Date: Mon, 5 May 2014 13:33:49 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-virtualization@freebsd.org Cc: Kurt Lidl <lidl@pix.net> Subject: Re: Problems with bhyve's kgdb support and loadable modules Message-ID: <201405051333.49758.jhb@freebsd.org> In-Reply-To: <5365AD6C.5060107@freebsd.org> References: <5363F399.40709@pix.net> <5365AD6C.5060107@freebsd.org>
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On Saturday, May 03, 2014 11:01:00 pm Peter Grehan 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@fbsd9-2:~ # kldload if_tap > root@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@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. I think you want to set solib-absolute-prefix to '/mnt'. That will cause kgdb to prepend '/mnt' to the paths it finds for each kld. Mounting /mnt directly on the host is a bit risky though. What I've done with bhyve is to build a test kernel on the host and install it to a directory on the host, then use the '-h' flag to bhyveload to export that directory to the loader and explicitly load the test kernel from the 'host0:' filesystem. I've even been able to build a cross kgdb-i386 to debug a cross-built kernel for an i386 guest and have this work ok. -- John Baldwin
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