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From:   Gabriel Krisman Bertazi <krisman@collabora.com>
To:     Shreeya Patel <shreeya.patel@collabora.com>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca, jaegeuk@kernel.org,
        chao@kernel.org, ebiggers@google.com, drosen@google.com,
        ebiggers@kernel.org, yuchao0@huawei.com,
        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-fsdevel@vger.kernel.org, kernel@collabora.com,
        andre.almeida@collabora.com
Subject: Re: [PATCH v5 4/4] fs: unicode: Add utf8 module and a unicode layer
Organization: Collabora
References: <20210329204240.359184-1-shreeya.patel@collabora.com>
        <20210329204240.359184-5-shreeya.patel@collabora.com>
Date:   Mon, 29 Mar 2021 17:20:18 -0400
In-Reply-To: <20210329204240.359184-5-shreeya.patel@collabora.com> (Shreeya
        Patel's message of "Tue, 30 Mar 2021 02:12:40 +0530")
Message-ID: <87o8f1r71p.fsf@collabora.com>
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Shreeya Patel <shreeya.patel@collabora.com> writes:

> 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 load this large table in the kernel if no
> filesystem is using it, hence make UTF-8 encoding loadable by converting
> it into a module.
> 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.
> Also, indirect calls using function pointers are slow, use static calls to
> avoid overhead caused in case of repeated indirect calls. Static calls
> improves the performance by directly calling the functions as opposed to
> indirect calls.
>
> Signed-off-by: Shreeya Patel <shreeya.patel@collabora.com>
> ---
> Changes in v5
>   - Rename global variables and default static call functions for better
>     understanding
>   - Make only config UNICODE_UTF8 visible and config UNICODE to be always
>     enabled provided UNICODE_UTF8 is enabled.  
>   - Improve the documentation for Kconfig
>   - Improve the commit message.
>  
> Changes in v4
>   - Return error from the static calls instead of doing nothing and
>     succeeding even without loading the module.
>   - Remove the complete usage of utf8_ops and use static calls at all
>     places.
>   - Restore the static calls to default values when module is unloaded.
>   - Decrement the reference of module after calling the unload function.
>   - Remove spinlock as there will be no race conditions after removing
>     utf8_ops.
>
> Changes in v3
>   - Add a patch which checks if utf8 is loaded before calling utf8_unload()
>     in ext4 and f2fs filesystems
>   - Return error if strscpy() returns value < 0
>   - Correct the conditions to prevent NULL pointer dereference while
>     accessing functions via utf8_ops variable.
>   - Add spinlock to avoid race conditions.
>   - Use static_call() for preventing speculative execution attacks.
>
> Changes in v2
>   - Remove the duplicate file from the last patch.
>   - 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.
>   - Resolve the warning reported by kernel test robot.
>   - Resolve all the checkpatch.pl warnings.
>
>
>  fs/unicode/Kconfig        |  17 ++-
>  fs/unicode/Makefile       |   5 +-
>  fs/unicode/unicode-core.c | 241 +++++++----------------------------
>  fs/unicode/unicode-utf8.c | 256 ++++++++++++++++++++++++++++++++++++++
>  include/linux/unicode.h   | 123 +++++++++++++++---
>  5 files changed, 426 insertions(+), 216 deletions(-)
>  create mode 100644 fs/unicode/unicode-utf8.c
>
> diff --git a/fs/unicode/Kconfig b/fs/unicode/Kconfig
> index 2c27b9a5cd6c..ad4b837f2eb2 100644
> --- a/fs/unicode/Kconfig
> +++ b/fs/unicode/Kconfig
> @@ -2,13 +2,26 @@
>  #
>  # UTF-8 normalization
>  #
> +# CONFIG_UNICODE will be automatically enabled if CONFIG_UNICODE_UTF8
> +# is enabled. This config option adds the unicode subsystem layer which loads
> +# the UTF-8 module whenever any filesystem needs it.
>  config UNICODE
> -	bool "UTF-8 normalization and casefolding support"
> +	bool
> +
> +# utf8data.h_shipped has a large database table which is an auto-generated
> +# decodification trie for the unicode normalization functions and it is not
> +# necessary to carry this large table in the kernel.
> +# Enabling UNICODE_UTF8 option will allow UTF-8 encoding to be built as a
> +# module and this module will be loaded by the unicode subsystem layer only
> +# when any filesystem needs it.
> +config UNICODE_UTF8
> +	tristate "UTF-8 module"
>  	help
>  	  Say Y here to enable UTF-8 NFD normalization and NFD+CF casefolding
>  	  support.
> +	select UNICODE
>  
>  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..49d50083e6ee 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 := unicode-utf8.o utf8-norm.o
>  
>  $(obj)/utf8-norm.o: $(obj)/utf8data.h
> +$(obj)/unicode-utf8.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 730dbaedf593..07d42f471e42 100644
> --- a/fs/unicode/unicode-core.c
> +++ b/fs/unicode/unicode-core.c
> @@ -1,237 +1,80 @@
>  /* 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"
> +static struct module *utf8mod;
>  
> -int unicode_validate(const struct unicode_map *um, const struct qstr *str)
> -{
> -	const struct utf8data *data = utf8nfdi(um->version);
> +DEFINE_STATIC_CALL(_unicode_validate, unicode_validate_default);
> +EXPORT_STATIC_CALL(_unicode_validate);
>  
> -	if (utf8nlen(data, str->name, str->len) < 0)
> -		return -1;
> -	return 0;
> -}
> -EXPORT_SYMBOL(unicode_validate);
> +DEFINE_STATIC_CALL(_unicode_strncmp, unicode_strncmp_default);
> +EXPORT_STATIC_CALL(_unicode_strncmp);
>  
> -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;
> +DEFINE_STATIC_CALL(_unicode_strncasecmp, unicode_strncasecmp_default);
> +EXPORT_STATIC_CALL(_unicode_strncasecmp);

Why are these here if the _default functions are defined in the header
file?  I think the definitions could be in this file. No?

> -	if (utf8ncursor(&cur1, data, s1->name, s1->len) < 0)
> -		return -EINVAL;
> +DEFINE_STATIC_CALL(_unicode_strncasecmp_folded, unicode_strncasecmp_folded_default);
> +EXPORT_STATIC_CALL(_unicode_strncasecmp_folded);
>  
> -	if (utf8ncursor(&cur2, data, s2->name, s2->len) < 0)
> -		return -EINVAL;
> +DEFINE_STATIC_CALL(_unicode_normalize, unicode_normalize_default);
> +EXPORT_STATIC_CALL(_unicode_normalize);
>  
> -	do {
> -		c1 = utf8byte(&cur1);
> -		c2 = utf8byte(&cur2);
> +DEFINE_STATIC_CALL(_unicode_casefold, unicode_casefold_default);
> +EXPORT_STATIC_CALL(_unicode_casefold);
>  
> -		if (c1 < 0 || c2 < 0)
> -			return -EINVAL;
> -		if (c1 != c2)
> -			return 1;
> -	} while (c1);
> +DEFINE_STATIC_CALL(_unicode_casefold_hash, unicode_casefold_hash_default);
> +EXPORT_STATIC_CALL(_unicode_casefold_hash);
>  
> -	return 0;
> -}
> -EXPORT_SYMBOL(unicode_strncmp);
> +DEFINE_STATIC_CALL(_unicode_load, unicode_load_default);
> +EXPORT_STATIC_CALL(_unicode_load);
>  
> -int unicode_strncasecmp(const struct unicode_map *um,
> -			const struct qstr *s1, const struct qstr *s2)
> +static int unicode_load_module(void)
>  {
> -	const struct utf8data *data = utf8nfdicf(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_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)
> -{
> -	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;
> -
> -	for (nlen = 0; nlen < dlen; nlen++) {
> -		int c = utf8byte(&cur);
> -
> -		dest[nlen] = c;
> -		if (!c)
> -			return nlen;
> -		if (c == -1)
> -			break;
> -	}
> -	return -EINVAL;
> -}
> -EXPORT_SYMBOL(unicode_casefold);
> +	int ret = request_module("utf8");
>  
> -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);
> +	if (ret) {
> +		pr_err("Failed to load UTF-8 module\n");
> +		return ret;
>  	}
> -	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;
> -
> -	if (utf8ncursor(&cur, data, str->name, str->len) < 0)
> -		return -EINVAL;
> +	int ret = unicode_load_module();

Splitting this in two functions sound unnecessary, since the other
function just calls request_module.  By the way, is there any protection
against calling request_module if the module is already loaded?  Surely
that's not necessary, perhaps try_then_request_module(utf8mod, "utf8")?

> -	for (nlen = 0; nlen < dlen; nlen++) {
> -		int c = utf8byte(&cur);
> +	if (ret)
> +		return ERR_PTR(ret);
>  
> -		dest[nlen] = c;
> -		if (!c)
> -			return nlen;
> -		if (c == -1)
> -			break;
> -	}
> -	return -EINVAL;
> +	if (!try_module_get(utf8mod))

Can't module_unregister be called in between the register_module and
here, and then you have a bogus utf8mod pointer? true, try_module_get
checks for NULL, but if you are unlucky module_is_live will it breaks.
I still think utf8mod needs to be protected while you don't have a
reference to the module.

> +		return ERR_PTR(-ENODEV);
> +	else
> +		return static_call(_unicode_load)(version);
>  }
> -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}
> -	};
> -	int ret = strscpy(version_string, version, sizeof(version_string));
> -
> -	if (ret < 0)
> -		return ret;
> -
> -	if (match_token(version_string, token, args) != 1)
> -		return -EINVAL;
> -
> -	if (match_int(&args[0], maj) || match_int(&args[1], min) ||
> -	    match_int(&args[2], rev))
> -		return -EINVAL;
> +	kfree(um);
>  
> -	return 0;
> +	if (utf8mod)
> +		module_put(utf8mod);
>  }
> +EXPORT_SYMBOL(unicode_unload);
>  
> -struct unicode_map *unicode_load(const char *version)
> +void unicode_register(struct module *owner)
>  {
> -	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;
> +	utf8mod = owner;
>  }
> -EXPORT_SYMBOL(unicode_load);
> +EXPORT_SYMBOL(unicode_register);
>  
> -void unicode_unload(struct unicode_map *um)
> +void unicode_unregister(void)
>  {
> -	kfree(um);
> +	utf8mod = NULL;
>  }
> -EXPORT_SYMBOL(unicode_unload);
> +EXPORT_SYMBOL(unicode_unregister);
>  
>  MODULE_LICENSE("GPL v2");
> diff --git a/fs/unicode/unicode-utf8.c b/fs/unicode/unicode-utf8.c
> new file mode 100644
> index 000000000000..9c6b58239067
> --- /dev/null
> +++ b/fs/unicode/unicode-utf8.c
> @@ -0,0 +1,256 @@
> +// 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 <linux/static_call.h>
> +
> +#include "utf8n.h"
> +
> +static int utf8_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;
> +}
> +
> +static int utf8_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;
> +}
> +
> +static int utf8_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;
> +
> +	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;
> +}
> +
> +/* String cf is expected to be a valid UTF-8 casefolded
> + * string.
> + */
> +static int utf8_strncasecmp_folded(const struct unicode_map *um,
> +				   const struct qstr *cf,
> +				   const struct qstr *s1)
> +{
> +	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;
> +}
> +
> +static int utf8_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;
> +
> +	for (nlen = 0; nlen < dlen; nlen++) {
> +		int c = utf8byte(&cur);
> +
> +		dest[nlen] = c;
> +		if (!c)
> +			return nlen;
> +		if (c == -1)
> +			break;
> +	}
> +	return -EINVAL;
> +}
> +
> +static int utf8_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;
> +}
> +
> +static int utf8_normalize(const struct unicode_map *um, const struct qstr *str,
> +			  unsigned char *dest, size_t dlen)
> +{
> +	const struct utf8data *data = utf8nfdi(um->version);
> +	struct utf8cursor cur;
> +	ssize_t nlen = 0;
> +
> +	if (utf8ncursor(&cur, data, str->name, str->len) < 0)
> +		return -EINVAL;
> +
> +	for (nlen = 0; nlen < dlen; nlen++) {
> +		int c = utf8byte(&cur);
> +
> +		dest[nlen] = c;
> +		if (!c)
> +			return nlen;
> +		if (c == -1)
> +			break;
> +	}
> +	return -EINVAL;
> +}
> +
> +static int utf8_parse_version(const char *version, unsigned int *maj,
> +			      unsigned int *min, unsigned int *rev)
> +{
> +	substring_t args[3];
> +	char version_string[12];
> +	static const struct match_token token[] = {
> +		{1, "%d.%d.%d"},
> +		{0, NULL}
> +	};
> +
> +	int ret = strscpy(version_string, version, sizeof(version_string));
> +
> +	if (ret < 0)
> +		return ret;
> +
> +	if (match_token(version_string, token, args) != 1)
> +		return -EINVAL;
> +
> +	if (match_int(&args[0], maj) || match_int(&args[1], min) ||
> +	    match_int(&args[2], rev))
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
> +static struct unicode_map *utf8_load(const char *version)
> +{
> +	struct unicode_map *um = NULL;
> +	int unicode_version;
> +
> +	if (version) {
> +		unsigned int maj, min, rev;
> +
> +		if (utf8_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();
> +		pr_warn("UTF-8 version not specified. Assuming latest supported version (%d.%d.%d).",
> +			(unicode_version >> 16) & 0xff,
> +			(unicode_version >> 8) & 0xff,
> +			(unicode_version & 0xfe));
> +	}
> +
> +	um = kzalloc(sizeof(*um), GFP_KERNEL);
> +	if (!um)
> +		return ERR_PTR(-ENOMEM);
> +
> +	um->charset = "UTF-8";
> +	um->version = unicode_version;
> +
> +	return um;
> +}
> +
> +static int __init utf8_init(void)
> +{
> +	static_call_update(_unicode_validate, utf8_validate);
> +	static_call_update(_unicode_strncmp, utf8_strncmp);
> +	static_call_update(_unicode_strncasecmp, utf8_strncasecmp);
> +	static_call_update(_unicode_strncasecmp_folded, utf8_strncasecmp_folded);
> +	static_call_update(_unicode_normalize, utf8_normalize);
> +	static_call_update(_unicode_casefold, utf8_casefold);
> +	static_call_update(_unicode_casefold_hash, utf8_casefold_hash);
> +	static_call_update(_unicode_load, utf8_load);
> +
> +	unicode_register(THIS_MODULE);
> +	return 0;
> +}
> +
> +static void __exit utf8_exit(void)
> +{
> +	static_call_update(_unicode_validate, unicode_validate_default);
> +	static_call_update(_unicode_strncmp, unicode_strncmp_default);
> +	static_call_update(_unicode_strncasecmp, unicode_strncasecmp_default);
> +	static_call_update(_unicode_strncasecmp_folded, unicode_strncasecmp_folded_default);
> +	static_call_update(_unicode_normalize, unicode_normalize_default);
> +	static_call_update(_unicode_casefold, unicode_casefold_default);
> +	static_call_update(_unicode_casefold_hash, unicode_casefold_hash_default);
> +	static_call_update(_unicode_load, unicode_load_default);
> +
> +	unicode_unregister();
> +}
> +
> +module_init(utf8_init);
> +module_exit(utf8_exit);
> +
> +MODULE_LICENSE("GPL v2");
> diff --git a/include/linux/unicode.h b/include/linux/unicode.h
> index de23f9ee720b..18a1d3db9de5 100644
> --- a/include/linux/unicode.h
> +++ b/include/linux/unicode.h
> @@ -4,33 +4,128 @@
>  
>  #include <linux/init.h>
>  #include <linux/dcache.h>
> +#include <linux/static_call.h>
> +
>  
>  struct unicode_map {
>  	const char *charset;
>  	int version;
>  };
>  
> -int unicode_validate(const struct unicode_map *um, const struct qstr *str);
> +static int unicode_warn_on(void)
> +{
> +	WARN_ON(1);
> +	return -EIO;
> +}

