Re: [RESEND PATCH v9 1/3] libfs: Introduce case-insensitive string comparison helper

[Gabriel Krisman Bertazi]

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.
>
> Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
> Signed-off-by: Eugen Hristev <eugen.hristev@xxxxxxxxxxxxx>

Hi Eugen,

Thanks for picking this up.  Please,  CC me in future versions.

> ---
>  fs/libfs.c         | 68 ++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h |  4 +++
>  2 files changed, 72 insertions(+)
>
> diff --git a/fs/libfs.c b/fs/libfs.c
> index bb18884ff20e..f80cb982ac89 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -1773,6 +1773,74 @@ 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.
> + * @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.
> + *
> + *
> + * 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.

Can we add a note that this is a filesystem helper for comparison with
directory entries, and VFS' ->d_compare should use generic_ci_d_compare.

> + *
> + * 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, match = false;

I know I originally wrote it this way, but match is an integer, so
let's use integers instead of false/true.

> +
> +	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;
> +	}
> +
> +	if (folded_name->name)
> +		res = utf8_strncasecmp_folded(um, folded_name, &dirent);
> +	else
> +		res = utf8_strncasecmp(um, name, &dirent);

Similar to

  https://git.kernel.org/pub/scm/linux/kernel/git/krisman/unicode.git/commit/?h=for-next&id=367122c529f35b4655acbe33c0cc4d6d3b32ba71

We should be checking for an exact-match first to avoid the utf8
comparison cost unnecessarily.  The only problem is that we need to
ensure we fail for an invalid utf-8 de_name in "strict mode".

Fortunately, if folded_name->name exists, we know the name-under-lookup
was validated when initialized, so an exact-match of de_name must also
be valid.  If folded_name is NULL, though, we might either have an
invalid utf-8 dentry-under-lookup or the allocation itself failed, so we
need to utf8_validate it.

Honestly, I don't care much about this !folded_name->name case, since
utf8_strncasecmp will do the right thing and an invalid utf8 on
case-insensitive directories should be an exception, not the norm.  but
the code might get simpler if we do both:

(untested)

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);
} 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);
}

> +
> +	if (!res)
> +		match = true;
> +	else if (res < 0 && !sb_has_strict_encoding(sb)) {
> +		/*
> +		 * In non-strict mode, fallback to a byte comparison if
> +		 * the names have invalid characters.
> +		 */
> +		res = 0;
> +		match = ((name->len == dirent.len) &&
> +			 !memcmp(name->name, dirent.name, dirent.len));
> +	}

This goes away entirely.

> +
> +out:
> +	kfree(decrypted_name.name);
> +	return (res >= 0) ? match : res;

and this becomes:

return res;

-- 
Gabriel Krisman Bertazi