GDB Notes

I am studying on low-level security.

Of course, GNU Debugger (GDB) is my best friend to help me understand what is going on on the low-level side. Simply, GDB is a powerful debugger ( C and C++). Gdb can use debugging symbols that are generated by GCC ( -g) option.

As usual, this is not a full tutorial. I jotted down what is important to me:)

gdb  – args ./main args1 // start gdb with argument 

p/x variable // hexadecimal print 

x memory_address // read memory address

p/t variable // binary print 

b // set a breakpoint. It can be put on a function, or specific line in a file */ 

watch // set a watchpoint, act on variables

finish // runs until the current function is finished

bt // backtrace

thread apply all bt // print the backtrace of all threads, it is so useful to solve deadlock problem. 

info threads // print summarised version of above commands

info registers // print the register value

info all-registers rsp // print the rsp register value 

i r rsp // shortage 

info all-registers // print all registers

info locals // print the local variable 

handle SIGSEGV nostop // don't stop the program in case of SEGFAULT. I needed to handle SIGSEGV functions, but the gdb doesn't allow me. 

info breakpoint // print all breakpoint

si // step by machine instructions rather than source lines

set disable-randomization off // ASLR is disabled at default, it can be open with this command
set scheduler-locking // in multithreading application to debug just for one thread. 
    off  == no locking (threads may preempt at any time)
    on   == full locking (no thread except the current thread may run)
    step == scheduler locked during every single-step operation.
        In this mode, no other thread may run during a step command.
        Other threads may run while stepping over a function call

p $_siginfo // to print the last signal info

If you want to connect your application with GDB while it is deadlock situation, please follow the blog.

DeadLock detecting using GDB

gdb --args env LD_PRELOAD=/usr/local/lib/libstderred.so ls -l
set exec-wrapper env 'LD_PRELOAD=../../playground/sud-library-concept/libsud.so'

Compiler Explorer

Quite usefull for understanding the low level stuff

To remote host, target remote localhost:1234 can be used. More target subcommand is here.

List of target subcommands:
target core - Use a core file as a target.
target exec - Use an executable file as a target.
target extended-remote - Use a remote computer via a serial line, using a gdb-specific protocol.
target native - Native process (started by the "run" command).
target record-btrace - Collect control-flow trace and provide the execution history.
target record-core - Log program while executing and replay execution from log.
target record-full - Log program while executing and replay execution from log.
target remote - Use a remote computer via a serial line, using a gdb-specific protocol.
target tfile - Use a trace file as a target.

Have a nice debugging :)