Creating this extra function adds the same number of lines than if you
write `WARN_ON(1); return -EIO;` in each of the few handlers below, but
the later would be more clear, and you already do it for
unicode_load_default anyway. :)
> +
> +static int unicode_validate_default(const struct unicode_map *um,
> +				    const struct qstr *str)
> +{
> +	return unicode_warn_on();
> +}
> +
> +static int unicode_strncmp_default(const struct unicode_map *um,
> +				   const struct qstr *s1,
> +				   const struct qstr *s2)
> +{
> +	return unicode_warn_on();
> +}
> +
> +static int unicode_strncasecmp_default(const struct unicode_map *um,
> +				       const struct qstr *s1,
> +				       const struct qstr *s2)
> +{
> +	return unicode_warn_on();
> +}
> +
> +static int unicode_strncasecmp_folded_default(const struct unicode_map *um,
> +					      const struct qstr *cf,
> +					      const struct qstr *s1)
> +{
> +	return unicode_warn_on();
> +}
> +
> +static int unicode_normalize_default(const struct unicode_map *um,
> +				     const struct qstr *str,
> +				     unsigned char *dest, size_t dlen)
> +{
> +	return unicode_warn_on();
> +}
> +
> +static int unicode_casefold_default(const struct unicode_map *um,
> +				    const struct qstr *str,
> +				    unsigned char *dest, size_t dlen)
> +{
> +	return unicode_warn_on();
> +}
>  
> -int unicode_strncmp(const struct unicode_map *um,
> -		    const struct qstr *s1, const struct qstr *s2);
> +static int unicode_casefold_hash_default(const struct unicode_map *um,
> +					 const void *salt, struct qstr *str)
> +{
> +	return unicode_warn_on();
> +}

