Kernel Debugging using Qemu

[Aruna Hewapathirane]

Hi Saket,

Thank you so much for the clear instructions. I really wanted to get qemu
to work on my 64bit system so I kept trying and finally I got things to
work.

After following your instructions I could get further but gdb complained
about a "Remote 'g' packet reply is too long".Your way works fine but I
needed to debug a kernel module so what finally worked for me is this.

I am enclosing the steps it may help others some day:

After Googling into the late hours of the morning I saw someone else had
similar issues and he resolved this way:
http://lists.gnu.org/archive/html/qemu-discuss/2014-10/msg00069.html

I am using GNU gdb (GDB) 7.9 the file is remote.c and I commented out lines:

 /* Further sanity checks, with knowledge of the architecture.  */
6157  //if (buf_len > 2 * rsa->sizeof_g_packet)
6158  //  error (_("Remote 'g' packet reply is too long: %s"), rs->buf);

make then make install gdm, then proceed as usual but you need to specify
the -initrd else cannot debug kernel modules. You can still step into the
kernel no issues there but modules nope.

qemu-system-x86_64 -s -S -kernel arch/x86/boot/bzImage -initrd
/boot/initrd.img-4.1.2-vtaqemu+ /dev/zero
xvnc4viewer :5900 ( Your port # may differ.. )

gdb vmlinux
target remote localhost:1234

(gdb) add-symbol-file /home/aruna/kmod/misc.ko 0xffffffffa057e000 -s .data
0xffffffffa0580000 -s .bss 0xffffffffa05802c8

----------gdb
output---------------------------------------------------------------------------------
add symbol table from file "/home/aruna/kmod/misc.ko" at
    .text_addr = 0xffffffffa057e000
    .data_addr = 0xffffffffa0580000
    .bss_addr = 0xffffffffa05802c8
(y or n) y
Reading symbols from /home/aruna/kmod/misc.ko...done.

(gdb) b hello_write

...and it triggers when I echo "testing123" > /dev/aruna

Breakpoint 2 at 0xffffffffa057e050: file /home/aruna/kmod/misc.c, line 29.
----end--gdb
output---------------------------------------------------------------------------------

Thank you very much for your help I am extremely grateful, without your
guidance I would have not figured things out :)

Aruna
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