hfsplus: correct usage of HFSPLUS_ATTR_MAX_STRLEN for non-English attributes

HFSPLUS_ATTR_MAX_STRLEN (=127) is the limit of attribute names for the
number of unicode character (UTF-16BE) storable in the HFS+ file system.
Almost all the current usage of it is wrong, in relation to NLS to
on-disk conversion.

Except for one use calling hfsplus_asc2uni (which should stay the same)
and its uses in calling hfsplus_uni2asc (which was corrected in the
earlier patch in this series concerning usage of hfsplus_uni2asc), all
the other uses are of the forms:

- char buffer[size]

- bound check: "if (namespace_adjusted_input_length > size) return failure;"

Conversion between on-disk unicode representation and NLS char strings
(in whichever direction) always needs to accommodate the worst-case NLS
conversion, so all char buffers of that size need to have a
NLS_MAX_CHARSET_SIZE x .

The bound checks are all wrong, since they compare nls_length derived
from strlen() to a unicode length limit.

It turns out that all the bound-checks do is to protect
hfsplus_asc2uni(), which can fail if the input is too large.

There is only one usage of it as far as attributes are concerned, in
hfsplus_attr_build_key().  It is in turn used by hfsplus_find_attr(),
hfsplus_create_attr(), hfsplus_delete_attr().  Thus making sure that
errors from hfsplus_asc2uni() is caught in hfsplus_attr_build_key() and
propagated is sufficient to replace all the bound checks.

Unpropagated errors from hfsplus_asc2uni() in the file catalog code was
addressed recently in an independent patch "hfsplus: fix longname
handling" by Sougata Santra.

Before this patch, trying to set a 55 CJK character (in a UTF-8 locale,
> 127/3=42) attribute plus user prefix fails with:

    $ setfattr -n user.`cat testing-string` -v `cat testing-string` \
        testing-string
    setfattr: testing-string: Operation not supported

and retrieving a stored long attributes is particular ugly(!):

    find /mnt/* -type f -exec getfattr -d {} \;
    getfattr: /mnt/testing-string: Input/output error

with console log:
    [268008.389781] hfsplus: unicode conversion failed

After the patch, both of the above works.

FYI, the test attribute string is prepared with:

echo -e -n \
"\xe9\x80\x99\xe6\x98\xaf\xe4\xb8\x80\xe5\x80\x8b\xe9\x9d\x9e\xe5" \
"\xb8\xb8\xe6\xbc\xab\xe9\x95\xb7\xe8\x80\x8c\xe6\xa5\xb5\xe5\x85" \
"\xb6\xe4\xb9\x8f\xe5\x91\xb3\xe5\x92\x8c\xe7\x9b\xb8\xe7\x95\xb6" \
"\xe7\x84\xa1\xe8\xb6\xa3\xe3\x80\x81\xe4\xbb\xa5\xe5\x8f\x8a\xe7" \
"\x84\xa1\xe7\x94\xa8\xe7\x9a\x84\xe3\x80\x81\xe5\x86\x8d\xe5\x8a" \
"\xa0\xe4\xb8\x8a\xe6\xaf\xab\xe7\x84\xa1\xe6\x84\x8f\xe7\xbe\xa9" \
"\xe7\x9a\x84\xe6\x93\xb4\xe5\xb1\x95\xe5\xb1\xac\xe6\x80\xa7\xef" \
"\xbc\x8c\xe8\x80\x8c\xe5\x85\xb6\xe5\x94\xaf\xe4\xb8\x80\xe5\x89" \
"\xb5\xe5\xbb\xba\xe7\x9b\xae\xe7\x9a\x84\xe5\x83\x85\xe6\x98\xaf" \
"\xe7\x82\xba\xe4\xba\x86\xe6\xb8\xac\xe8\xa9\xa6\xe4\xbd\x9c\xe7" \
"\x94\xa8\xe3\x80\x82" | tr -d ' '

(= "pointlessly long attribute for testing", elaborate Chinese in
UTF-8 enoding).

However, it is not possible to set double the size (110 + 5 is still
under 127) in a UTF-8 locale:

    $setfattr -n user.`cat testing-string testing-string` -v \
        `cat testing-string testing-string` testing-string
    setfattr: testing-string: Numerical result out of range

110 CJK char in UTF-8 is 330 bytes - the generic get/set attribute
system call code in linux/fs/xattr.c imposes a 255 byte limit.  One can
use a combination of iconv to encode content, changing terminal locale
for viewing, and an nls=cp932/cp936/cp949/cp950 mount option to fully
use 127-unicode attribute in a double-byte locale.

Also, as an additional information, it is possible to (mis-)use unicode
half-width/full-width forms (U+FFxx) to write attributes which looks
like english but not actually ascii.

Thanks Anton Altaparmakov for reviewing the earlier ideas behind this
change.

[akpm@linux-foundation.org: fix build]
[akpm@linux-foundation.org: fix build]
Signed-off-by: Hin-Tak Leung <htl10@users.sourceforge.net>
Cc: Anton Altaparmakov <anton@tuxera.com>
Cc: Vyacheslav Dubeyko <slava@dubeyko.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Sougata Santra <sougata@tuxera.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>