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Debugging The Linux 2.4 Kernel
To be able to debug the Linux 2.4 kernel, you need to enable
the kgdb support in the kernel configuration, and select a
suitable debug port, and then rebuild the kernel (with 'make
images' for example). Note that when you enable the kgdb
support, the Linux kernel will be built with -g and so may
increase significantly in size. When you boot, something like
this should show up on the serial port (use sermon or some
other serial sniffer):
Uncompressing Linux...
Done. Now booting the
kernel.
$O4c696e75782076657273696f#e3$O6e20322e342e3220286f726a616e66#81
[lots more]
The garbled stuff is the gdb stub dumping information ($
start a packet, # ends the packet, and then
there's a 2 hex digit checksum). Now start gdb-cris with the
vmlinux file as argument:
gdb-cris vmlinux
Then
(gdb) target remote
/dev/ttyS0
Remote debugging using
/dev/ttyS0
breakpoint () at
kgdb.c:1530
1530 __asm__ volatile
("break 8"); /* Jump to handle_breakpoint. */
(gdb)
A quick backtrace will tell you where you are:
(gdb) backtrace
#0 breakpoint () at
kgdb.c:1530
#1 0x6004a8ca in
init_IRQ () at irq.c:487
#2 0x600bc49c in
start_kernel () at init/main.c:535
A couple of things to remember when debugging the kernel:
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Turn off the watchdog (make menuconfig,
under General setup). It will bite you
otherwise.
-
The kernel is compiled with the -O2 flag.
The compiler is likely to have rearranged the statements
in the code, and variables may have been optimized away.
-
The Linux kernel contains many inlined functions,
multi-statement macros and assembly code. Be prepared to
disassemble to really understand what's going on.
-
Things happen under your feet. For example, interrupts
caused by the timer occur while you are debugging.
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Some parts of the code don't really like being stepped
through, like the somewhat critical stack frame setup in
entry.S.
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