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_posts/2023-09-05-kernel-debugging.markdown

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+---
+layout: post
+title:  "Kernel debugging through a Segger J-Link JTAG adapter"
+date:   2023-08-25 00:00:00 -0400
+---
+
+# 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.
+```
+
+