Cannot insert breakpoint in nullfs code

[Aijaz Baig]

I am trying to get more information about the VFS layer of FreeBSD for the
moment. My setup includes two VMs, one configured as the server (target)
and the other as client (debugger). I used named pipes as a serial port to
establish the connection.

I am trying to debug the nullfs kernel module but I am having a hard time
getting the KGDB to create breakpoints where I want due to the infamous
'cannot access memory at address' error. Below are my steps:

On the target machine:
kldstat gives
Id Refs Address            Size     Name
 1   10 0xffffffff80200000 17e10c8  kernel
 2    1 0xffffffff819e2000 4cf0     vmxnet.ko
 3    1 0xffffffff819e7000 16e0     echo.ko
 4    1 0xffffffff81c11000 23dc     vmmemctl.ko
 5    1 0xffffffff81c14000 641b     nullfs.ko

nm /boot/kernel/nullfs.ko | grep mount
00000000000018f0 t null_getwritemount
0000000000000540 t nullfs_mount
0000000000000930 t nullfs_unmount
                 U vfs_mountedfrom
                 U vop_getwritemount_desc

On the local machine (which connects to the target via a named pipe acting
as a serial console:

(kgdb) tr0
kdb_sysctl_enter (oidp=<value optimized out>, arg1=<value optimized out>,
arg2=0xfffffe004e7cc7f0,
    req=<value optimized out>) at /usr/src/sys/kern/subr_kdb.c:446
446                     kdb_why = KDB_WHY_UNSET;
Current language:  auto; currently minimal
(kgdb) getsyms
During symbol reading, Incomplete CFI data; unspecified registers at
0xffffffff8099497a.
Id Refs Address    Size     Name
 1   10 0x80200000 17e10c8  kernel
 2    1 0x819e2000 4cf0     vmxnet.ko
 3    1 0x819e7000 16e0     echo.ko
 4    1 0x81c11000 23dc     vmmemctl.ko
 5    1 0x81c14000 641b     nullfs.ko
Select the list above with the mouse, paste into the screen
and then press ^D.  Yes, this is annoying.
 5    1 0x81c14000 641b     nullfs.ko
add symbol table from file "/usr/obj/usr/src/sys/AIJAZ-DE
BUG/modules/usr/src/sys/modules/nullfs/nullfs.ko.debug" at
        .text_addr = 0x81c14000
        .data_addr = 0x81c14000
        .bss_addr = 0x81c14000
(kgdb) add-kld nullfs.ko
add symbol table from file "/boot/kernel/nullfs.ko.symbols" at
        .text_addr = 0xffffffff81c14000
        set_sysinit_set_addr = 0xffffffff81c15c90
        set_sysuninit_set_addr = 0xffffffff81c15cb0
        .rodata.str1.1_addr = 0xffffffff81c15cc8
        set_modmetadata_set_addr = 0xffffffff81c15e48
        set_sysctl_set_addr = 0xffffffff81c15e58
        .data_addr = 0xffffffff81c15e60
        .bss_addr = 0xffffffff81c16360
(y or n) y
Reading symbols from /boot/kernel/nullfs.ko.symbols...
location expression too complex...done.

(kgdb) b nullfs_mount
Cannot access memory at address 0x81c14540

As one can see from the output of 'nm' and 'kldstat' above, the addresses
are indeed proper.

I even tried setting a "hardware breakpoint" at the above address
(kgdb) hbreak *0x81c14540
Hardware assisted breakpoint 1 at 0x81c14540: file
/usr/src/sys/modules/nullfs/../../fs/nullfs/null_vfsops.c, line 74.

(kgdb) c
Continuing.
Warning:
Cannot insert breakpoint 1.
Error accessing memory address 0x81c14540: Input/output error.

-- 

Best Regards,
Aijaz Baig