nfsd4: allow 4.0 clients to change callback path

[safe/jmp/linux-2.6] / Documentation / BUG-HUNTING

diff --git a/Documentation/BUG-HUNTING b/Documentation/BUG-HUNTING
index ca29242..65022a8 100644 (file)
--- a/Documentation/BUG-HUNTING
+++ b/Documentation/BUG-HUNTING
@@ -1,6 +1,59 @@
+Table of contents
+=================
+
+Last updated: 20 December 2005
+
+Contents
+========
+
+- Introduction
+- Devices not appearing
+- Finding patch that caused a bug
+-- Finding using git-bisect
+-- Finding it the old way
+- Fixing the bug
+
+Introduction
+============
+
+Always try the latest kernel from kernel.org and build from source. If you are
+not confident in doing that please report the bug to your distribution vendor
+instead of to a kernel developer.
+
+Finding bugs is not always easy. Have a go though. If you can't find it don't
+give up. Report as much as you have found to the relevant maintainer. See
+MAINTAINERS for who that is for the subsystem you have worked on.
+
+Before you submit a bug report read REPORTING-BUGS.
+
+Devices not appearing
+=====================
+
+Often this is caused by udev. Check that first before blaming it on the
+kernel.
+
+Finding patch that caused a bug
+===============================
+
+
+
+Finding using git-bisect
+------------------------
+
+Using the provided tools with git makes finding bugs easy provided the bug is
+reproducible.
+
+Steps to do it:
+- start using git for the kernel source
+- read the man page for git-bisect
+- have fun
+
+Finding it the old way
+----------------------
+

 [Sat Mar  2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)]
 
 [Sat Mar  2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)]
 

-This is how to track down a bug if you know nothing about kernel hacking.  
+This is how to track down a bug if you know nothing about kernel hacking.

 It's a brute force approach but it works pretty well.
 
 You need:
 It's a brute force approach but it works pretty well.
 
 You need:


@@ -13,12 +66,12 @@ You will then do:
 
         . Rebuild a revision that you believe works, install, and verify that.
         . Do a binary search over the kernels to figure out which one
 
         . Rebuild a revision that you believe works, install, and verify that.
         . Do a binary search over the kernels to figure out which one

-          introduced the bug.  I.e., suppose 1.3.28 didn't have the bug, but 
+          introduced the bug.  I.e., suppose 1.3.28 didn't have the bug, but

           you know that 1.3.69 does.  Pick a kernel in the middle and build
           that, like 1.3.50.  Build & test; if it works, pick the mid point
           between .50 and .69, else the mid point between .28 and .50.
         . You'll narrow it down to the kernel that introduced the bug.  You
           you know that 1.3.69 does.  Pick a kernel in the middle and build
           that, like 1.3.50.  Build & test; if it works, pick the mid point
           between .50 and .69, else the mid point between .28 and .50.
         . You'll narrow it down to the kernel that introduced the bug.  You

-          can probably do better than this but it gets tricky.  
+          can probably do better than this but it gets tricky.

 
         . Narrow it down to a subdirectory
 
 
         . Narrow it down to a subdirectory
 


@@ -28,27 +81,27 @@ You will then do:
             directories:
 
                 Copy the non-working directory next to the working directory
             directories:
 
                 Copy the non-working directory next to the working directory

-                as "dir.63".  
+                as "dir.63".

                 One directory at time, try moving the working directory to
                 One directory at time, try moving the working directory to

-                "dir.62" and mv dir.63 dir"time, try 
+                "dir.62" and mv dir.63 dir"time, try

 
                         mv dir dir.62
                         mv dir.63 dir
                         find dir -name '*.[oa]' -print | xargs rm -f
 
                 And then rebuild and retest.  Assuming that all related
 
                         mv dir dir.62
                         mv dir.63 dir
                         find dir -name '*.[oa]' -print | xargs rm -f
 
                 And then rebuild and retest.  Assuming that all related

-                changes were contained in the sub directory, this should 
-                isolate the change to a directory.  
+                changes were contained in the sub directory, this should
+                isolate the change to a directory.

 
                 Problems: changes in header files may have occurred; I've
 
                 Problems: changes in header files may have occurred; I've

-                found in my case that they were self explanatory - you may 
+                found in my case that they were self explanatory - you may

                 or may not want to give up when that happens.
 
         . Narrow it down to a file
 
           - You can apply the same technique to each file in the directory,
                 or may not want to give up when that happens.
 
         . Narrow it down to a file
 
           - You can apply the same technique to each file in the directory,

-            hoping that the changes in that file are self contained.  
-            
+            hoping that the changes in that file are self contained.
+

         . Narrow it down to a routine
 
           - You can take the old file and the new file and manually create
         . Narrow it down to a routine
 
           - You can take the old file and the new file and manually create


@@ -77,7 +130,7 @@ You will then do:
             that makes the difference.
 
