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A 35-Year-Old Bug in Patch Found in Efforts to Restore 29 Year Old 2.11BSD

posted by Fnord666 on Friday August 21 2020, @05:21AM   Printer-friendly
from the cleaning-the-window-sill dept.

An Anonymous Coward writes:

A 35-year-old bug in patch found in efforts to restore 29 year old 2.11BSD:

Larry Wall posted patch 1.3 to mod.sources on May 8, 1985. A number of versions followed over the years. It's been a faithful alley [sic] for a long, long time. I've never had a problem with patch until I embarked on the 2.11BSD restoration project. In going over the logs very carefully, I've discovered a bug that bites this effort twice. It's quite interesting to use 27 year old patches to find this bug while restoring a 29 year old OS...

After some careful research, this turned out to be a fairly obscure bug in an odd edge case caused by "the state of email in the 1980s." which can be relegated to the dustbin of history...

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  • (Score: 2) by FatPhil on Friday August 21 2020, @07:25AM (3 children)

    by FatPhil (863) <reversethis-{if.fdsa} {ta} {tnelyos-cp}> on Friday August 21 2020, @07:25AM (#1039808) Homepage
    As so often, the bug is associated with (not caused by, as such) a magic number:
    "So why '4' in the second 'if' in the quoted code above? what's so magic about it? Indeed, if we hack the patch to have 6 lines of context instead of 3, it applies correctly."

    Had that code said:
    /* Sometimes we're missing blank lines at the end of patches, magic them back. */
    #define MISSING_LINES_FIXUP 4
    ...
    if(p_max - p_end < MISSING_LINES_FIXUP) {
    ...
    (With a better comment and a better name for the macro) then that would be a honking great warning sign that this might be where the dragons are.

    I do think this is progress, the prior fixup was a kludge. However, you're also right, it's asking for the same hackish non-solution to be recreated at a future point in time. The ultimate fix is to programatically detect which of the two behaviours is most likely to be required, and - akin to the fuzzing messages - at least warn when an assumption has been programatically made, so that you can know that the program attempted to Do The Right Thing(tm) in the face of ambiguity.
    --
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  • (Score: 2) by Mojibake Tengu on Friday August 21 2020, @02:16PM (2 children)

    by Mojibake Tengu (8598) on Friday August 21 2020, @02:16PM (#1039879) Journal

    If patch/diff was designed with some initial sanity on mind (technically, it is a strict imperative command language), it would use separate checksum for every command issued at least.

    Instead, while email/news did not guarantee a fixed transfer of text files, Unix freaks imposed a space hell on themselves, including tabs hell in makefiles.

    A magic number like this is a shameless shame.

    --
    Respect Authorities. Know your social status. Woke responsibly.

    • (Score: 2) by FatPhil on Friday August 21 2020, @03:42PM (1 child)

      by FatPhil (863) <reversethis-{if.fdsa} {ta} {tnelyos-cp}> on Friday August 21 2020, @03:42PM (#1039932) Homepage
      Oh, yeah, adding semantic meaning to differences in whitespace was a terrible idea. Thank goodness nobody ever repeated that misnake.
      --
      Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves