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phoenix |
[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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