linux-ext4 - Re: [PATCH v7 1/8] unicode: Add utf8_casefold

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Date:   Tue, 11 Feb 2020 19:38:00 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Daniel Rosenberg <drosen@...gle.com>
Cc:     Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fscrypt@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Richard Weinberger <richard@....at>,
        linux-mtd@...ts.infradead.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 1/8] unicode: Add utf8_casefold_iter

On Fri, Feb 07, 2020 at 05:35:45PM -0800, Daniel Rosenberg wrote:
> This function will allow other uses of unicode to act upon a casefolded
> string without needing to allocate their own copy of one.
> 
> The actor function can return an nonzero value to exit early.
> 
> Signed-off-by: Daniel Rosenberg <drosen@...gle.com>
> ---
>  fs/unicode/utf8-core.c  | 25 ++++++++++++++++++++++++-
>  include/linux/unicode.h | 10 ++++++++++
>  2 files changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/unicode/utf8-core.c b/fs/unicode/utf8-core.c
> index 2a878b739115d..db050bf59a32b 100644
> --- a/fs/unicode/utf8-core.c
> +++ b/fs/unicode/utf8-core.c
> @@ -122,9 +122,32 @@ int utf8_casefold(const struct unicode_map *um, const struct qstr *str,
>  	}
>  	return -EINVAL;
>  }
> -
>  EXPORT_SYMBOL(utf8_casefold);
>  
> +int utf8_casefold_iter(const struct unicode_map *um, const struct qstr *str,
> +		    struct utf8_itr_context *ctx)
> +{
> +	const struct utf8data *data = utf8nfdicf(um->version);
> +	struct utf8cursor cur;
> +	int c;
> +	int res = 0;
> +	int pos = 0;
> +
> +	if (utf8ncursor(&cur, data, str->name, str->len) < 0)
> +		return -EINVAL;
> +
> +	while ((c = utf8byte(&cur))) {
> +		if (c < 0)
> +			return c;
> +		res = ctx->actor(ctx, c, pos);
> +		pos++;
> +		if (res)
> +			return res;
> +	}
> +	return res;
> +}
> +EXPORT_SYMBOL(utf8_casefold_iter);

Indirect function calls are expensive these days for various reasons, including
Spectre mitigations and CFI.  Are you sure it's okay from a performance
perspective to make an indirect call for every byte of the pathname?

> +typedef int (*utf8_itr_actor_t)(struct utf8_itr_context *, int byte, int pos);

The byte argument probably should be 'u8', to avoid confusion about whether it's
a byte or a Unicode codepoint.

- Eric