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Message-ID: <87msrwbj18.fsf@mailhost.krisman.be>
Date: Mon, 19 Feb 2024 09:55:31 -0500
From: Gabriel Krisman Bertazi <krisman@...e.de>
To: Eugen Hristev <eugen.hristev@...labora.com>
Cc: tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
jaegeuk@...nel.org, chao@...nel.org,
linux-f2fs-devel@...ts.sourceforge.net, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, kernel@...labora.com,
viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz, Gabriel
Krisman Bertazi <krisman@...labora.com>
Subject: Re: [PATCH v10 3/8] libfs: Introduce case-insensitive string
comparison helper
Eugen Hristev <eugen.hristev@...labora.com> writes:
> On 2/16/24 18:12, Gabriel Krisman Bertazi wrote:
>> Eugen Hristev <eugen.hristev@...labora.com> writes:
>>
>>> From: Gabriel Krisman Bertazi <krisman@...labora.com>
>>>
>>> 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@...labora.com>
>>> [eugen.hristev@...labora.com: rework to first test the exact match]
>>> Signed-off-by: Eugen Hristev <eugen.hristev@...labora.com>
>>> ---
>>> 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
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