Kernel debugging through a Segger J-Link JTAG adapter

Kernel debugging through J-Link

A Linux kernel can be debugged using GDB. Here are some notes of what worked for me.

Tested:

• AT91SAM9260-EK evaluation board (console through DBGU serial ttyS0)
• Segger J-Link EDU JTAG adapter.
• Segger GDB server 7.88 (free download)

Steps:

• Run J-Link GDB Server and configure it with the correct CPU type
• Compile the kernel with the following options:

  CONFIG_GDB_SCRIPTS=y
  CONFIG_FRAME_POINTER=y
  CONFIG_KGDB=y
  CONFIG_KGDB_SERIAL_CONSOLE=y
  CONFIG_KGDB_KDB=y
  

• Enable kgdboc against the console ttyS0 echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
• Trigger SysRQ (this will hang the kerneli and go into KDB mode) echo g > /proc/sysrq-trigger
• Switch KDB to KGDB kgdb
• Now close any terminal emulators on the host to release the serial port. (Close teraterm/minicom/screen etc)
• Run gdb (gdb ./vmlinux). I used my buildroot host gdb in the kernel build dir: /test/host/bin/arm-buildroot-linux-uclibcgnueabi-gdb vmlinux
• This was on a machine different from where GDBServer was running so: target remote 192.168.2.51:2331
• Running lx-symbols should load symbols, modules and you can start debugging stuff:

  (gdb) lx-symbols
  loading vmlinux
  scanning for modules in /test/build/linux-4.19.292
  loading @0xbf04a000: /test/build/linux-4.19.292/crypto/algif_skcipher.ko
  loading @0xbf042000: /test/build/linux-4.19.292/crypto/md5.ko
  loading @0xbf03a000: /test/build/linux-4.19.292/crypto/algif_hash.ko
  loading @0xbf032000: /test/build/linux-4.19.292/drivers/misc/atmel-ssc.ko
  loading @0xbf028000: /test/build/linux-4.19.292/drivers/iio/adc/at91_adc.ko
  loading @0xbf021000: /test/build/linux-4.19.292/drivers/iio/buffer/industrialio-triggered-buffer.ko
  loading @0xbf01a000: /test/build/linux-4.19.292/drivers/iio/buffer/kfifo_buf.ko
  loading @0xbf000000: /test/build/linux-4.19.292/drivers/iio/industrialio.ko
  (gdb) apropos lx
  function lx_current -- Return current task.
  function lx_module -- Find module by name and return the module variable.
  function lx_per_cpu -- Return per-cpu variable.
  function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable.
  function lx_thread_info -- Calculate Linux thread_info from task variable.
  function lx_thread_info_by_pid -- Calculate Linux thread_info from task variable found by pid
  lx-cmdline --  Report the Linux Commandline used in the current kernel.
  lx-cpus -- List CPU status arrays
  lx-dmesg -- Print Linux kernel log buffer.
  lx-fdtdump -- Output Flattened Device Tree header and dump FDT blob to the filename
  lx-iomem -- Identify the IO memory resource locations defined by the kernel
  lx-ioports -- Identify the IO port resource locations defined by the kernel
  lx-list-check -- Verify a list consistency
  lx-lsmod -- List currently loaded modules.
  lx-mounts -- Report the VFS mounts of the current process namespace.
  lx-ps -- Dump Linux tasks.
  lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules.
  lx-version --  Report the Linux Version of the current kernel.