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

[Eugen Hristev]

On 2/19/24 16:55, Gabriel Krisman Bertazi wrote:
> 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.
> 


Okay, I am changing it.

By the way, is this supposed to work like this on case-insensitive directories ?

user@debian-rockchip-rock5b-rk3588:~$ ls -la /media/CI_dir/*cuc
ls: cannot access '/media/CI_dir/*cuc': No such file or directory
user@debian-rockchip-rock5b-rk3588:~$ ls -la /media/CI_dir/*CUC
-rw-r--r-- 1 root root 0 Feb 12 17:47 /media/CI_dir/CUC
user@debian-rockchip-rock5b-rk3588:~$ ls -la /media/CI_dir/cuc
-rw-r--r-- 1 root root 0 Feb 12 17:47 /media/CI_dir/cuc
user@debian-rockchip-rock5b-rk3588:~$


basically wildcards don't work.

Thanks,
Eugen