 Finally, you take all the info that you have, kernel revisions, bug
             that makes the difference.
 
 Finally, you take all the info that you have, kernel revisions, bug

-description, the extent to which you have narrowed it down, and pass 
+description, the extent to which you have narrowed it down, and pass

 that off to whomever you believe is the maintainer of that section.
 A post to linux.dev.kernel isn't such a bad idea if you've done some
 work to narrow it down.
 that off to whomever you believe is the maintainer of that section.
 A post to linux.dev.kernel isn't such a bad idea if you've done some
 work to narrow it down.


@@ -90,3 +143,104 @@ it does work and it lets non-hackers help fix bugs.  And it is cool
 because Linux snapshots will let you do this - something that you can't
 do with vendor supplied releases.
 
 because Linux snapshots will let you do this - something that you can't
 do with vendor supplied releases.
 

+Fixing the bug
+==============
+
+Nobody is going to tell you how to fix bugs. Seriously. You need to work it
+out. But below are some hints on how to use the tools.
+
+To debug a kernel, use objdump and look for the hex offset from the crash
+output to find the valid line of code/assembler. Without debug symbols, you
+will see the assembler code for the routine shown, but if your kernel has
+debug symbols the C code will also be available. (Debug symbols can be enabled
+in the kernel hacking menu of the menu configuration.) For example:
+
+    objdump -r -S -l --disassemble net/dccp/ipv4.o
+
+NB.: you need to be at the top level of the kernel tree for this to pick up
+your C files.
+
+If you don't have access to the code you can also debug on some crash dumps
+e.g. crash dump output as shown by Dave Miller.
+
+>    EIP is at ip_queue_xmit+0x14/0x4c0
+>     ...
+>    Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
+>    00 00 55 57  56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
+>    <8b> 83 3c 01 00 00 89 44  24 14 8b 45 28 85 c0 89 44 24 18 0f 85
+>
+>    Put the bytes into a "foo.s" file like this:
+>
+>           .text
+>           .globl foo
+>    foo:
+>           .byte  .... /* bytes from Code: part of OOPS dump */
+>
+>    Compile it with "gcc -c -o foo.o foo.s" then look at the output of
+>    "objdump --disassemble foo.o".
+>
+>    Output:
+>
+>    ip_queue_xmit:
+>        push       %ebp
+>        push       %edi
+>        push       %esi
+>        push       %ebx
+>        sub        $0xbc, %esp
+>        mov        0xd0(%esp), %ebp        ! %ebp = arg0 (skb)
+>        mov        0x8(%ebp), %ebx         ! %ebx = skb->sk
+>        mov        0x13c(%ebx), %eax       ! %eax = inet_sk(sk)->opt
+
+In addition, you can use GDB to figure out the exact file and line
+number of the OOPS from the vmlinux file. If you have
+CONFIG_DEBUG_INFO enabled, you can simply copy the EIP value from the
+OOPS:
+
+ EIP:    0060:[<c021e50e>]    Not tainted VLI
+
+And use GDB to translate that to human-readable form:
+
+  gdb vmlinux
+  (gdb) l *0xc021e50e
+
+If you don't have CONFIG_DEBUG_INFO enabled, you use the function
+offset from the OOPS:
+
+ EIP is at vt_ioctl+0xda8/0x1482
+
+And recompile the kernel with CONFIG_DEBUG_INFO enabled:
+
+  make vmlinux
+  gdb vmlinux
+  (gdb) p vt_ioctl
+  (gdb) l *(0x<address of vt_ioctl> + 0xda8)
+or, as one command
+  (gdb) l *(vt_ioctl + 0xda8)
+
+If you have a call trace, such as :-
+>Call Trace:
+> [<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
+> [<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
+> [<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
+> ...
+this shows the problem in the :jbd: module. You can load that module in gdb
+and list the relevant code.
+  gdb fs/jbd/jbd.ko
+  (gdb) p log_wait_commit
+  (gdb) l *(0x<address> + 0xa3)
+or
+  (gdb) l *(log_wait_commit + 0xa3)
+
+
+Another very useful option of the Kernel Hacking section in menuconfig is
+Debug memory allocations. This will help you see whether data has been
+initialised and not set before use etc. To see the values that get assigned
+with this look at mm/slab.c and search for POISON_INUSE. When using this an
+Oops will often show the poisoned data instead of zero which is the default.
+
+Once you have worked out a fix please submit it upstream. After all open
+source is about sharing what you do and don't you want to be recognised for
+your genius?
+
+Please do read Documentation/SubmittingPatches though to help your code get
+accepted.