linux.git/fs/ext4/hash.c, branch v5.7

ext4: fix kernel oops caused by spurious casefold flag

2019-09-03T05:43:17+00:00

If an directory has the a casefold flag set without the casefold
feature set, s_encoding will not be initialized, and this will cause
the kernel to dereference a NULL pointer.  In addition to adding
checks to avoid these kernel oops, attempts to load inodes with the
casefold flag when the casefold feature is not enable will cause the
file system to be declared corrupted.

Signed-off-by: Theodore Ts'o

ext4: Support case-insensitive file name lookups

2019-04-25T18:12:08+00:00

This patch implements the actual support for case-insensitive file name
lookups in ext4, based on the feature bit and the encoding stored in the
superblock.

A filesystem that has the casefold feature set is able to configure
directories with the +F (EXT4_CASEFOLD_FL) attribute, enabling lookups
to succeed in that directory in a case-insensitive fashion, i.e: match
a directory entry even if the name used by userspace is not a byte per
byte match with the disk name, but is an equivalent case-insensitive
version of the Unicode string.  This operation is called a
case-insensitive file name lookup.

The feature is configured as an inode attribute applied to directories
and inherited by its children.  This attribute can only be enabled on
empty directories for filesystems that support the encoding feature,
thus preventing collision of file names that only differ by case.

* dcache handling:

For a +F directory, Ext4 only stores the first equivalent name dentry
used in the dcache. This is done to prevent unintentional duplication of
dentries in the dcache, while also allowing the VFS code to quickly find
the right entry in the cache despite which equivalent string was used in
a previous lookup, without having to resort to ->lookup().

d_hash() of casefolded directories is implemented as the hash of the
casefolded string, such that we always have a well-known bucket for all
the equivalencies of the same string. d_compare() uses the
utf8_strncasecmp() infrastructure, which handles the comparison of
equivalent, same case, names as well.

For now, negative lookups are not inserted in the dcache, since they
would need to be invalidated anyway, because we can't trust missing file
dentries.  This is bad for performance but requires some leveraging of
the vfs layer to fix.  We can live without that for now, and so does
everyone else.

* on-disk data:

Despite using a specific version of the name as the internal
representation within the dcache, the name stored and fetched from the
disk is a byte-per-byte match with what the user requested, making this
implementation 'name-preserving'. i.e. no actual information is lost
when writing to storage.

DX is supported by modifying the hashes used in +F directories to make
them case/encoding-aware.  The new disk hashes are calculated as the
hash of the full casefolded string, instead of the string directly.
This allows us to efficiently search for file names in the htree without
requiring the user to provide an exact name.

* Dealing with invalid sequences:

By default, when a invalid UTF-8 sequence is identified, ext4 will treat
it as an opaque byte sequence, ignoring the encoding and reverting to
the old behavior for that unique file.  This means that case-insensitive
file name lookup will not work only for that file.  An optional bit can
be set in the superblock telling the filesystem code and userspace tools
to enforce the encoding.  When that optional bit is set, any attempt to
create a file name using an invalid UTF-8 sequence will fail and return
an error to userspace.

* Normalization algorithm:

The UTF-8 algorithms used to compare strings in ext4 is implemented
lives in fs/unicode, and is based on a previous version developed by
SGI.  It implements the Canonical decomposition (NFD) algorithm
described by the Unicode specification 12.1, or higher, combined with
the elimination of ignorable code points (NFDi) and full
case-folding (CF) as documented in fs/unicode/utf8_norm.c.

NFD seems to be the best normalization method for EXT4 because:

  - It has a lower cost than NFC/NFKC (which requires
    decomposing to NFD as an intermediary step)
  - It doesn't eliminate important semantic meaning like
    compatibility decompositions.

Although:

  - This implementation is not completely linguistic accurate, because
  different languages have conflicting rules, which would require the
  specialization of the filesystem to a given locale, which brings all
  sorts of problems for removable media and for users who use more than
  one language.

