gdb fails to insert a breakpoint even if compiled with -no-pie - c++

Question

I'm trying to get gdb working with C++ programs on Ubuntu 20.04. What I need is to be able to set a breakpoint (for example, break main.cpp:3 gdb command) and then run until the breakpoint, but at the moment both start and run fail because they "Cannot insert breakpoint" and "Cannot access memory". For me gdb fails even with very simple examples. This is main.cpp content:
#include <iostream>
int main() {
std::cout << "Hello World!";
return 0;
}
I found somewhere that using -no-pie might help to get gdb working (with breakpoints), so I compile the program by running g++ -ggdb3 -no-pie -o main main.cpp (I also tried -g instead of -ggdb3, and -fno-PIE in addition to -no-pie). When I try to use gdb, it complains "Cannot insert breakpoint 1":
gdb -q main
Reading symbols from main...
(gdb) start
Temporary breakpoint 1 at 0x1189: file main.cpp, line 3.
Starting program: /tmp/main
Warning:
Cannot insert breakpoint 1.
Cannot access memory at address 0x1189
Without -no-pie result is the same. Only thing that changes with or without -no-pie is the hexadecimal address, without -no-pie it is low like 0x1189 (as shown above), with -no-pie it can be 0x401176, but everything else exactly the same, I keep getting the "Cannot access memory at address" warning in both cases.
If I use starti instead of start, it works at first, but after a few nexti iterations it prints usual message "Cannot insteart breakpoint":
gdb -q main
Reading symbols from main...
(gdb) starti
Starting program: /tmp/main
Program stopped.
0x00007ffff7fd0100 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) nexti
0x00007ffff7fd0103 in ?? () from /lib64/ld-linux-x86-64.so.2
...
(gdb) nexti
Warning:
Cannot insert breakpoint 0.
Cannot access memory at address 0x4
0x00007ffff7fd0119 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) nexti
0x00007ffff7fd011c in ?? () from /lib64/ld-linux-x86-64.so.2
...
(gdb) nexti
Warning:
Cannot insert breakpoint 0.
Cannot access memory at address 0x1c
0x000055555556ca22 in ?? ()
(gdb) nexti
[Detaching after fork from child process 3829827]
...
[Detaching after fork from child process 3829840]
Hello World![Inferior 1 (process 3819010) exited normally]
So I can use nexti, but cannot use next and obviously cannot insert breakpoints.
I tried -Wl,-no-pie (by running g++ -Wl,-no-pie -ggdb3 -o main main.cpp; adding -no-pie does not change anything) but this option causes a strange linker error:
/usr/bin/ld: cannot find -lgcc_s
/usr/bin/ld: cannot find -lgcc_s
collect2: error: ld returned 1 exit status
When I google the error, I only found advice to try -no-pie instead of -Wl,-no-pie, and no other solutions. Since debugging C++ programs is very common activity, I feel like I'm missing something obvious but I found no solution so far.
To make it easier to understand what exact commands I use and to make it clear I'm not mixing up directories and to show what versions of g++ and gdb I'm using, here is full terminal log:
$ ls
main.cpp
$ g++ --version | grep Ubuntu
g++ (Ubuntu 9.3.0-10ubuntu2) 9.3.0
$ g++ -ggdb3 -no-pie -o main main.cpp
$ ls
main main.cpp
$ gdb --version | grep Ubuntu
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
$ readelf -h main | grep 'Type: .*EXEC'
Type: EXEC (Executable file)
$ gdb -q main
Reading symbols from main...
(gdb) start
Temporary breakpoint 1 at 0x401176: file main.cpp, line 3.
Starting program: /tmp/main/main
Warning:
Cannot insert breakpoint 1.
Cannot access memory at address 0x401176
For completeness, I tried the same without -no-pie:
$ rm main
$ g++ -ggdb3 -o main main.cpp
$ readelf -h main | grep 'Type: .*'
Type: DYN (Shared object file)
$ gdb -q main
Reading symbols from main...
(gdb) start
Temporary breakpoint 1 at 0x1189: file main.cpp, line 3.
Starting program: /tmp/main/main
Warning:
Cannot insert breakpoint 1.
Cannot access memory at address 0x1189
As you can see the only difference with or without -no-pie is the memory address, but the issue and warnings are the same. Without -no-pie this may be expected, but I do not understand why this is happening if I compiled with -no-pie and what else I can try to solve the issue.

Answer 1

This:
g++ -ggdb3 -no-pie -o main main.cpp
should produce a non-PIE executable. You should be able to verify that it non-PIE by looking at readelf -h main | grep 'Type: .*EXEC' (PIE binaries have ET_DYN type).
This:
Temporary breakpoint 1 at 0x1189: file main.cpp, line 3.
is unambiguously a PIE binary (a non-PIE binary will not have any code below 0x40000 on x86_64 Linux).
Conclusion: you are either debugging the wrong binary (e.g. you are compiling main in a different directory from the one in which you are debugging), or you are not telling is the whole story.