1: # Introduction
2:
3: This HOWTO explains how to set up a test environment for symbolic
4: debugging of the NetBSD kernel using a pair of QEMU virtual machines.
5:
6: ## Prerequisites
7:
8: You need a computer running an OS capable of cross-building NetBSD
9: (the "host system").
10: This can be NetBSD itself, Linux, or some other Unix-like OS.
11: These instructions have been tested with NetBSD/amd64 6.1.4 and
12: Debian 7 hosts. There should be at least 20 gigabytes of available
13: disk space.
14:
15: If your host system is running NetBSD, install the following packages
16: from pkgsrc:
17:
18: * emulators/qemu >= 2.0.0nb4
19: * misc/py-anita
20:
21: If your host system uses a package system other than pkgsrc,
22: use that to install cvs, make, gcc, qemu, the Python pexpect
23: library, and genisoimage or mkisofs. Also download and
24: install the most recent anita package from
25: <http://www.gson.org/netbsd/anita/download/>.
26:
27: ## Building the target system
28:
29: Check out the NetBSD-current sources from CVS and build a full release of
30: NetBSD-current/i386 or /amd64 with debug symbols using the build.sh
31: script. The i386 and amd64 ports are the preferred test platforms because
32: sparc, the only other port supported by anita, is affected by a known bug,
33: [[qemu bug 1399943|https://bugs.launchpad.net/qemu/+bug/1399943]].
34: If you do the build in a directory other than /usr/src,
35: use the -fdebug-prefix-map option to ensure that the source file names embedded
36: in the debug symbols point to /usr/src, which is where the sources will be
37: installed on the target system. For example:
38:
39: [[!template id=programlisting text="""
40: $ CVSROOT=anoncvs@anoncvs.NetBSD.org:/cvsroot cvs checkout -A -P src
41: $ cd src
42: $ ./build.sh -j 4 -V MKDEBUG=YES -V COPTS="-g -fdebug-prefix-map=$(pwd)=/usr/src" -O ../obj -m i386 -U release sourcesets
43: """]]
44:
45: For best performance, change the number after "-j" to the number of CPU cores
46: you have, or slightly more.
47:
48: ## Installing the target system
49:
50: Install the system in a virtual machine, including the debug symbols and source code:
51:
52: [[!template id=programlisting text="""
53: $ cd ..
54: $ anita --workdir work --disk-size 4G --memory-size 256M \
55: --sets kern-GENERIC,modules,base,etc,comp,debug,games,man,misc,tests,text,syssrc,src,sharesrc,gnusrc \
56: install $(pwd)/obj/releasedir/i386/
57: """]]
58:
59: ## Booting the VMs
60:
61: Next, start two qemu virtual machines, one to run the kernel being
62: debugged (the "target VM") and another to run gdb (the "gdb VM").
63:
64: The two VMs could be run on separate physical machines, but in this
65: example, they are run on the same physical machine and share the same
66: hard disk image. This sharing is made possible by the "-snapshot"
67: option to qemu, which ensures that the disk image is not written to by
68: either VM.
69:
70: First start the target VM, enabling qemu's built-in GDB target stub
71: on TCP port 1234:
72:
73: [[!template id=programlisting text="""
74: $ qemu-system-i386 -nographic -snapshot -hda work/wd0.img -gdb tcp::1234
75: """]]
76:
77: If you don't want everyone on the Internet to be able to debug your
78: target, make sure incoming connections on port 1234 are blocked in
79: your firewall.
80:
81: In a second terminal window, start the gdb VM:
82:
83: [[!template id=programlisting text="""
84: $ qemu-system-i386 -nographic -snapshot -hda work/wd0.img
85: """]]
86:
87: Log in to the gdb VM as root and set up the network:
88:
89: [[!template id=programlisting text="""
90: login: root
91: # dhcpcd
92: """]]
93:
94: Start gdb on the gdb VM and connect to the target:
95:
96: [[!template id=programlisting text="""
97: # gdb /netbsd
98: (gdb) target remote my.host.name:1234
99: """]]
100:
101: where my.host.name is the domain name or IP address of the
102: host system.
103:
104: Now you should be able to get a stack trace and start debugging
105: with full debug symbols and access to the source code:
106:
107: [[!template id=programlisting text="""
108: (gdb) where
109: (gdb) list
110: """]]
111:
112: If the stack trace prints very slowly (like 30 seconds per stack
113: frame), it's likely because you are using a version of qemu where
114: the user-mode networking code fails to disable the Nagle algorithm.
115: This is fixed in the qemu in pkgsrc, but you may run into it if your
116: qemu is not installed via pkgsrc.
117:
118: ## Qemu tips
119:
120: Here is a couple of useful qemu commands to know:
121:
122: * Ctrl-a b will send a break which will make the NetBSD VM enter the ddb kernel debugger.
123:
124: * Ctrl-a c will switch to the qemu monitor where you can enter commands like "quit" to exit qemu,
125: or do things like saving/restoring the VM to/from a file.
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