FreeBSD/Linux Kernel Cross Reference
sys/Documentation/BUG-HUNTING

     [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.  
     It's a brute force approach but it works pretty well.
     
     You need:
     
             . A reproducible bug - it has to happen predictably (sorry)
             . All the kernel tar files from a revision that worked to the
              revision that doesn't
    
    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
              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
              can probably do better than this but it gets tricky.  
    
            . Narrow it down to a subdirectory
    
              - Copy kernel that works into "test".  Let's say that 3.62 works,
                but 3.63 doesn't.  So you diff -r those two kernels and come
                up with a list of directories that changed.  For each of those
                directories:
    
                    Copy the non-working directory next to the working directory
                    as "dir.63".  
                    One directory at time, try moving the working directory to
                    "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
                    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
                    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,
                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
                a merged file that has
    
                    #ifdef VER62
                    routine()
                    {
                            ...
                    }
                    #else
                    routine()
                    {
                            ...
                    }
                    #endif
    
                And then walk through that file, one routine at a time and
                prefix it with
    
                    #define VER62
                    /* both routines here */
                    #undef VER62
    
                Then recompile, retest, move the ifdefs until you find the one
                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 
    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.
    
    If you get it down to a routine, you'll probably get a fix in 24 hours.
    
    My apologies to Linus and the other kernel hackers for describing this
    brute force approach, it's hardly what a kernel hacker would do.  However,
    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.
    

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