2 The purpose of this document is to introduce the reader with vkernel debugging.
3 The vkernel architecture allows us to run DragonFly kernels in userland. These virtual
4 kernels can be paniced or otherwise abused, without affecting the host operating system.
6 To make things a bit more interesting, we will use a real life example.
9 ... I wrote a simple program that used the AIO interface. As it turned out we don't support
10 this feature, but at that point I didn't know.
12 $ gcc t_aio.c -o t_aio -Wall -ansi -pedantic
14 aio_read: Function not implemented
17 Ktrace'ing the process and seeing with my own eyes what was going on, seemed like a good idea.
18 Here comes the fun. I misread the [ktrace(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=ktrace§ion=1) man page and typed:
24 (My intention was to track the system calls of t_aio, but what I typed would actually disable all traces from all process to the t_aio file.)
27 To setup a vkernel, please consult this [man page](http://leaf.dragonflybsd.org/cgi/web-man?command=vkernel§ion=ANY).
28 It's very straightforward.
30 # Reproduce the problem
31 We boot into our vkernel:
34 # ./boot/kernel -m 64m -r rootimg.01 -I auto:bridge0
38 And then try to reproduce the system freeze:
42 Fatal trap 12: page fault while in kernel mode
43 mp_lock = 00000001; cpuid = 1
44 fault virtual address = 0x0
45 fault code = supervisor read, page not present
46 instruction pointer = 0x1f:0x80aca52
47 stack pointer = 0x10:0x5709d914
48 frame pointer = 0x10:0x5709dbe0
49 processor eflags = interrupt enabled, resume, IOPL = 0
50 current process = 692 (ktrace)
51 current thread = pri 6
53 kernel: type 12 trap, code=4
55 CPU1 stopping CPUs: 0x00000001
57 Stopped at 0x80aca52: movl 0(%eax),%eax
60 This db> prompt is from [ddb(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=ddb§ion=4), the interactive kernel debugger.
63 fault virtual address = 0x0
65 field is indicative of a NULL pointer dereference inside the kernel.
67 Let's get a trace of what went wrong:
70 ktrdestroy(57082700,5709dc5c,0,57082700,5709dca0) at 0x80aca52
71 allproc_scan(80aca14,5709dc5c,be,2,0) at 0x80b2e91
72 sys_ktrace(5709dca0,6,0,0,57082700) at 0x80acffe
73 syscall2(5709dd40,6,57082700,0,0) at 0x8214b6d
74 user_trap(5709dd40,570940e8,8214185,0,8215462) at 0x8214d9c
75 go_user(5709dd38,0,0,7b,0) at 0x82151ac
78 sys_ktrace, allproc_scan, etc represent the function names. The hex values in parentheses are the first five items on the stack.
79 The last hex value is the instruction address. Since ddb doesn't really know how many arguments a function takes, it always prints five.
83 Quoting from [vkernel(7)](http://leaf.dragonflybsd.org/cgi/web-man?command=vkernel§ion=7):
85 It is possible to directly gdb the virtual kernel's process. It is recommended that you do a `handle SIGSEGV noprint' to ignore page faults processed by the virtual kernel itself and `handle SIGUSR1 noprint' to ignore signals used for simulating inter-processor interrupts (SMP build only).
