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From:   Gabriel Krisman Bertazi <krisman@collabora.com>
To:     Shreeya Patel <shreeya.patel@collabora.com>
Cc:     jaegeuk@kernel.org, yuchao0@huawei.com, tytso@mit.edu,
        adilger.kernel@dilger.ca, drosen@google.com, ebiggers@google.com,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        kernel@collabora.com, andre.almeida@collabora.com
Subject: Re: [PATCH v2 4/4] fs: unicode: Add utf8 module and a unicode layer
Organization: Collabora
References: <20210318133305.316564-1-shreeya.patel@collabora.com>
        <20210318133305.316564-5-shreeya.patel@collabora.com>
Date:   Thu, 18 Mar 2021 15:57:03 -0400
In-Reply-To: <20210318133305.316564-5-shreeya.patel@collabora.com> (Shreeya
        Patel's message of "Thu, 18 Mar 2021 19:03:05 +0530")
Message-ID: <87sg4si6b4.fsf@collabora.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)
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Precedence: bulk


Shreeya,


> utf8data.h_shipped has a large database table which is an auto-generated
> decodification trie for the unicode normalization functions.
> It is not necessary to carry this large table in the kernel hence make
>

Thanks for the v2.  This is looking much better!  I have a few comments
inline.

I also have a question about how this patchset was tested.  Did you run
the normalization test (UNICODE_NORMALIZATION_SELFTEST)?  What about
actual filesystems?

Minor nit:

"It is not necessary to load this large table in the kernel in the
kernel if no file system is using it"

> UTF-8 encoding loadable by converting it into a module.
> Also, modify the file called unicode-core which will act as a layer for
> unicode subsystem. It will load the UTF-8 module and access it's functions
> whenever any filesystem that needs unicode is mounted.
>
> Signed-off-by: Shreeya Patel <shreeya.patel@collabora.com>
> ---
>
> Changes in v2
>   - Remove the duplicate file utf8-core.c
>   - Make the wrapper functions inline.
>   - Remove msleep and use try_module_get() and module_put()
>     for ensuring that module is loaded correctly and also
>     doesn't get unloaded while in use.

