[Bug 239315] Forth loader misreads nextboot.conf files over 80 bytes

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239315

            Bug ID: 239315
           Summary: Forth loader misreads nextboot.conf files over 80
                    bytes
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: cgull+l-freebsd-bugzilla at glup.org

Created attachment 205904
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=205904&action=edit
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
foo="xxxxxxxxxxxxxxnextboot_enable="YES"
                                    ^

/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.

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