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Documentation / oops-tracing.txt

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Based on kernel version 4.9. Page generated on 2016-12-21 14:36 EST.

1	NOTE: ksymoops is useless on 2.6.  Please use the Oops in its original format
2	(from dmesg, etc).  Ignore any references in this or other docs to "decoding
3	the Oops" or "running it through ksymoops".  If you post an Oops from 2.6 that
4	has been run through ksymoops, people will just tell you to repost it.
6	Quick Summary
7	-------------
9	Find the Oops and send it to the maintainer of the kernel area that seems to be
10	involved with the problem.  Don't worry too much about getting the wrong person.
11	If you are unsure send it to the person responsible for the code relevant to
12	what you were doing.  If it occurs repeatably try and describe how to recreate
13	it.  That's worth even more than the oops.
15	If you are totally stumped as to whom to send the report, send it to 
16	linux-kernel@vger.kernel.org. Thanks for your help in making Linux as
17	stable as humanly possible.
19	Where is the Oops?
20	----------------------
22	Normally the Oops text is read from the kernel buffers by klogd and
23	handed to syslogd which writes it to a syslog file, typically
24	/var/log/messages (depends on /etc/syslog.conf).  Sometimes klogd dies,
25	in which case you can run dmesg > file to read the data from the kernel
26	buffers and save it.  Or you can cat /proc/kmsg > file, however you
27	have to break in to stop the transfer, kmsg is a "never ending file".
28	If the machine has crashed so badly that you cannot enter commands or
29	the disk is not available then you have three options :-
31	(1) Hand copy the text from the screen and type it in after the machine
32	    has restarted.  Messy but it is the only option if you have not
33	    planned for a crash. Alternatively, you can take a picture of
34	    the screen with a digital camera - not nice, but better than
35	    nothing.  If the messages scroll off the top of the console, you
36	    may find that booting with a higher resolution (eg, vga=791)
37	    will allow you to read more of the text. (Caveat: This needs vesafb,
38	    so won't help for 'early' oopses)
40	(2) Boot with a serial console (see Documentation/serial-console.txt),
41	    run a null modem to a second machine and capture the output there
42	    using your favourite communication program.  Minicom works well.
44	(3) Use Kdump (see Documentation/kdump/kdump.txt),
45	    extract the kernel ring buffer from old memory with using dmesg
46	    gdbmacro in Documentation/kdump/gdbmacros.txt.
49	Full Information
50	----------------
52	NOTE: the message from Linus below applies to 2.4 kernel.  I have preserved it
53	for historical reasons, and because some of the information in it still
54	applies.  Especially, please ignore any references to ksymoops. 
56	From: Linus Torvalds <torvalds@osdl.org>
58	How to track down an Oops.. [originally a mail to linux-kernel]
60	The main trick is having 5 years of experience with those pesky oops 
61	messages ;-)
63	Actually, there are things you can do that make this easier. I have two 
64	separate approaches:
66		gdb /usr/src/linux/vmlinux
67		gdb> disassemble <offending_function>
69	That's the easy way to find the problem, at least if the bug-report is 
70	well made (like this one was - run through ksymoops to get the 
71	information of which function and the offset in the function that it 
72	happened in).
74	Oh, it helps if the report happens on a kernel that is compiled with the 
75	same compiler and similar setups.
77	The other thing to do is disassemble the "Code:" part of the bug report: 
78	ksymoops will do this too with the correct tools, but if you don't have
79	the tools you can just do a silly program:
81		char str[] = "\xXX\xXX\xXX...";
82		main(){}
84	and compile it with gcc -g and then do "disassemble str" (where the "XX" 
85	stuff are the values reported by the Oops - you can just cut-and-paste 
86	and do a replace of spaces to "\x" - that's what I do, as I'm too lazy 
87	to write a program to automate this all).