>  fs/unicode/Kconfig        |  11 +-
>  fs/unicode/Makefile       |   5 +-
>  fs/unicode/unicode-core.c | 229 +++++------------------------------
>  fs/unicode/utf8mod.c      | 246 ++++++++++++++++++++++++++++++++++++++
>  include/linux/unicode.h   |  73 ++++++++---
>  5 files changed, 346 insertions(+), 218 deletions(-)
>  create mode 100644 fs/unicode/utf8mod.c
>
> diff --git a/fs/unicode/Kconfig b/fs/unicode/Kconfig
> index 2c27b9a5cd6c..2961b0206b4d 100644
> --- a/fs/unicode/Kconfig
> +++ b/fs/unicode/Kconfig
> @@ -8,7 +8,16 @@ config UNICODE
>  	  Say Y here to enable UTF-8 NFD normalization and NFD+CF casefolding
>  	  support.
>  
> +# UTF-8 encoding can be compiled as a module using UNICODE_UTF8 option.
> +# Having UTF-8 encoding as a module will avoid carrying large
> +# database table present in utf8data.h_shipped into the kernel
> +# by being able to load it only when it is required by the filesystem.
> +config UNICODE_UTF8
> +	tristate "UTF-8 module"
> +	depends on UNICODE
> +	default m
> +
>  config UNICODE_NORMALIZATION_SELFTEST
>  	tristate "Test UTF-8 normalization support"
> -	depends on UNICODE
> +	depends on UNICODE_UTF8
>  	default n
> diff --git a/fs/unicode/Makefile b/fs/unicode/Makefile
> index fbf9a629ed0d..9dbb04194b32 100644
> --- a/fs/unicode/Makefile
> +++ b/fs/unicode/Makefile
> @@ -1,11 +1,14 @@
>  # SPDX-License-Identifier: GPL-2.0
>  
>  obj-$(CONFIG_UNICODE) += unicode.o
> +obj-$(CONFIG_UNICODE_UTF8) += utf8.o
>  obj-$(CONFIG_UNICODE_NORMALIZATION_SELFTEST) += utf8-selftest.o
>  
> -unicode-y := utf8-norm.o unicode-core.o
> +unicode-y := unicode-core.o
> +utf8-y := utf8mod.o utf8-norm.o
>  
>  $(obj)/utf8-norm.o: $(obj)/utf8data.h
> +$(obj)/utf8mod.o: $(obj)/utf8-norm.o
>  
>  # In the normal build, the checked-in utf8data.h is just shipped.
>  #
> diff --git a/fs/unicode/unicode-core.c b/fs/unicode/unicode-core.c
> index 287a8a48836c..945984a3fe9e 100644
> --- a/fs/unicode/unicode-core.c
> +++ b/fs/unicode/unicode-core.c
> @@ -1,235 +1,60 @@
>  /* SPDX-License-Identifier: GPL-2.0 */
>  #include <linux/module.h>
>  #include <linux/kernel.h>
> -#include <linux/string.h>
>  #include <linux/slab.h>
> -#include <linux/parser.h>
>  #include <linux/errno.h>
>  #include <linux/unicode.h>
> -#include <linux/stringhash.h>
>  
> -#include "utf8n.h"
>  
> -int unicode_validate(const struct unicode_map *um, const struct qstr *str)
> -{
> -	const struct utf8data *data = utf8nfdi(um->version);
> -
> -	if (utf8nlen(data, str->name, str->len) < 0)
> -		return -1;
> -	return 0;
> -}
> -EXPORT_SYMBOL(unicode_validate);
> -
> -int unicode_strncmp(const struct unicode_map *um,
> -		    const struct qstr *s1, const struct qstr *s2)
> -{
> -	const struct utf8data *data = utf8nfdi(um->version);
> -	struct utf8cursor cur1, cur2;
> -	int c1, c2;
> -
> -	if (utf8ncursor(&cur1, data, s1->name, s1->len) < 0)
> -		return -EINVAL;
> -
> -	if (utf8ncursor(&cur2, data, s2->name, s2->len) < 0)
> -		return -EINVAL;
> -
> -	do {
> -		c1 = utf8byte(&cur1);
> -		c2 = utf8byte(&cur2);
> -
> -		if (c1 < 0 || c2 < 0)
> -			return -EINVAL;
> -		if (c1 != c2)
> -			return 1;
> -	} while (c1);
> -
> -	return 0;
> -}
> -EXPORT_SYMBOL(unicode_strncmp);
> -
> -int unicode_strncasecmp(const struct unicode_map *um,
> -			const struct qstr *s1, const struct qstr *s2)
