Date: Sun, 21 May 2017 22:30:29 +0530 From: Aijaz Baig <aijazbaig1@gmail.com> To: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Debugging nullfs kernel module - cannot access memory at address Message-ID: <CAHB2L%2BcooPRtsWj5pfL=iNdb8fTpsDjzdVzDfWXcBd1=D1i4JA@mail.gmail.com>
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I am trying to debug the nullfs kernel module so to that end, I do the
following:
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 (I am using virtual machines):
(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-DEBUG/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.
On searching for this error on Linux, it appears that this is taken care of
by turning off CONFIG_DEBUG_RODATA as part of the kernel config (which as
per this:
http://elinux.org/Overwrite_detection_for_kernel_text_and_read-only_data
link appears to be some sort of a protection mechanism which detects when
the text section of the kernel is being overwritten for some reason). This
helps with the setting of software breakpoints which would otherwise be not
set.
Is there something similar for FreeBSD systems as well??
Keen to hear
--
Best Regards,
Aijaz Baig
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