89	Alternatively, you can use the shell script in scripts/decodecode.
90	Its usage is:  decodecode < oops.txt
92	The hex bytes that follow "Code:" may (in some architectures) have a series
93	of bytes that precede the current instruction pointer as well as bytes at and
94	following the current instruction pointer.  In some cases, one instruction
95	byte or word is surrounded by <> or (), as in "<86>" or "(f00d)".  These
96	<> or () markings indicate the current instruction pointer.  Example from
97	i386, split into multiple lines for readability:
99	Code: f9 0f 8d f9 00 00 00 8d 42 0c e8 dd 26 11 c7 a1 60 ea 2b f9 8b 50 08 a1
100	64 ea 2b f9 8d 34 82 8b 1e 85 db 74 6d 8b 15 60 ea 2b f9 <8b> 43 04 39 42 54
101	7e 04 40 89 42 54 8b 43 04 3b 05 00 f6 52 c0
103	Finally, if you want to see where the code comes from, you can do
105		cd /usr/src/linux
106		make fs/buffer.s 	# or whatever file the bug happened in
108	and then you get a better idea of what happens than with the gdb 
109	disassembly.
111	Now, the trick is just then to combine all the data you have: the C 
112	sources (and general knowledge of what it _should_ do), the assembly 
113	listing and the code disassembly (and additionally the register dump you 
114	also get from the "oops" message - that can be useful to see _what_ the 
115	corrupted pointers were, and when you have the assembler listing you can 
116	also match the other registers to whatever C expressions they were used 
117	for).
119	Essentially, you just look at what doesn't match (in this case it was the 
120	"Code" disassembly that didn't match with what the compiler generated). 
121	Then you need to find out _why_ they don't match. Often it's simple - you 
122	see that the code uses a NULL pointer and then you look at the code and 
123	wonder how the NULL pointer got there, and if it's a valid thing to do 
124	you just check against it..
126	Now, if somebody gets the idea that this is time-consuming and requires 
127	some small amount of concentration, you're right. Which is why I will 
128	mostly just ignore any panic reports that don't have the symbol table 
129	info etc looked up: it simply gets too hard to look it up (I have some 
130	programs to search for specific patterns in the kernel code segment, and 
131	sometimes I have been able to look up those kinds of panics too, but 
132	that really requires pretty good knowledge of the kernel just to be able 
133	to pick out the right sequences etc..)
135	_Sometimes_ it happens that I just see the disassembled code sequence 
136	from the panic, and I know immediately where it's coming from. That's when 
137	I get worried that I've been doing this for too long ;-)
139			Linus
142	---------------------------------------------------------------------------
143	Notes on Oops tracing with klogd:
145	In order to help Linus and the other kernel developers there has been
146	substantial support incorporated into klogd for processing protection
147	faults.  In order to have full support for address resolution at least
148	version 1.3-pl3 of the sysklogd package should be used.
150	When a protection fault occurs the klogd daemon automatically
151	translates important addresses in the kernel log messages to their
152	symbolic equivalents.  This translated kernel message is then
153	forwarded through whatever reporting mechanism klogd is using.  The
154	protection fault message can be simply cut out of the message files
155	and forwarded to the kernel developers.
157	Two types of address resolution are performed by klogd.  The first is
158	static translation and the second is dynamic translation.  Static
159	translation uses the System.map file in much the same manner that
160	ksymoops does.  In order to do static translation the klogd daemon
161	must be able to find a system map file at daemon initialization time.
162	See the klogd man page for information on how klogd searches for map
163	files.
165	Dynamic address translation is important when kernel loadable modules
166	are being used.  Since memory for kernel modules is allocated from the
167	kernel's dynamic memory pools there are no fixed locations for either
168	the start of the module or for functions and symbols in the module.
170	The kernel supports system calls which allow a program to determine
171	which modules are loaded and their location in memory.  Using these
172	system calls the klogd daemon builds a symbol table which can be used
173	to debug a protection fault which occurs in a loadable kernel module.
175	At the very minimum klogd will provide the name of the module which
176	generated the protection fault.  There may be additional symbolic
177	information available if the developer of the loadable module chose to
178	export symbol information from the module.
180	Since the kernel module environment can be dynamic there must be a
181	mechanism for notifying the klogd daemon when a change in module
182	environment occurs.  There are command line options available which
183	allow klogd to signal the currently executing daemon that symbol
184	information should be refreshed.  See the klogd manual page for more
185	information.
