Problems with bhyve's kgdb support and loadable modules

[Michael Berman]

On 5/3/14, 8:01 PM, "Peter Grehan" <grehan at freebsd.org> wrote:

>Hi Kurt,
>
>> Is there any reasonable tutorial for using kgdb with the
>> bvmdebug kernel option?
>
>  Not really (https://wiki.freebsd.org/BHyVe/gdb)
>
>  In any event, 9.2 doesn't have bvmdebug, though it would be a simple
>backport. It's also not strictly required - you can use the serial port
>same as on h/w; see below.
>
>> A couple of folks I know have run into issues trying to
>> debug a FreeBSD stable/9 kernel from their bhyve
>> hosting machine (running stable/10).
>>
>> In particular, the loadable modules that are in use in
>> the stable/9 kernel are being "troublesome" to get to
>> the point where source-level debugging actually works.
>>
>> Even a pointer to a couple of "worked" examples might be
>> useful.
>>
>> I've read this:
>> http://people.freebsd.org/~jhb/papers/bsdcan/2008/article/node4.html
>> but not all the techniques in there appear to work properly.
>
>  I tried to repro this with some success from a host running CURRENT.
>
>  Firstly, I installed a 9.2 VM, with source. I edited GENERIC and added
>options DDB and GDB, and reinstalled the kernel.
>
>  The disk was then copied, and mdconfig'd/mounted on the host to
>provide access to the just-buit 9.2 kernel syms and sources.
>
>  com2 was set up as a debug port by dropping to the bhyveload prompt and
>
>    hint.uart.1.flags="0x80"
>
>  (this could also have been done in the guest's /boot/loader.conf)
>
>  com2 was then set up in the bhyve command line to point to an nmdm
>device
>
>   -l com2,/dev/nmdm0A
>
>  The guest probed uart1 as a debug port:
>
>uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 flags 0x80 on acpi0
>ioapic0: routing intpin 3 (ISA IRQ 3) to lapic 0 vector 64
>uart1: fast interrupt
>uart1: debug port (9600,n,8,1)
>
>  After booting, I loaded the the tap device in the guest to provide a
>kld for kgdb to examine:
>
>root at fbsd9-2:~ # kldload if_tap
>root at fbsd9-2:~ # kldstat
>Id Refs Address            Size     Name
>  1    3 0xffffffff80200000 15f92d8  kernel
>  2    1 0xffffffff81a12000 59e9     if_tap.ko
>
>  Now time to try kgdb:
>
>root at fbsd9-2:~ # sysctl debug.kdb.enter=1
>debug.kdb.enter: 0KDB: enter: sysctl debug.kdb.enter
>[ thread pid 577 tid 100054 ]
>Stopped at      kdb_enter+0x3b: movq    $0,0xaf0362(%rip)
>db> gdb
>(ctrl-c will return control to ddb)
>Switching to gdb back-end
>Switching to gdb back-end
>
>  In another window, cd'd to the mounted copy of the 9.2 disk:
>
>kgdb -r /dev/nmdm0B kernel.debug
>...
>This GDB was configured as "amd64-marcel-freebsd"...Switching to remote
>protocol
>kdb_enter (why=0xffffffff80fbf671 "sysctl", msg=0x80 <Address 0x80 out
>of bounds>) at ../../../kern/subr_kdb.c:441
>441			kdb_why = KDB_WHY_UNSET;
>
>(kgdb)
>
>  After some experimentation, I found the way to get the correct symbols
>for the kld was to manually specify it:
>
>(kgdb) add-kld /mnt/boot/kernel/if_tap.ko
>add symbol table from file "/mnt/boot/kernel/if_tap.ko.symbols" at
>	.text_addr = 0xffffffff81a12000
>	.rodata.str1.8_addr = 0xffffffff81a13b50
>	.rodata.str1.1_addr = 0xffffffff81a13ddb
>	set_sysinit_set_addr = 0xffffffff81a13f68
>	set_modmetadata_set_addr = 0xffffffff81a13f80
>	set_sysctl_set_addr = 0xffffffff81a13f90
>	set_sysuninit_set_addr = 0xffffffff81a13fc0
>	.data_addr = 0xffffffff81a13fe0
>	.bss_addr = 0xffffffff81a14620
>(y or n) y
>
>  However, 'info sharedlibrary' didn't seem to reflect this:
>
>(kgdb) info sharedlibrary
> From                To                  Syms Read   Shared Object Library
>0xffffffff81a12000  0xffffffff81a13c04  No
>/boot/kernel/if_tap.ko.symbols
>
>  Might have been a bug there, since I was able to successfully set
>breakpoints in if_tap routines and have them trigger.
>
>  I did have some trouble getting the source path set up correctly, but
>never fully investigated that: seems like gdb is quite rich in that area
>and it should be possible to get sorted.
>
>later,
>
>Peter.

I¹ve had good results with gdb a few of different ways, first as Peter
described, but also using a .gdbinit file to set the substitute-path.

For example:
set substitute-path /usr/home/michael/src /usr/src

Another nice thing about the bhyve diskdev is that you can mount it,
here¹s a Bourne shell script snippet:
md=`sudo mdconfig -a -t vnode -f $diskdev`
        sudo mkdir -p /mnt/ts_guest
        sudo fsck -y -t ufs /dev/${md}p2
        sudo mount -t ufs /dev/${md}p2 /mnt/ts_guest


This means that with some careful gimmicks one can mount the guest such
that its source tree is at a useful place.

Depending on how/when you do this, I recommend a read only mount ;-)


Michael