> -{
> -	const struct utf8data *data = utf8nfdicf(um->version);
> -	struct utf8cursor cur1, cur2;
> -	int c1, c2;
> +struct unicode_ops *utf8_ops;
> +EXPORT_SYMBOL(utf8_ops);
>  
> -	if (utf8ncursor(&cur1, data, s1->name, s1->len) < 0)
> -		return -EINVAL;
> -
> -	if (utf8ncursor(&cur2, data, s2->name, s2->len) < 0)
> -		return -EINVAL;
> -
> -	do {
> -		c1 = utf8byte(&cur1);
> -		c2 = utf8byte(&cur2);
> -
> -		if (c1 < 0 || c2 < 0)
> -			return -EINVAL;
> -		if (c1 != c2)
> -			return 1;
> -	} while (c1);
> -
> -	return 0;
> -}
> -EXPORT_SYMBOL(unicode_strncasecmp);
> -
> -/* String cf is expected to be a valid UTF-8 casefolded
> - * string.
> - */
> -int unicode_strncasecmp_folded(const struct unicode_map *um,
> -			       const struct qstr *cf,
> -			       const struct qstr *s1)
> +static int unicode_load_module(void)
>  {
> -	const struct utf8data *data = utf8nfdicf(um->version);
> -	struct utf8cursor cur1;
> -	int c1, c2;
> -	int i = 0;
> -
> -	if (utf8ncursor(&cur1, data, s1->name, s1->len) < 0)
> -		return -EINVAL;
> -
> -	do {
> -		c1 = utf8byte(&cur1);
> -		c2 = cf->name[i++];
> -		if (c1 < 0)
> -			return -EINVAL;
> -		if (c1 != c2)
> -			return 1;
> -	} while (c1);
> -
> -	return 0;
> -}
> -EXPORT_SYMBOL(unicode_strncasecmp_folded);
> -
> -int unicode_casefold(const struct unicode_map *um, const struct qstr *str,
> -		     unsigned char *dest, size_t dlen)
> -{
> -	const struct utf8data *data = utf8nfdicf(um->version);
> -	struct utf8cursor cur;
> -	size_t nlen = 0;
> -
> -	if (utf8ncursor(&cur, data, str->name, str->len) < 0)
> -		return -EINVAL;
> +	int ret = request_module("utf8");
>  
> -	for (nlen = 0; nlen < dlen; nlen++) {
> -		int c = utf8byte(&cur);
> -
> -		dest[nlen] = c;
> -		if (!c)
> -			return nlen;
> -		if (c == -1)
> -			break;
> +	if (ret) {
> +		pr_err("Failed to load UTF-8 module\n");
> +		return ret;
>  	}
> -	return -EINVAL;
> -}
> -EXPORT_SYMBOL(unicode_casefold);
> -
> -int unicode_casefold_hash(const struct unicode_map *um, const void *salt,
> -			  struct qstr *str)
> -{
> -	const struct utf8data *data = utf8nfdicf(um->version);
> -	struct utf8cursor cur;
> -	int c;
> -	unsigned long hash = init_name_hash(salt);
>  
> -	if (utf8ncursor(&cur, data, str->name, str->len) < 0)
> -		return -EINVAL;
> -
> -	while ((c = utf8byte(&cur))) {
> -		if (c < 0)
> -			return -EINVAL;
> -		hash = partial_name_hash((unsigned char)c, hash);
> -	}
> -	str->hash = end_name_hash(hash);
>  	return 0;
>  }
> -EXPORT_SYMBOL(unicode_casefold_hash);
>  
> -int unicode_normalize(const struct unicode_map *um, const struct qstr *str,
> -		      unsigned char *dest, size_t dlen)
> +struct unicode_map *unicode_load(const char *version)
>  {
> -	const struct utf8data *data = utf8nfdi(um->version);
> -	struct utf8cursor cur;
> -	ssize_t nlen = 0;
> +	int ret = unicode_load_module();
>  
> -	if (utf8ncursor(&cur, data, str->name, str->len) < 0)
> -		return -EINVAL;
> +	if (ret)
> +		return ERR_PTR(ret);
>  
> -	for (nlen = 0; nlen < dlen; nlen++) {
> -		int c = utf8byte(&cur);
> +	if (utf8_ops && !try_module_get(utf8_ops->owner))
> +		return ERR_PTR(-ENODEV);
>  
> -		dest[nlen] = c;
> -		if (!c)
> -			return nlen;
> -		if (c == -1)
> -			break;
> -	}
> -	return -EINVAL;
> +	else
> +		return utf8_ops->load(version);

