Bug 239315 - Forth loader misreads nextboot.conf files over 80 bytes
Summary: Forth loader misreads nextboot.conf files over 80 bytes
Status: New
Alias: None
Product: Base System
Classification: Unclassified
Component: kern (show other bugs)
Version: CURRENT
Hardware: Any Any
: --- Affects Only Me
Assignee: freebsd-bugs (Nobody)
Keywords: patch
Depends on:
Reported: 2019-07-19 19:22 UTC by John Hood
Modified: 2019-08-25 12:58 UTC (History)
1 user (show)

See Also:

one-line fix (334 bytes, patch)
2019-07-19 19:22 UTC, John Hood
no flags Details | Diff

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Description John Hood 2019-07-19 19:22:09 UTC
Created attachment 205904 [details]
one-line fix

Large nextboot.conf files (over 80 bytes) are not read correctly by the Forth loader, causing file parsing to abort, and nextboot configuration fails to apply.

Simple repro:

nextboot -e foo=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
shutdown -r now

That will cause the bug to cause a parse failure but shouldn't otherwise affect the boot.  Depending on your loader configuration, you may also have to set beastie_disable and/or reduce the number of modules loaded to see the error on a small console screen.  12.0 or CURRENT users will also have to explicitly use the Forth loader instead of the Lua loader.  The error will look something like:

Warning: syntax error on file /boot/loader.conf.local

/boot/support.4th has crude file I/O buffering, which uses a buffer 'read_buffer', defined to be 80 bytes by the 'read_buffer_size' constant.  The loader first tastes nextboot.conf, reading and parsing the first line in it for nextboot_enable="YES".  If this is true, then it reopens the file and parses it like other loader .conf files.

Unfortunately, the file I/O buffering code does not fully reset the buffer state in the reset_line_reading word.  If the last file was read to the end, that doesn't matter; the file buffer is treated as empty anyway.  But in the nextboot.conf case, the loader will not read to the end of file if it is over 80 bytes, and the file buffer may be reused when reading the next file.  When the file is reread, the corrupt text may cause file parsing to abort on bad syntax (if the corrupt line has <>2 quotes in it), the wrong variable to be set, no variable to be set at all, or (if the splice happens to land at a line ending) something approximating normal operation.

The bug is very old, dating back to at least 2000 if not before, and is still present in 12.0 and CURRENT r345863 (though it is now hidden by the Lua loader by default).

Suggested one-line attached.  This does change the behavior of the reset_line_reading word, which is exported in the line-reading dictionary (though the export is not documented in loader man pages).  But repo history shows it was probably exported for the PNP support code, which was never included in the loader build, and was removed 5 months ago.

One thing that puzzles me: how has this bug gone unnoticed/unfixed for nearly 2 decades?  I find it hard to believe that nobody's tried to do something interesting with nextboot, like load a kernel and filesystem, which is what I'm doing.