87 You can add these two commands in your ~/.gdbinit to save yourself from typing them again and again.
90 handle SIGSEGV noprint
91 handle SIGUSR1 noprint
93 So we are going to attach to the vkernel process:
95 # ps aux | grep kernel
96 root 25408 0.0 2.3 1053376 17772 p0 IL+ 8:32PM 0:06.51 ./boot/kernel -m 64m -r rootimg.01 -I auto:bridge0
101 Let's get a trace from inside gdb:
104 #0 0x282d60d0 in read () from /usr/lib/libc.so.6
105 #1 0x2828389f in read () from /usr/lib/libthread_xu.so.2
106 #2 0x0821cd86 in vconsgetc (private=0x56758168) at /usr/src/sys/platform/vkernel/platform/console.c:373
107 #3 0x080e431d in cngetc () at /usr/src/sys/kern/tty_cons.c:482
108 #4 0x080813d0 in db_readline (lstart=0x82806a0 "", lsize=120) at /usr/src/sys/ddb/db_input.c:314
109 #5 0x08081c43 in db_read_line () at /usr/src/sys/ddb/db_lex.c:55
110 #6 0x080804ff in db_command_loop () at /usr/src/sys/ddb/db_command.c:467
111 #7 0x08082ef8 in db_trap (type=12, code=4) at /usr/src/sys/ddb/db_trap.c:71
112 #8 0x082125aa in kdb_trap (type=12, code=4, regs=0x5746c8cc) at /usr/src/sys/platform/vkernel/i386/db_interface.c:151
113 #9 0x082143e1 in trap_fatal (frame=0x5746c8cc, usermode=<value optimized out>, eva=0)
114 at /usr/src/sys/platform/vkernel/i386/trap.c:1031
115 #10 0x0821453e in trap_pfault (frame=0x5746c8cc, usermode=0, eva=0) at /usr/src/sys/platform/vkernel/i386/trap.c:948
116 #11 0x0821468d in kern_trap (frame=0x5746c8cc) at /usr/src/sys/platform/vkernel/i386/trap.c:709
117 #12 0x0821528c in exc_segfault (signo=11, info=0x5746cb98, ctxp=0x5746c8b8)
118 at /usr/src/sys/platform/vkernel/i386/exception.c:181
119 #13 <signal handler called>
120 #14 0x080aca52 in ktrace_clear_callback (p=0x567480c0, data=0x5746cc5c) at /usr/src/sys/kern/kern_ktrace.c:347
121 #15 0x080b2e91 in allproc_scan (callback=0x80aca14 <ktrace_clear_callback>, data=0x5746cc5c)
122 at /usr/src/sys/kern/kern_proc.c:533
123 #16 0x080acffe in sys_ktrace (uap=0x5746cca0) at /usr/src/sys/kern/kern_ktrace.c:276
124 #17 0x08214b6d in syscall2 (frame=0x5746cd40) at /usr/src/sys/platform/vkernel/i386/trap.c:1273
125 #18 0x08214d9c in user_trap (frame=0x5746cd40) at /usr/src/sys/platform/vkernel/i386/trap.c:413
126 #19 0x082151ac in go_user (frame=0x5746cd38) at /usr/src/sys/platform/vkernel/i386/trap.c:1473
127 #20 0x08215462 in pmsg4 () at /usr/src/sys/platform/vkernel/i386/fork_tramp.s:103
130 At this point we can examine the data of various variables. Keep in mind that bare addresses must be cast to the respective data type, prior to accessing. E.g.:
132 (gdb) print ((struct proc *)0x567480c0)->p_pid
137 Let's try this time to break into the kernel _before_ it crashes. sys_ktrace() seems like a good candidate.
142 (gdb) break sys_ktrace
143 Breakpoint 1 at 0x80acf43: file ./machine/thread.h, line 83.
146 Next we type 'c' in the gdb prompt to resume vkernel execution:
151 Now we go to our vkernel and type the offending command:
155 Gdb stops the execution of vkernel and a message pops up in gdb buffer:
157 Breakpoint 1, sys_ktrace (uap=0x573e2ca0) at ./machine/thread.h:83
158 83 __asm ("movl %%fs:globaldata,%0" : "=r" (gd) : "m"(__mycpu__dummy));
161 We navigate through source code with the 'step' and 'next' gdb commands. They are identical, except that 'step' follows function calls. When we meet this call:
163 276 allproc_scan(ktrace_clear_callback, &info);
165 we 'step' inside it. alloproc_scan() iterates through the process list and applies the ktrace_clear_callback() to each one of them.
168 347 if (p->p_tracenode->kn_vp == info->tracenode->kn_vp) {
170 Here p is a pointer to the current process:
173 $1 = (struct proc *) 0x57098c00
175 Let's see if this process is traced:
177 (gdb) print p->p_tracenode
178 $2 = (struct ktrace_node *) 0x0
181 Oops. There is no trace to a vnode for this process. The code will try to access p->p_tracenode and is bound to crash. This is the zero virtual address we saw before.
186 #0 0x282d4c10 in sigsuspend () from /usr/lib/libc.so.6
187 #1 0x28287eb2 in sigsuspend () from /usr/lib/libthread_xu.so.2
188 #2 0x0821530a in stopsig (nada=24, info=0x40407d2c, ctxp=0x40407a4c) at /usr/src/sys/platform/vkernel/i386/exception.c:112
189 #3 <signal handler called>
190 #4 0x282d4690 in umtx_sleep () from /usr/lib/libc.so.6
191 #5 0x08213bde in cpu_idle () at /usr/src/sys/platform/vkernel/i386/cpu_regs.c:722
192 #6 0x00000000 in ?? ()
195 Why does it differ from the ddb's trace ?
196 Well, when the vkernel is sitting at a db> prompt all vkernel threads representing virtual cpu's except the one handling the db> prompt itself will be suspended in stopsig(). The backtrace only sees one of the N threads.