Signed-off-by: Gabriel Krisman Bertazi 
Signed-off-by: Theodore Ts'o

ext4: annotate implicit fall throughs

2019-02-21T15:49:53+00:00

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1). Fix them up.

This commit remove the following warnings:

  fs/ext4/hash.c:233:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
  fs/ext4/hash.c:246:15: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre 
Signed-off-by: Theodore Ts'o 
Reviewed-by: Andreas Dilger

ext4: fix up remaining files with SPDX cleanups

2017-12-18T03:00:59+00:00

A number of ext4 source files were skipped due because their copyright
permission statements didn't match the expected text used by the
automated conversion utilities.  I've added SPDX tags for the rest.

While looking at some of these files, I've noticed that we have quite
a bit of variation on the licenses that were used --- in particular
some of the Red Hat licenses on the jbd2 files use a GPL2+ license,
and we have some files that have a LGPL-2.1 license (which was quite
surprising).

I've not attempted to do any license changes.  Even if it is perfectly
legal to relicense to GPL 2.0-only for consistency's sake, that should
be done with ext4 developer community discussion.

Signed-off-by: Theodore Ts'o

ext4: remove useless test and assignment in strtohash functions

2017-08-24T19:11:34+00:00

On transformation of str to hash, computed value is initialised before
first byte modulo 4. But it is already initialised before entering loop
and after processing last byte modulo 4. So the corresponding test and
initialisation could be removed.

Signed-off-by: Damien Guibouret 
Signed-off-by: Theodore Ts'o

ext4: move halfmd4 into hash.c directly

2017-02-02T16:52:14+00:00

The "half md4" transform should not be used by any new code. And
fortunately, it's only used now by ext4. Since ext4 supports several
hashing methods, at some point it might be desirable to move to
something like SipHash. As an intermediate step, remove half md4 from
cryptohash.h and lib, and make it just a local function in ext4's
hash.c. There's precedent for doing this; the other function ext can use
for its hashes -- TEA -- is also implemented in the same place. Also, by
being a local function, this might allow gcc to perform some additional
optimizations.

Signed-off-by: Jason A. Donenfeld 
Reviewed-by: Andreas Dilger 
Cc: Theodore Ts'o 
Signed-off-by: Theodore Ts'o

ext4: remove unused header files

2015-04-03T03:47:42+00:00

Remove unused header files and header files which are included in
ext4.h.

Signed-off-by: Sheng Yong 
Signed-off-by: Theodore Ts'o

ext4: reduce one "if" comparison in ext4_dirhash()

2013-02-02T03:33:21+00:00

It is unnecessary to check i<4 after the loop; just do it before the
break.

Signed-off-by: Cong Ding 
Signed-off-by: "Theodore Ts'o"

ext4: return 32/64-bit dir name hash according to usage type

2012-03-19T02:44:40+00:00

Traditionally ext2/3/4 has returned a 32-bit hash value from llseek()
to appease NFSv2, which can only handle a 32-bit cookie for seekdir()
and telldir().  However, this causes problems if there are 32-bit hash
collisions, since the NFSv2 server can get stuck resending the same
entries from the directory repeatedly.

Allow ext4 to return a full 64-bit hash (both major and minor) for
telldir to decrease the chance of hash collisions.  This still needs
integration on the NFS side.

Patch-updated-by: Bernd Schubert 
(blame me if something is not correct)

Signed-off-by: Fan Yong 
Signed-off-by: Andreas Dilger 
Signed-off-by: Bernd Schubert 
Signed-off-by: "Theodore Ts'o"

ext4: Add support for non-native signed/unsigned htree hash algorithms

2008-10-28T17:21:44+00:00

The original ext3 hash algorithms assumed that variables of type char
were signed, as God and K&R intended.  Unfortunately, this assumption
is not true on some architectures.  Userspace support for marking
filesystems with non-native signed/unsigned chars was added two years
ago, but the kernel-side support was never added (until now).

Signed-off-by: "Theodore Ts'o"