adoc

kgdb.adoc

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+[[kgdb]]
+= KGDB
+
+KGDB is kernel dark magic that allows you to GDB the kernel on real
+hardware without any extra hardware support.
+
+It is useless with QEMU since we already have full system visibility
+with `-gdb`, but this is a good way to learn it.
+
+Cheaper than JTAG (free) and easier to setup (all you need is serial),
+but with less visibility as it depends on the kernel working, so e.g.:
+dies on panic, does not see boot sequence.
+
+Usage:
+
+....
+./run -k
+./rungdb -k
+....
+
+In GDB:
+
+....
+c
+....
+
+In QEMU:
+
+....
+/count.sh &
+/kgdb.sh
+....
+
+In GDB:
+
+....
+b sys_write
+c
+c
+c
+c
+....
+
+And now you can count from GDB!
+
+If you do: `b sys_write` immediately after `./rungdb -k`, it fails with
+`KGDB: BP remove failed: <address>`. I think this is because it would
+break too early on the boot sequence, and KGDB is not yet ready.
+
+See also:
+
+* https://github.com/torvalds/linux/blob/v4.9/Documentation/DocBook/kgdb.tmpl
+* https://stackoverflow.com/questions/22004616/qemu-kernel-debugging-with-kgdb/44197715#44197715
+
+[[kgdb-kernel-modules]]
+== KGDB kernel modules
+
+In QEMU:
+
+....
+/kgdb-mod.sh
+....
+
+In GDB:
+
+....
+lx-symbols ../kernel_module-1.0/
+b fop_write
+c
+c
+c
+....
+
+and you now control the count.
+
+TODO: if I `-ex lx-symbols` to the `gdb` command, just like done for
+QEMU `-gdb`, the kernel oops. How to automate this step?
+
+[[kdb]]
+== KDB
+
+If you modify `runqemu` to use:
+
+....
+-append kgdboc=kbd
+....
+
+instead of `kgdboc=ttyS0,115200`, you enter a different debugging mode
+called KDB.
+
+Usage: in QEMU:
+
+....
+[0]kdb> go
+....
+
+Boot finishes, then:
+
+....
+/kgdb.sh
+....
+
+And you are back in KDB. Now you can:
+
+....
+[0]kdb> help
+[0]kdb> bp sys_write
+[0]kdb> go
+....
+
+And you will break whenever `sys_write` is hit.
+
+The other KDB commands allow you to instruction steps, view memory,
+registers and some higher level kernel runtime data.
+
+But TODO I don't think you can see where you are in the kernel source
+code and line step as from GDB, since the kernel source is not available
+on guest (ah, if only debugging information supported full source).