Method to do live debugging with GDB along with source code

Get source code in the machine where you do debugging using mount

Refer https://www.digitalocean.com/community/tutorials/how-to-set-up-an-nfs-mount-on-ubuntu-16-04

Configure source code machine (10.106.102.141)

Install nfs in ubuntu machine

root#apt-get install nfs-kernel-server

root#apt-get install nfs-common

Enable remote access via mount

root@ubuntu:~/ws1/rs_120/usr.src# cat /etc/exports

# /etc/exports: the access control list for filesystems which may be exported

# to NFS clients. See exports(5).

# Example for NFSv2 and NFSv3:

# /srv/homes hostname1(rw,sync,no_subtree_check) hostname2(ro,sync,no_subtree_check)

# Example for NFSv4:

# /srv/nfs4 gss/krb5i(rw,sync,fsid=0,crossmnt,no_subtree_check)

# /srv/nfs4/homes gss/krb5i(rw,sync,no_subtree_check)

/root/ 10.102.53.248(rw,no_root_squash,sync,no_subtree_check)

root#/etc/init.d/nfs-kernel-server restart

Mount source code to the machine where you do GDB(10.102.53.248)

root#mount -t nfs 10.106.102.141:/root/ /var/code/

root# mount | grep 10.106.102.141

10.106.102.141:/root on /var/code type nfs4 (rw,relatime,vers=4.0,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=10.102.53.248,local_lock=none,addr=10.106.102.141)

root@ubuntu:~/personal/sslcrash/assert32bit# ls /var/code/

emacs op personal pid_file rb.log ws1

Configure GDB

Get test binary

root@ubuntu:/var/code/ws1/mytest# cat test.c

#include<stdio.h>

int main()

{

printf("Hello world");

return 0;

}

root@ubuntu:/var/code/ws1/mytest# gcc -ggdb3 test.c -o test

root@ubuntu:/var/code/ws1/mytest# ls

test test.c

root@ubuntu:/var/code/ws1/mytest# cd ../../

root@ubuntu:/var/code# cp ws1/mytest/test .

root@ubuntu:/var/code# file test

test: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=182c25a6f444b7aa7d1b8b7c6d11cadb6bbb1e75, not stripped

Enable source code walkthrough in GDB

root@ubuntu:/var/code/ws1/mytest# cd /root/personal/testdir/

root@ubuntu:~/personal/testdir# ls

test

root@ubuntu:~/personal# gdb testdir/test

GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1

License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software: you are free to change and redistribute it.

There is NO WARRANTY, to the extent permitted by law. Type "show copying"

and "show warranty" for details.

This GDB was configured as "x86_64-linux-gnu".

Type "show configuration" for configuration details.

For bug reporting instructions, please see:

<http://www.gnu.org/software/gdb/bugs/>.

Find the GDB manual and other documentation resources online at:

<http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".

Type "apropos word" to search for commands related to "word"...

Reading symbols from testdir/test...done.

(gdb) help dir

Add directory DIR to beginning of search path for source files.

Forget cached info on source file locations and line positions.

DIR can also be $cwd for the current working directory, or $cdir for the

directory in which the source file was compiled into object code.

With no argument, reset the search path to $cdir:$cwd, the default.

(gdb) dir /var/code/ws1/mytest/

Source directories searched: /var/code/ws1/mytest:$cdir:$cwd

(gdb) show dir

Source directories searched: /var/code/ws1/mytest:$cdir:$cwd

(gdb) layout src

Screenshot of debugging

Reference

http://stackoverflow.com/questions/16254546/gdb-can-not-open-shared-object-file

https://sourceware.org/gdb/onlinedocs/gdb/Forks.html