On Sat, Mar 18, 2023 at 11:42:56AM +1100, Dave Chinner wrote: > On Thu, Mar 16, 2023 at 09:48:26AM -0700, Darrick J. Wong wrote: > > From: Darrick J. Wong <djwong@xxxxxxxxxx> > > > > 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 mailing list. None of > > the developers noticed until AUTOSEL made an announcement. > > > > 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 will be > > followed up in xfsprogs with a similar patch. > > > > Link: https://lore.kernel.org/linux-ext4/Y8bpkm3jA3bDm3eL@debian-BULLSEYE-live-builder-AMD64/ > > Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx> > > I'm going to trust that your binary tables exercise the hash in the > manner needed because I don't have time right now to manually > decode it. With that caveat, everything else looks fine. Yep. The kernel and userspace use the same 4k buffer of arbitrary bytes, and the test tables are identical. I don't know if the dahash function is *correct* mathematically speaking, but at least this will demonstrate consistency in behavior between the kernel and userspace. --D > Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx> > -- > Dave Chinner > david@xxxxxxxxxxxxx
