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

[Eugen Hristev]

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.

Eugen
> 
> 
>> +	} else {
>> +		if (dirent.len == name->len &&
>> +		    !memcmp(name->name, dirent.name, dirent.len) &&
>> +		    (!sb_has_strict_encoding(sb) || !utf8_validate(um, name))) {
>> +			res = 1;
>> +			goto out;
>> +		}
>> +		res = !utf8_strncasecmp(um, name, &dirent);
>> +	}
>> +
>> +out:
>> +	kfree(decrypted_name.name);
>> +	return res;
>> +}
>> +EXPORT_SYMBOL(generic_ci_match);
>>  #endif
>>  
>>  #ifdef CONFIG_FS_ENCRYPTION
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 820b93b2917f..7af691ff8d44 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -3296,6 +3296,10 @@ extern int generic_file_fsync(struct file *, loff_t, loff_t, int);
>>  extern int generic_check_addressable(unsigned, u64);
>>  
>>  extern void generic_set_encrypted_ci_d_ops(struct dentry *dentry);
>> +extern 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);
>>  
>>  static inline bool sb_has_encoding(const struct super_block *sb)
>>  {
>