187	A patch is included with the sysklogd distribution which modifies the
188	modules-2.0.0 package to automatically signal klogd whenever a module
189	is loaded or unloaded.  Applying this patch provides essentially
190	seamless support for debugging protection faults which occur with
191	kernel loadable modules.
193	The following is an example of a protection fault in a loadable module
194	processed by klogd:
195	---------------------------------------------------------------------------
196	Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
197	Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
198	Aug 29 09:51:01 blizard kernel: *pde = 00000000
199	Aug 29 09:51:01 blizard kernel: Oops: 0002
200	Aug 29 09:51:01 blizard kernel: CPU:    0
201	Aug 29 09:51:01 blizard kernel: EIP:    0010:[oops:_oops+16/3868]
202	Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
203	Aug 29 09:51:01 blizard kernel: eax: 315e97cc   ebx: 003a6f80   ecx: 001be77b   edx: 00237c0c
204	Aug 29 09:51:01 blizard kernel: esi: 00000000   edi: bffffdb3   ebp: 00589f90   esp: 00589f8c
205	Aug 29 09:51:01 blizard kernel: ds: 0018   es: 0018   fs: 002b   gs: 002b   ss: 0018
206	Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
207	Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001 
208	Aug 29 09:51:01 blizard kernel:        00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00 
209	Aug 29 09:51:01 blizard kernel:        bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036 
210	Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128] 
211	Aug 29 09:51:01 blizard kernel: Code: c7 00 05 00 00 00 eb 08 90 90 90 90 90 90 90 90 89 ec 5d c3 
212	---------------------------------------------------------------------------
214	Dr. G.W. Wettstein           Oncology Research Div. Computing Facility
215	Roger Maris Cancer Center    INTERNET: greg@wind.rmcc.com
216	820 4th St. N.
217	Fargo, ND  58122
218	Phone: 701-234-7556
221	---------------------------------------------------------------------------
222	Tainted kernels:
224	Some oops reports contain the string 'Tainted: ' after the program
225	counter. This indicates that the kernel has been tainted by some
226	mechanism.  The string is followed by a series of position-sensitive
227	characters, each representing a particular tainted value.
229	  1: 'G' if all modules loaded have a GPL or compatible license, 'P' if
230	     any proprietary module has been loaded.  Modules without a
231	     MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
232	     insmod as GPL compatible are assumed to be proprietary.
234	  2: 'F' if any module was force loaded by "insmod -f", ' ' if all
235	     modules were loaded normally.
237	  3: 'S' if the oops occurred on an SMP kernel running on hardware that
238	     hasn't been certified as safe to run multiprocessor.
239	     Currently this occurs only on various Athlons that are not
240	     SMP capable.
242	  4: 'R' if a module was force unloaded by "rmmod -f", ' ' if all
243	     modules were unloaded normally.
245	  5: 'M' if any processor has reported a Machine Check Exception,
246	     ' ' if no Machine Check Exceptions have occurred.
248	  6: 'B' if a page-release function has found a bad page reference or
249	     some unexpected page flags.
251	  7: 'U' if a user or user application specifically requested that the
252	     Tainted flag be set, ' ' otherwise.
254	  8: 'D' if the kernel has died recently, i.e. there was an OOPS or BUG.
256	  9: 'A' if the ACPI table has been overridden.
258	 10: 'W' if a warning has previously been issued by the kernel.
259	     (Though some warnings may set more specific taint flags.)
261	 11: 'C' if a staging driver has been loaded.
263	 12: 'I' if the kernel is working around a severe bug in the platform
264	     firmware (BIOS or similar).
266	 13: 'O' if an externally-built ("out-of-tree") module has been loaded.
268	 14: 'E' if an unsigned module has been loaded in a kernel supporting
269	     module signature.
271	 15: 'L' if a soft lockup has previously occurred on the system.
273	 16: 'K' if the kernel has been live patched.
275	The primary reason for the 'Tainted: ' string is to tell kernel
276	debuggers if this is a clean kernel or if anything unusual has
277	occurred.  Tainting is permanent: even if an offending module is
278	unloaded, the tainted value remains to indicate that the kernel is not
279	trustworthy.
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