[debugging fails with illegal instruction]

Good day,

 

I'm developing on the Allwinner A20 & A33 CPUs, which contains ARM cores.

I've first noticed the segfault during remote debugging, using gdbserver,

from my host PC to the target. Debugging the the application directly on

the target did not cause this problem. Performing the remote debugging from

a 32-bit host to the 32-bit target made no difference. That seemed to 

eliminate the possibility of a 64/32 bit compatibility issue.

 

My setup:

Host PC:

  Kubuntu 16.04 64-bit

  Toolchain: gcc-linaro-6.1.1-2016.08

           : gdb version (Linaro_GDB-2016.08) 7.11.1.20160801-git

            Obtained from: https://releases.linaro.org/components/toolchain/binaries         

Target:

  Ubuntu 16.04 ARMHF

  : GDB version: (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1

 

Illustrating the problem

------------------------------

Program: main.c

  #include <stdio.h>

  int main (void)

  {

    printf("Hallo world!\n");

    return 0;

  }

Compile on host PC:

  $ arm-linux-gnueabihf-gcc -g main.c -o main_32

File signature:

  $ file main_32

    main_32: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked,

          interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 2.6.32, 

          BuildID[sha1]=cc1a4a1ff17da63f9f8362c84dffe54ee7fff4ff, not stripped

 

Remote target:

The executable (main_32) needs to be copied to the remote target

 

$ gdbserver :5555 main_32  

  Process main_32 created; pid = 904

  Listening on port 5555

  Remote debugging from host 192.168.7.2

 

Host PC:

$ arm-linux-gnueabihf-gdb ./main_32

 

(gdb) target remote 192.168.7.1:5555 

  Remote debugging using 192.168.7.1:5555

  Reading /lib/ld-linux-armhf.so.3 from remote target...

  warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.

  Reading /lib/ld-linux-armhf.so.3 from remote target...

  Reading symbols from target:/lib/ld-linux-armhf.so.3...Reading /lib/ld-2.23.so from remote target...

  Reading /lib/.debug/ld-2.23.so from remote target...

  (no debugging symbols found)...done.

  0xb6fd7a40 in ?? () from target:/lib/ld-linux-armhf.so.3

 

(gdb) continue

  Continuing.

  Program received signal SIGSEGV, Segmentation fault.

  0xb6fd9822 in ?? () from target:/lib/ld-linux-armhf.so.3

 

To try and figure out where the segfault occurred I figured that the debug symbols needed to be loaded.

The gdb command 'set debug-file-directory' can be used to accomplish this. It seems that this command 

is only relative to the host PC filesystem and not according to the remote target file system.

With 'set sysroot' the debugger can also be instructed to load all data locally instead of transferring it

runtime over the network.

 

I've copied the full contents of the SD card residing on the Allwinner A20 rootfs to a local directory on

the host PC: /opt/gnueabihf_rootfs

 

Illustrating the solution

-------------------------------

Required packages: apt-get install libc6-dbg

 

Remote target:

$ gdbserver :5555 main_32 

  Process main_32 created; pid = 912

  Listening on port 5555

  Remote debugging from host 192.168.7.2

  

  Hallo world!

  Child exited with status 0

 

Host PC:

$ arm-linux-gnueabihf-gdb ./main_32

  (gdb) set debug-file-directory /opt/gnueabihf_rootfs/usr/lib/debug

  (gdb) set sysroot /opt/gnueabihf_rootfs

  (gdb) target remote 192.168.7.1:5555

 

  Remote debugging using 192.168.7.1:5555

  Reading symbols from /opt/gnueabihf_rootfs/lib/ld-linux-armhf.so.3...done.

  Reading symbols from /opt/gnueabihf_rootfs/usr/lib/debug/lib/arm-linux-gnueabihf/ld-2.23.so...done.

  0xb6fd7a40 in _start ()

   

  (gdb) continue

  Continuing.

  Cannot parse expression `.L966 4@r4'.

  warning: Probes-based dynamic linker interface failed.

  Reverting to original interface.

 

Even though there is a warning regarding "Probes-based dynamic linker interface failed", the program

keeps on running correctly. The thread describing the GDB patch about the linker interface can be

read at: https://sourceware.org/ml/gdb-patches/2015-08/msg00629.html

 

It appears the work around was introduced around August 2015, but if you do not specify the path to

the debug symbols, the debugger still segfaults.

 

The 'set debug-file-directory' and 'set sysroot' can be added to the .gdbinit file to be applied

automatically when you start a remote debug session.

 

Regards,

Frikkie Thirion