xfs: stabilize the dirent name transformation function used for ascii-ci dir hash...

xfs: stabilize the dirent name transformation function used for ascii-ci dir hash computation

Back in the old days, the "ascii-ci" feature was created to implement
case-insensitive directory entry lookups for latin1-encoded names and
remove the large overhead of Samba's case-insensitive lookup code.  UTF8
names were not allowed, but nobody explicitly wrote in the documentation
that this was only expected to work if the system used latin1 names.
The kernel tolower function was selected to prepare names for hashed
lookups.

There's a major discrepancy in the function that computes directory entry
hashes for filesystems that have ASCII case-insensitive lookups enabled.
The root of this is that the kernel and glibc's tolower implementations
have differing behavior for extended ASCII accented characters.  I wrote
a program to spit out characters for which the tolower() return value is
different from the input:

glibc tolower:
65:A 66:B 67:C 68:D 69:E 70:F 71:G 72:H 73:I 74:J 75:K 76:L 77:M 78:N
79:O 80:P 81:Q 82:R 83:S 84:T 85:U 86:V 87:W 88:X 89:Y 90:Z

kernel tolower:
65:A 66:B 67:C 68:D 69:E 70:F 71:G 72:H 73:I 74:J 75:K 76:L 77:M 78:N
79:O 80:P 81:Q 82:R 83:S 84:T 85:U 86:V 87:W 88:X 89:Y 90:Z 192:À 193:Á
194:Â 195:Ã 196:Ä 197:Å 198:Æ 199:Ç 200:È 201:É 202:Ê 203:Ë 204:Ì 205:Í
206:Î 207:Ï 208:Ð 209:Ñ 210:Ò 211:Ó 212:Ô 213:Õ 214:Ö 215:× 216:Ø 217:Ù
218:Ú 219:Û 220:Ü 221:Ý 222:Þ

Which means that the kernel and userspace do not agree on the hash value
for a directory filename that contains those higher values.  The hash
values are written into the leaf index block of directories that are
larger than two blocks in size, which means that xfs_repair will flag
these directories as having corrupted hash indexes and rewrite the index
with hash values that the kernel now will not recognize.

Because the ascii-ci feature is not frequently enabled and the kernel
touches filesystems far more frequently than xfs_repair does, fix this
by encoding the kernel's toupper predicate and tolower functions into
libxfs.  Give the new functions less provocative names to make it really
obvious that this is a pre-hash name preparation function, and nothing
else.  This change makes userspace's behavior consistent with the
kernel.

Found by auditing obfuscate_name in xfs_metadump as part of working on
parent pointers, wondering how it could possibly work correctly with ci
filesystems, writing a test tool to create a directory with
hash-colliding names, and watching xfs_repair flag it.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>