fsck_msdosfs: add tests for the 32-bit boot block field decodingHEADmain

Exercise each of the 32-bit BIOS Parameter Block and FSInfo fields that
readboot() decodes, using values whose most significant byte has its high
bit set.  Each case checks two things: that fsck_msdosfs(8) reports the
full unsigned 32-bit value back on stdout, and that nothing writes a
sanitizer runtime error to stderr.

The second check is what catches a byte-at-a-time decode.  Shifting such
a byte left by 24 is undefined, but every compiler we use wraps it into
the same bit pattern, so the decoded value alone cannot tell a correct
decode from an overflowing one.  In a WITH_UBSAN build bsd.sanitizer.mk
compiles with -fsanitize=undefined and -fsanitize-recover=undefined, so
the shift is reported on stderr and execution continues, which the test
can then assert on.  Against the byte-at-a-time decode these cases fail
in a WITH_UBSAN build and pass without it.

Note that the stderr check also fails on unrelated undefined behavior
that these images reach anywhere in fsck_msdosfs(8), which is intended.

MFC after:	1 week

0 files changed, 0 insertions, 0 deletions