linux-kernel - Re: [PATCH v13 3/9] libfs: Introduce case-insensitive string comparison helper

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Message-ID: <87il1pk9hz.fsf@mailhost.krisman.be>
Date: Wed, 13 Mar 2024 19:16:56 -0400
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 v13 3/9] libfs: Introduce case-insensitive string
 comparison helper

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         | 81 ++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h |  4 +++
>  2 files changed, 85 insertions(+)
>
> diff --git a/fs/libfs.c b/fs/libfs.c
> index c297953db948..c107c24f33b9 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -1776,6 +1776,87 @@ static const struct dentry_operations generic_ci_dentry_ops = {
>  	.d_revalidate = fscrypt_d_revalidate,
>  #endif
>  };
> +
> +/**
> + * 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.
> + *
> + * 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, match = 0;
> +
> +	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.
> +	 */
> +	if (folded_name->name) {
> +		if (dirent.len == folded_name->len &&
> +		    !memcmp(folded_name->name, dirent.name, dirent.len)) {
> +			match = 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))) {
> +			match = 1;
> +			goto out;
> +		}
> +		res = utf8_strncasecmp(um, name, &dirent);
> +	}
> +
> +out:
> +	kfree(decrypted_name.name);

> +	if (match) /* matched by direct comparison */
> +		return 1;
> +	else if (!res) /* matched by utf8 comparison */
> +		return 1;
> +	else if (res < 0) /* error on utf8 comparison */
> +		return res;
> +	return 0; /* no match */
> +}

I think I've made this comment before, but I'd prefer this to be written
in a much simpler way

if (res < 0)
   return res;
return (match || !res);

Other than that, this looks good to me.

-- 
Gabriel Krisman Bertazi