Re: [PATCH v10 3/8] libfs: Introduce case-insensitive string comparison helper

[Gabriel Krisman Bertazi]

Eugen Hristev <eugen.hristev@xxxxxxxxxxxxx> writes:

> On 2/16/24 18:12, Gabriel Krisman Bertazi wrote:
>> Eugen Hristev <eugen.hristev@xxxxxxxxxxxxx> writes:
>> 
>>> From: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
>>>
>>> generic_ci_match can be used by case-insensitive filesystems to compare
>>> strings under lookup with dirents in a case-insensitive way.  This
>>> function is currently reimplemented by each filesystem supporting
>>> casefolding, so this reduces code duplication in filesystem-specific
>>> code.
>>>
>>> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
>>> [eugen.hristev@xxxxxxxxxxxxx: rework to first test the exact match]
>>> Signed-off-by: Eugen Hristev <eugen.hristev@xxxxxxxxxxxxx>
>>> ---
>>>  fs/libfs.c         | 80 ++++++++++++++++++++++++++++++++++++++++++++++
>>>  include/linux/fs.h |  4 +++
>>>  2 files changed, 84 insertions(+)
>>>
>>> diff --git a/fs/libfs.c b/fs/libfs.c
>>> index bb18884ff20e..82871fa1b066 100644
>>> --- a/fs/libfs.c
>>> +++ b/fs/libfs.c
>>> @@ -1773,6 +1773,86 @@ static const struct dentry_operations generic_ci_dentry_ops = {
>>>  	.d_hash = generic_ci_d_hash,
>>>  	.d_compare = generic_ci_d_compare,
>>>  };
>>> +
>>> +/**
>>> + * generic_ci_match() - Match a name (case-insensitively) with a dirent.
>>> + * This is a filesystem helper for comparison with directory entries.
>>> + * generic_ci_d_compare should be used in VFS' ->d_compare instead.
>>> + *
>>> + * @parent: Inode of the parent of the dirent under comparison
>>> + * @name: name under lookup.
>>> + * @folded_name: Optional pre-folded name under lookup
>>> + * @de_name: Dirent name.
>>> + * @de_name_len: dirent name length.
>>> + *
>>> + *
>> 
>> Since this need a respin, mind dropping the extra empty line here?
>> 
>>> + * Test whether a case-insensitive directory entry matches the filename
>>> + * being searched.  If @folded_name is provided, it is used instead of
>>> + * recalculating the casefold of @name.
>>> + *
>>> + * Return: > 0 if the directory entry matches, 0 if it doesn't match, or
>>> + * < 0 on error.
>>> + */
>>> +int generic_ci_match(const struct inode *parent,
>>> +		     const struct qstr *name,
>>> +		     const struct qstr *folded_name,
>>> +		     const u8 *de_name, u32 de_name_len)
>>> +{
>>> +	const struct super_block *sb = parent->i_sb;
>>> +	const struct unicode_map *um = sb->s_encoding;
>>> +	struct fscrypt_str decrypted_name = FSTR_INIT(NULL, de_name_len);
>>> +	struct qstr dirent = QSTR_INIT(de_name, de_name_len);
>>> +	int res;
>>> +
>>> +	if (IS_ENCRYPTED(parent)) {
>>> +		const struct fscrypt_str encrypted_name =
>>> +			FSTR_INIT((u8 *) de_name, de_name_len);
>>> +
>>> +		if (WARN_ON_ONCE(!fscrypt_has_encryption_key(parent)))
>>> +			return -EINVAL;
>>> +
>>> +		decrypted_name.name = kmalloc(de_name_len, GFP_KERNEL);
>>> +		if (!decrypted_name.name)
>>> +			return -ENOMEM;
>>> +		res = fscrypt_fname_disk_to_usr(parent, 0, 0, &encrypted_name,
>>> +						&decrypted_name);
>>> +		if (res < 0)
>>> +			goto out;
>>> +		dirent.name = decrypted_name.name;
>>> +		dirent.len = decrypted_name.len;
>>> +	}
>>> +
>>> +	/*
>>> +	 * Attempt a case-sensitive match first. It is cheaper and
>>> +	 * should cover most lookups, including all the sane
>>> +	 * applications that expect a case-sensitive filesystem.
>>> +	 *
>> 
>> 
>>> +	 * This comparison is safe under RCU because the caller
>>> +	 * guarantees the consistency between str and len. See
>>> +	 * __d_lookup_rcu_op_compare() for details.
>>> +	 */
>> 
>> This paragraph doesn't really make sense here.  It is originally from
>> the d_compare hook, which can be called under RCU, but there is no RCU
>> here.  Also, here we are comparing the dirent with the
>> name-under-lookup, name which is already safe.
>> 
>> 
>>> +	if (folded_name->name) {
>>> +		if (dirent.len == folded_name->len &&
>>> +		    !memcmp(folded_name->name, dirent.name, dirent.len)) {
>>> +			res = 1;
>>> +			goto out;
>>> +		}
>>> +		res = !utf8_strncasecmp_folded(um, folded_name, &dirent);
>> 
>> Hmm, second thought on this.  This will ignore errors from utf8_strncasecmp*,
>> which CAN happen for the first time here, if the dirent itself is
>> corrupted on disk (exactly why we have patch 6).  Yes, ext4_match will drop the
>> error, but we want to propagate it from here, such that the warning on
>> patch 6 can trigger.
>> 
>> This is why I did that match dance on the original submission.  Sorry
>> for suggesting it.  We really want to get the error from utf8 and
>> propagate it if it is negative. basically:
>> 
>>         res > 0: match
>>         res == 0: no match.
>>         res < 0: propagate error and let the caller handle it
>
> In that case I will revert to the original v9 implementation and send a v11 to
> handle that.

Please, note that the memcmp optimization is still valid. On match, we
know the name is valid utf8.  It is just a matter of propagating the
error code from utf8 to the caller if we need to call it.

-- 
Gabriel Krisman Bertazi