Re: [PATCH v5 4/8] ext4: Reuse generic_ci_match for ci comparisons

[Gabriel Krisman Bertazi <krisman@collabora.com>]

Eric Biggers <ebiggers@kernel.org> writes:

> On Wed, May 18, 2022 at 01:23:16PM -0400, Gabriel Krisman Bertazi wrote:
>> Instead of reimplementing ext4_match_ci, use the new libfs helper.
>> 
>> Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
>> ---
> [...]
>>  int ext4_fname_setup_ci_filename(struct inode *dir, const struct qstr *iname,
>>  				  struct ext4_filename *name)
>>  {
>> @@ -1432,20 +1380,25 @@ static bool ext4_match(struct inode *parent,
>>  #if IS_ENABLED(CONFIG_UNICODE)
>>  	if (parent->i_sb->s_encoding && IS_CASEFOLDED(parent) &&
>>  	    (!IS_ENCRYPTED(parent) || fscrypt_has_encryption_key(parent))) {
>> -		if (fname->cf_name.name) {
>> -			if (IS_ENCRYPTED(parent)) {
>> -				if (fname->hinfo.hash != EXT4_DIRENT_HASH(de) ||
>> -					fname->hinfo.minor_hash !=
>> -						EXT4_DIRENT_MINOR_HASH(de)) {
>> +		int ret;
>>  
>> -					return false;
>> -				}
>> -			}
>> -			return !ext4_ci_compare(parent, &fname->cf_name,
>> -						de->name, de->name_len, true);
>> +		if (IS_ENCRYPTED(parent) &&
>> +		    (fname->hinfo.hash != EXT4_DIRENT_HASH(de) ||
>> +		     fname->hinfo.minor_hash != EXT4_DIRENT_MINOR_HASH(de)))
>> +			return false;
>> +
>> +		ret = generic_ci_match(parent, fname->usr_fname,
>> +				       &fname->cf_name, de->name,
>> +				       de->name_len);
>> +		if (ret < 0) {
>> +			/*
>> +			 * Treat comparison errors as not a match.  The
>> +			 * only case where it happens is on a disk
>> +			 * corruption or ENOMEM.
>> +			 */
>> +			return false;
>>  		}
>> -		return !ext4_ci_compare(parent, fname->usr_fname, de->name,
>> -						de->name_len, false);
>> +		return ret;
>>  	}
>
> This needs an explanation for why it's okay to remove
> 'fname->cf_name.name != NULL' from the condition for doing the hash comparison
> for an encrypted+casefolded directory entry.

Hi Eric,

The reason is that the only two ways for fname->cf_name to be NULL on a
case-insensitive lookup is 1) if name under lookup has an invalid
encoding and the FS is not in strict mode; or 2) if the directory is
encrypted and we don't have the key.  For case 1, it doesn't
matter, because the lookup hash will be generated with fname->usr_name,
the same as the disk (fallback to invalid encoding behavior on !strict
mode).  Case 2 is caught by the previous check
(!IS_ENCRYPTED(parent) || fscrypt_has_encryption_key(parent)), so we
never reach this code.

I'll add the above rationale to the commit message.

-- 
Gabriel Krisman Bertazi