Date: Sat, 03 May 2014 20:01:00 -0700 From: Peter Grehan <grehan@freebsd.org> To: Kurt Lidl <lidl@pix.net> Cc: freebsd-virtualization@freebsd.org Subject: Re: Problems with bhyve's kgdb support and loadable modules Message-ID: <5365AD6C.5060107@freebsd.org> In-Reply-To: <5363F399.40709@pix.net> References: <5363F399.40709@pix.net>
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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. later, Peter.
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