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246 lines
8.1 KiB
246 lines
8.1 KiB
Table of contents
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=================
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Last updated: 20 December 2005
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Contents
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========
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- Introduction
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- Devices not appearing
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- Finding patch that caused a bug
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-- Finding using git-bisect
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-- Finding it the old way
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- Fixing the bug
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Introduction
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============
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Always try the latest kernel from kernel.org and build from source. If you are
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not confident in doing that please report the bug to your distribution vendor
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instead of to a kernel developer.
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Finding bugs is not always easy. Have a go though. If you can't find it don't
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give up. Report as much as you have found to the relevant maintainer. See
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MAINTAINERS for who that is for the subsystem you have worked on.
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Before you submit a bug report read REPORTING-BUGS.
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Devices not appearing
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=====================
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Often this is caused by udev. Check that first before blaming it on the
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kernel.
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Finding patch that caused a bug
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===============================
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Finding using git-bisect
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------------------------
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Using the provided tools with git makes finding bugs easy provided the bug is
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reproducible.
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Steps to do it:
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- start using git for the kernel source
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- read the man page for git-bisect
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- have fun
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Finding it the old way
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----------------------
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[Sat Mar 2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)]
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This is how to track down a bug if you know nothing about kernel hacking.
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It's a brute force approach but it works pretty well.
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You need:
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. A reproducible bug - it has to happen predictably (sorry)
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. All the kernel tar files from a revision that worked to the
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revision that doesn't
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You will then do:
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. Rebuild a revision that you believe works, install, and verify that.
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. Do a binary search over the kernels to figure out which one
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introduced the bug. I.e., suppose 1.3.28 didn't have the bug, but
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you know that 1.3.69 does. Pick a kernel in the middle and build
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that, like 1.3.50. Build & test; if it works, pick the mid point
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between .50 and .69, else the mid point between .28 and .50.
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. You'll narrow it down to the kernel that introduced the bug. You
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can probably do better than this but it gets tricky.
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. Narrow it down to a subdirectory
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- Copy kernel that works into "test". Let's say that 3.62 works,
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but 3.63 doesn't. So you diff -r those two kernels and come
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up with a list of directories that changed. For each of those
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directories:
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Copy the non-working directory next to the working directory
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as "dir.63".
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One directory at time, try moving the working directory to
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"dir.62" and mv dir.63 dir"time, try
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mv dir dir.62
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mv dir.63 dir
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find dir -name '*.[oa]' -print | xargs rm -f
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And then rebuild and retest. Assuming that all related
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changes were contained in the sub directory, this should
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isolate the change to a directory.
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Problems: changes in header files may have occurred; I've
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found in my case that they were self explanatory - you may
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or may not want to give up when that happens.
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. Narrow it down to a file
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- You can apply the same technique to each file in the directory,
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hoping that the changes in that file are self contained.
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. Narrow it down to a routine
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- You can take the old file and the new file and manually create
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a merged file that has
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#ifdef VER62
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routine()
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{
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...
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}
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#else
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routine()
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{
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...
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}
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#endif
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And then walk through that file, one routine at a time and
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prefix it with
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#define VER62
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/* both routines here */
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#undef VER62
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Then recompile, retest, move the ifdefs until you find the one
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that makes the difference.
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Finally, you take all the info that you have, kernel revisions, bug
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description, the extent to which you have narrowed it down, and pass
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that off to whomever you believe is the maintainer of that section.
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A post to linux.dev.kernel isn't such a bad idea if you've done some
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work to narrow it down.
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If you get it down to a routine, you'll probably get a fix in 24 hours.
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My apologies to Linus and the other kernel hackers for describing this
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brute force approach, it's hardly what a kernel hacker would do. However,
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it does work and it lets non-hackers help fix bugs. And it is cool
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because Linux snapshots will let you do this - something that you can't
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do with vendor supplied releases.
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Fixing the bug
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==============
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Nobody is going to tell you how to fix bugs. Seriously. You need to work it
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out. But below are some hints on how to use the tools.
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To debug a kernel, use objdump and look for the hex offset from the crash
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output to find the valid line of code/assembler. Without debug symbols, you
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will see the assembler code for the routine shown, but if your kernel has
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debug symbols the C code will also be available. (Debug symbols can be enabled
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in the kernel hacking menu of the menu configuration.) For example:
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objdump -r -S -l --disassemble net/dccp/ipv4.o
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NB.: you need to be at the top level of the kernel tree for this to pick up
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your C files.
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If you don't have access to the code you can also debug on some crash dumps
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e.g. crash dump output as shown by Dave Miller.
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> EIP is at ip_queue_xmit+0x14/0x4c0
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> ...
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> Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
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> 00 00 55 57 56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
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> <8b> 83 3c 01 00 00 89 44 24 14 8b 45 28 85 c0 89 44 24 18 0f 85
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>
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> Put the bytes into a "foo.s" file like this:
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>
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> .text
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> .globl foo
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> foo:
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> .byte .... /* bytes from Code: part of OOPS dump */
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>
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> Compile it with "gcc -c -o foo.o foo.s" then look at the output of
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> "objdump --disassemble foo.o".
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>
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> Output:
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>
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> ip_queue_xmit:
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> push %ebp
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> push %edi
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> push %esi
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> push %ebx
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> sub $0xbc, %esp
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> mov 0xd0(%esp), %ebp ! %ebp = arg0 (skb)
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> mov 0x8(%ebp), %ebx ! %ebx = skb->sk
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> mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt
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In addition, you can use GDB to figure out the exact file and line
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number of the OOPS from the vmlinux file. If you have
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CONFIG_DEBUG_INFO enabled, you can simply copy the EIP value from the
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OOPS:
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EIP: 0060:[<c021e50e>] Not tainted VLI
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And use GDB to translate that to human-readable form:
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gdb vmlinux
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(gdb) l *0xc021e50e
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If you don't have CONFIG_DEBUG_INFO enabled, you use the function
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offset from the OOPS:
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EIP is at vt_ioctl+0xda8/0x1482
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And recompile the kernel with CONFIG_DEBUG_INFO enabled:
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make vmlinux
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gdb vmlinux
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(gdb) p vt_ioctl
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(gdb) l *(0x<address of vt_ioctl> + 0xda8)
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or, as one command
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(gdb) l *(vt_ioctl + 0xda8)
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If you have a call trace, such as :-
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>Call Trace:
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> [<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
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> [<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
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> [<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
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> ...
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this shows the problem in the :jbd: module. You can load that module in gdb
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and list the relevant code.
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gdb fs/jbd/jbd.ko
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(gdb) p log_wait_commit
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(gdb) l *(0x<address> + 0xa3)
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or
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(gdb) l *(log_wait_commit + 0xa3)
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Another very useful option of the Kernel Hacking section in menuconfig is
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Debug memory allocations. This will help you see whether data has been
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initialised and not set before use etc. To see the values that get assigned
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with this look at mm/slab.c and search for POISON_INUSE. When using this an
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Oops will often show the poisoned data instead of zero which is the default.
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Once you have worked out a fix please submit it upstream. After all open
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source is about sharing what you do and don't you want to be recognised for
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your genius?
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Please do read Documentation/SubmittingPatches though to help your code get
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accepted.
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