This patch is a bit hard to review with the way git separated the
hunks. I think you should be able to regenerate it in a simpler form
by tinkering with git-format-patch parameters.

Here, it seems that if utf8_ops is NULL (the first condition of the if
leg), the else leg dereferences a NULL pointer by calling
utf8_ops->load(), which can't be right.

Maybe, the if leg should be:

if (!utf8_ops || !try_module_get(utf8_ops->owner)
   return ERR_PTR(-ENODEV)

But this is still racy, since you are not protecting utf8_ops before
acquiring the reference.  If you race with module removal here, a
NULL ptr dereference can still occur.  See below.

>  }
> -EXPORT_SYMBOL(unicode_normalize);
> +EXPORT_SYMBOL(unicode_load);
>  
> -static int unicode_parse_version(const char *version, unsigned int *maj,
> -				 unsigned int *min, unsigned int *rev)
> +void unicode_unload(struct unicode_map *um)
>  {
> -	substring_t args[3];
> -	char version_string[12];
> -	static const struct match_token token[] = {
> -		{1, "%d.%d.%d"},
> -		{0, NULL}
> -	};
> -
> -	strscpy(version_string, version, sizeof(version_string));
> -
> -	if (match_token(version_string, token, args) != 1)
> -		return -EINVAL;
> +	if (utf8_ops)
> +		module_put(utf8_ops->owner);
>

How can we have a unicode_map to free if utf8_ops is NULL?  that seems
to be an invalid use of API, which suggests a bug elsewhere
in the kernel.  maybe this should read like this:

void unicode_unload(struct unicode_map *um)
{
  if (WARN_ON(!utf8_ops))
    return;

  module_put(utf8_ops->owner);
  kfree(um);
}

> -	if (match_int(&args[0], maj) || match_int(&args[1], min) ||
> -	    match_int(&args[2], rev))
> -		return -EINVAL;
> -
> -	return 0;
> +	kfree(um);
>  }
> +EXPORT_SYMBOL(unicode_unload);
>  
> -struct unicode_map *unicode_load(const char *version)
> +void unicode_register(struct unicode_ops *ops)
>  {
> -	struct unicode_map *um = NULL;
> -	int unicode_version;
> -
> -	if (version) {
> -		unsigned int maj, min, rev;
> -
> -		if (unicode_parse_version(version, &maj, &min, &rev) < 0)
> -			return ERR_PTR(-EINVAL);
> -
> -		if (!utf8version_is_supported(maj, min, rev))
> -			return ERR_PTR(-EINVAL);
> -
> -		unicode_version = UNICODE_AGE(maj, min, rev);
> -	} else {
> -		unicode_version = utf8version_latest();
> -		printk(KERN_WARNING"UTF-8 version not specified. "
> -		       "Assuming latest supported version (%d.%d.%d).",
> -		       (unicode_version >> 16) & 0xff,
> -		       (unicode_version >> 8) & 0xff,
> -		       (unicode_version & 0xff));
> -	}
> -
> -	um = kzalloc(sizeof(struct unicode_map), GFP_KERNEL);
> -	if (!um)
> -		return ERR_PTR(-ENOMEM);
> -
> -	um->charset = "UTF-8";
> -	um->version = unicode_version;
> -
> -	return um;
> +	utf8_ops = ops;

While we guarantee that utf8_ops isn't modified when a unicode_map is
in-use by acquiring the module reference, registering/unregistering the
module should be protected, to avoid that race I mentioned above.

>  }
> -EXPORT_SYMBOL(unicode_load);
> +EXPORT_SYMBOL(unicode_register);
>  
> -void unicode_unload(struct unicode_map *um)
> +void unicode_unregister(void)
>  {
> -	kfree(um);
> +	utf8_ops = NULL;
>  }
> -EXPORT_SYMBOL(unicode_unload);
> +EXPORT_SYMBOL(unicode_unregister);
>  
>  MODULE_LICENSE("GPL v2");
> diff --git a/fs/unicode/utf8mod.c b/fs/unicode/utf8mod.c
> new file mode 100644
> index 000000000000..9981960da863
> --- /dev/null
> +++ b/fs/unicode/utf8mod.c

This is a bit nitpicky, but can you follow the naming scheme and call
this file unicode-utf8.c or maybe utf8-mod.c ?

-- 
Gabriel Krisman Bertazi