Date: Sat, 27 May 2017 14:05:59 +0530 From: Aijaz Baig <aijazbaig1@gmail.com> To: freebsd-scsi@freebsd.org Subject: Cannot insert breakpoint in nullfs code Message-ID: <CAHB2L%2BcV_i3z6AVnCb0NFPf5fkneqSqJtWO4kQbwpwOm1u_bpQ@mail.gmail.com>
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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
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