Again, why isn't this in a .c ?  Does it need to be here?

>  
> -int unicode_strncasecmp(const struct unicode_map *um,
> -			const struct qstr *s1, const struct qstr *s2);
> -int unicode_strncasecmp_folded(const struct unicode_map *um,
> -			       const struct qstr *cf,
> -			       const struct qstr *s1);
> +static struct unicode_map *unicode_load_default(const char *version)
> +{
> +	unicode_warn_on();
> +	return ERR_PTR(-EIO);
> +}
>  
> -int unicode_normalize(const struct unicode_map *um, const struct qstr *str,
> -		      unsigned char *dest, size_t dlen);
> +DECLARE_STATIC_CALL(_unicode_validate, unicode_validate_default);
> +DECLARE_STATIC_CALL(_unicode_strncmp, unicode_strncmp_default);
> +DECLARE_STATIC_CALL(_unicode_strncasecmp, unicode_strncasecmp_default);
> +DECLARE_STATIC_CALL(_unicode_strncasecmp_folded, unicode_strncasecmp_folded_default);
> +DECLARE_STATIC_CALL(_unicode_normalize, unicode_normalize_default);
> +DECLARE_STATIC_CALL(_unicode_casefold, unicode_casefold_default);
> +DECLARE_STATIC_CALL(_unicode_casefold_hash, unicode_casefold_hash_default);
> +DECLARE_STATIC_CALL(_unicode_load, unicode_load_default);

nit: I hate this functions starting with a single  _ .  they are not common in the
rest of the kernel either.

> -int unicode_casefold(const struct unicode_map *um, const struct qstr *str,
> -		     unsigned char *dest, size_t dlen);
> +static inline int unicode_validate(const struct unicode_map *um, const struct qstr *str)
> +{
> +	return static_call(_unicode_validate)(um, str);
> +}
>  
> -int unicode_casefold_hash(const struct unicode_map *um, const void *salt,
> -			  struct qstr *str);
> +static inline int unicode_strncmp(const struct unicode_map *um,
> +				  const struct qstr *s1, const struct qstr *s2)
> +{
> +	return static_call(_unicode_strncmp)(um, s1, s2);
> +}
> +
> +static inline int unicode_strncasecmp(const struct unicode_map *um,
> +				      const struct qstr *s1, const struct qstr *s2)
> +{
> +	return static_call(_unicode_strncasecmp)(um, s1, s2);
> +}
> +
> +static inline int unicode_strncasecmp_folded(const struct unicode_map *um,
> +					     const struct qstr *cf,
> +					     const struct qstr *s1)
> +{
> +	return static_call(_unicode_strncasecmp_folded)(um, cf, s1);
> +}
> +
> +static inline int unicode_normalize(const struct unicode_map *um, const struct qstr *str,
> +				    unsigned char *dest, size_t dlen)
> +{
> +	return static_call(_unicode_normalize)(um, str, dest, dlen);
> +}
> +
> +static inline int unicode_casefold(const struct unicode_map *um, const struct qstr *str,
> +				   unsigned char *dest, size_t dlen)
> +{
> +	return static_call(_unicode_casefold)(um, str, dest, dlen);
> +}
> +
> +static inline int unicode_casefold_hash(const struct unicode_map *um, const void *salt,
> +					struct qstr *str)
> +{
> +	return static_call(_unicode_casefold_hash)(um, salt, str);
> +}
>  
>  struct unicode_map *unicode_load(const char *version);
>  void unicode_unload(struct unicode_map *um);
>  
> +void unicode_register(struct module *owner);
> +void unicode_unregister(void);
> +
>  #endif /* _LINUX_UNICODE_H */

-- 
Gabriel Krisman Bertazi