About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / BUG-HUNTING




Custom Search

Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.

1	Table of contents
2	=================
3	
4	Last updated: 20 December 2005
5	
6	Contents
7	========
8	
9	- Introduction
10	- Devices not appearing
11	- Finding patch that caused a bug
12	-- Finding using git-bisect
13	-- Finding it the old way
14	- Fixing the bug
15	
16	Introduction
17	============
18	
19	Always try the latest kernel from kernel.org and build from source. If you are
20	not confident in doing that please report the bug to your distribution vendor
21	instead of to a kernel developer.
22	
23	Finding bugs is not always easy. Have a go though. If you can't find it don't
24	give up. Report as much as you have found to the relevant maintainer. See
25	MAINTAINERS for who that is for the subsystem you have worked on.
26	
27	Before you submit a bug report read REPORTING-BUGS.
28	
29	Devices not appearing
30	=====================
31	
32	Often this is caused by udev. Check that first before blaming it on the
33	kernel.
34	
35	Finding patch that caused a bug
36	===============================
37	
38	
39	
40	Finding using git-bisect
41	------------------------
42	
43	Using the provided tools with git makes finding bugs easy provided the bug is
44	reproducible.
45	
46	Steps to do it:
47	- start using git for the kernel source
48	- read the man page for git-bisect
49	- have fun
50	
51	Finding it the old way
52	----------------------
53	
54	[Sat Mar  2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)]
55	
56	This is how to track down a bug if you know nothing about kernel hacking.
57	It's a brute force approach but it works pretty well.
58	
59	You need:
60	
61	        . A reproducible bug - it has to happen predictably (sorry)
62	        . All the kernel tar files from a revision that worked to the
63	          revision that doesn't
64	
65	You will then do:
66	
67	        . Rebuild a revision that you believe works, install, and verify that.
68	        . Do a binary search over the kernels to figure out which one
69	          introduced the bug.  I.e., suppose 1.3.28 didn't have the bug, but
70	          you know that 1.3.69 does.  Pick a kernel in the middle and build
71	          that, like 1.3.50.  Build & test; if it works, pick the mid point
72	          between .50 and .69, else the mid point between .28 and .50.
73	        . You'll narrow it down to the kernel that introduced the bug.  You
74	          can probably do better than this but it gets tricky.
75	
76	        . Narrow it down to a subdirectory
77	
78	          - Copy kernel that works into "test".  Let's say that 3.62 works,
79	            but 3.63 doesn't.  So you diff -r those two kernels and come
80	            up with a list of directories that changed.  For each of those
81	            directories:
82	
83	                Copy the non-working directory next to the working directory
84	                as "dir.63".
85	                One directory at time, try moving the working directory to
86	                "dir.62" and mv dir.63 dir"time, try
87	
88	                        mv dir dir.62
89	                        mv dir.63 dir
90	                        find dir -name '*.[oa]' -print | xargs rm -f
91	
92	                And then rebuild and retest.  Assuming that all related
93	                changes were contained in the sub directory, this should
94	                isolate the change to a directory.
95	
96	                Problems: changes in header files may have occurred; I've
97	                found in my case that they were self explanatory - you may
98	                or may not want to give up when that happens.
99	
100	        . Narrow it down to a file
101	
102	          - You can apply the same technique to each file in the directory,
103	            hoping that the changes in that file are self contained.
104	
105	        . Narrow it down to a routine
106	
107	          - You can take the old file and the new file and manually create
108	            a merged file that has
109	
110	                #ifdef VER62
111	                routine()
112	                {
113	                        ...
114	                }
115	                #else
116	                routine()
117	                {
118	                        ...
119	                }
120	                #endif
121	
122	            And then walk through that file, one routine at a time and
123	            prefix it with
124	
125	                #define VER62
126	                /* both routines here */
127	                #undef VER62
128	
129	            Then recompile, retest, move the ifdefs until you find the one
