Back in the 6.2-rc1 days, Eric Whitney reported a fstests regression in
ext4 against generic/454.  The cause of this test failure was the
unfortunate combination of setting an xattr name containing UTF8 encoded
emoji, an xattr hash function that accepted a char pointer with no
explicit signedness, signed type extension of those chars to an int, and
the 6.2 build tools maintainers deciding to mandate -funsigned-char
across the board.  As a result, the ondisk extended attribute structure
written out by 6.1 and 6.2 were not the same.

This discrepancy, in fact, had been noticeable if a filesystem with such
an xattr were moved between any two architectures that don't employ the
same signedness of a raw "char" declaration.  The only reason anyone
noticed is that x86 gcc defaults to signed, and no such -funsigned-char
update was made to e2fsprogs, so e2fsck immediately started reporting
data corruption.

After a day and a half of discussing how to handle this use case (xattrs
with bit 7 set anywhere in the name) without breaking existing users,
Linus merged his own patch and didn't tell the maintainer.  None of the
ext4 developers realized this until AUTOSEL announced that the commit
had been backported to stable.

In the end, this problem could have been detected much earlier if there
had been any useful tests of hash function(s) in use inside ext4 to make
sure that they always produce the same outputs given the same inputs.

The XFS dirent/xattr name hash takes a uint8_t*, so I don't think it's
vulnerable to this problem.  However, let's avoid all this drama by
adding our own self test to check that the da hash produces the same
outputs for a static pile of inputs on various platforms.  This enables
us to fix any breakage that may result in a controlled fashion.  The
buffer and test data are identical to the patches submitted to xfsprogs.

Link: https://lore.kernel.org/linux-ext4/Y8bpkm3jA3bDm3eL@debian-BULLSEYE-live-builder-AMD64/
Link: https://lore.kernel.org/linux-xfs/ZBUKCRR7xvIqPrpX@destitution/T/#md38272cc684e2c0d61494435ccbb91f022e8dee4Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>