1Table of contents 2================= 3 4Last updated: 20 December 2005 5 6Contents 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 16Introduction 17============ 18 19Always try the latest kernel from kernel.org and build from source. If you are 20not confident in doing that please report the bug to your distribution vendor 21instead of to a kernel developer. 22 23Finding bugs is not always easy. Have a go though. If you can't find it don't 24give up. Report as much as you have found to the relevant maintainer. See 25MAINTAINERS for who that is for the subsystem you have worked on. 26 27Before you submit a bug report read REPORTING-BUGS. 28 29Devices not appearing 30===================== 31 32Often this is caused by udev. Check that first before blaming it on the 33kernel. 34 35Finding patch that caused a bug 36=============================== 37 38 39 40Finding using git-bisect 41------------------------ 42 43Using the provided tools with git makes finding bugs easy provided the bug is 44reproducible. 45 46Steps to do it: 47- start using git for the kernel source 48- read the man page for git-bisect 49- have fun 50 51Finding it the old way 52---------------------- 53 54[Sat Mar 2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)] 55 56This is how to track down a bug if you know nothing about kernel hacking. 57It's a brute force approach but it works pretty well. 58 59You 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 65You 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 132Finally, you take all the info that you have, kernel revisions, bug 133description, the extent to which you have narrowed it down, and pass 134that off to whomever you believe is the maintainer of that section. 135A post to linux.dev.kernel isn't such a bad idea if you've done some 136work to narrow it down. 137 138If you get it down to a routine, you'll probably get a fix in 24 hours. 139 140My apologies to Linus and the other kernel hackers for describing this 141brute force approach, it's hardly what a kernel hacker would do. However, 142it does work and it lets non-hackers help fix bugs. And it is cool 143because Linux snapshots will let you do this - something that you can't 144do with vendor supplied releases. 145 146Fixing the bug 147============== 148 149Nobody is going to tell you how to fix bugs. Seriously. You need to work it 150out. But below are some hints on how to use the tools. 151 152To debug a kernel, use objdump and look for the hex offset from the crash 153output to find the valid line of code/assembler. Without debug symbols, you 154will see the assembler code for the routine shown, but if your kernel has 155debug symbols the C code will also be available. (Debug symbols can be enabled 156in the kernel hacking menu of the menu configuration.) For example: 157 158 objdump -r -S -l --disassemble net/dccp/ipv4.o 159 160NB.: you need to be at the top level of the kernel tree for this to pick up 161your C files. 162 163If you don't have access to the code you can also debug on some crash dumps 164e.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 194In addition, you can use GDB to figure out the exact file and line 195number of the OOPS from the vmlinux file. If you have 196CONFIG_DEBUG_INFO enabled, you can simply copy the EIP value from the 197OOPS: 198 199 EIP: 0060:[<c021e50e>] Not tainted VLI 200 201And use GDB to translate that to human-readable form: 202 203 gdb vmlinux 204 (gdb) l *0xc021e50e 205 206If you don't have CONFIG_DEBUG_INFO enabled, you use the function 207offset from the OOPS: 208 209 EIP is at vt_ioctl+0xda8/0x1482 210 211And 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) 217or, as one command 218 (gdb) l *(vt_ioctl + 0xda8) 219 220If 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> ... 226this shows the problem in the :jbd: module. You can load that module in gdb 227and list the relevant code. 228 gdb fs/jbd/jbd.ko 229 (gdb) p log_wait_commit 230 (gdb) l *(0x<address> + 0xa3) 231or 232 (gdb) l *(log_wait_commit + 0xa3) 233 234 235Another very useful option of the Kernel Hacking section in menuconfig is 236Debug memory allocations. This will help you see whether data has been 237initialised and not set before use etc. To see the values that get assigned 238with this look at mm/slab.c and search for POISON_INUSE. When using this an 239Oops will often show the poisoned data instead of zero which is the default. 240 241Once you have worked out a fix please submit it upstream. After all open 242source is about sharing what you do and don't you want to be recognised for 243your genius? 244 245Please do read Documentation/SubmittingPatches though to help your code get 246accepted. 247