130	            that makes the difference.
131	
132	Finally, you take all the info that you have, kernel revisions, bug
133	description, the extent to which you have narrowed it down, and pass
134	that off to whomever you believe is the maintainer of that section.
135	A post to linux.dev.kernel isn't such a bad idea if you've done some
136	work to narrow it down.
137	
138	If you get it down to a routine, you'll probably get a fix in 24 hours.
139	
140	My apologies to Linus and the other kernel hackers for describing this
141	brute force approach, it's hardly what a kernel hacker would do.  However,
142	it does work and it lets non-hackers help fix bugs.  And it is cool
143	because Linux snapshots will let you do this - something that you can't
144	do with vendor supplied releases.
145	
146	Fixing the bug
147	==============
148	
149	Nobody is going to tell you how to fix bugs. Seriously. You need to work it
150	out. But below are some hints on how to use the tools.
151	
152	To debug a kernel, use objdump and look for the hex offset from the crash
153	output to find the valid line of code/assembler. Without debug symbols, you
154	will see the assembler code for the routine shown, but if your kernel has
155	debug symbols the C code will also be available. (Debug symbols can be enabled
156	in the kernel hacking menu of the menu configuration.) For example:
157	
158	    objdump -r -S -l --disassemble net/dccp/ipv4.o
159	
160	NB.: you need to be at the top level of the kernel tree for this to pick up
161	your C files.
162	
163	If you don't have access to the code you can also debug on some crash dumps
164	e.g. crash dump output as shown by Dave Miller.
165	
166	>    EIP is at ip_queue_xmit+0x14/0x4c0
167	>     ...
168	>    Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
169	>    00 00 55 57  56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
170	>    <8b> 83 3c 01 00 00 89 44  24 14 8b 45 28 85 c0 89 44 24 18 0f 85
171	>
172	>    Put the bytes into a "foo.s" file like this:
173	>
174	>           .text
175	>           .globl foo
176	>    foo:
177	>           .byte  .... /* bytes from Code: part of OOPS dump */
178	>
179	>    Compile it with "gcc -c -o foo.o foo.s" then look at the output of
180	>    "objdump --disassemble foo.o".
181	>
182	>    Output:
183	>
184	>    ip_queue_xmit:
185	>        push       %ebp
186	>        push       %edi
187	>        push       %esi
188	>        push       %ebx
189	>        sub        $0xbc, %esp
190	>        mov        0xd0(%esp), %ebp        ! %ebp = arg0 (skb)
191	>        mov        0x8(%ebp), %ebx         ! %ebx = skb->sk
192	>        mov        0x13c(%ebx), %eax       ! %eax = inet_sk(sk)->opt
193	
194	In addition, you can use GDB to figure out the exact file and line
195	number of the OOPS from the vmlinux file. If you have
196	CONFIG_DEBUG_INFO enabled, you can simply copy the EIP value from the
197	OOPS:
198	
199	 EIP:    0060:[<c021e50e>]    Not tainted VLI
200	
201	And use GDB to translate that to human-readable form:
202	
203	  gdb vmlinux
204	  (gdb) l *0xc021e50e
205	
206	If you don't have CONFIG_DEBUG_INFO enabled, you use the function
207	offset from the OOPS:
208	
209	 EIP is at vt_ioctl+0xda8/0x1482
210	
211	And recompile the kernel with CONFIG_DEBUG_INFO enabled:
212	
213	  make vmlinux
214	  gdb vmlinux
215	  (gdb) p vt_ioctl
216	  (gdb) l *(0x<address of vt_ioctl> + 0xda8)
217	or, as one command
218	  (gdb) l *(vt_ioctl + 0xda8)
219	
220	If you have a call trace, such as :-
221	>Call Trace:
222	> [<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
223	> [<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
224	> [<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
225	> ...
226	this shows the problem in the :jbd: module. You can load that module in gdb
227	and list the relevant code.
228	  gdb fs/jbd/jbd.ko
229	  (gdb) p log_wait_commit
230	  (gdb) l *(0x<address> + 0xa3)
231	or
232	  (gdb) l *(log_wait_commit + 0xa3)
233	
234	
235	Another very useful option of the Kernel Hacking section in menuconfig is
236	Debug memory allocations. This will help you see whether data has been
237	initialised and not set before use etc. To see the values that get assigned
238	with this look at mm/slab.c and search for POISON_INUSE. When using this an
239	Oops will often show the poisoned data instead of zero which is the default.
240	
241	Once you have worked out a fix please submit it upstream. After all open
242	source is about sharing what you do and don't you want to be recognised for
243	your genius?
244	
245	Please do read Documentation/SubmittingPatches though to help your code get
246	accepted.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.