linux-ext4 - Re: [PATCH v2] unicode: Expose available encodings in sysfs

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Date:   Mon, 13 Apr 2020 14:50:13 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>,
        linux-fsdevel@...r.kernel.org
Cc:     linux-ext4@...r.kernel.org, kernel@...labora.com,
        Theodore Ts'o <tytso@....edu>, Jaegeuk Kim <jaegeuk@...nel.org>
Subject: Re: [PATCH v2] unicode: Expose available encodings in sysfs

Hey Gabriel,

On Mon, 2020-04-13 at 12:53 -0400, Gabriel Krisman Bertazi wrote:
> A filesystem configuration utility has no way to detect which filename
> encodings are supported by the running kernel.  This means, for
> instance, mkfs has no way to tell if the generated filesystem will be
> mountable in the current kernel or not.  Also, users have no easy way to
> know if they can update the encoding in their filesystems and still have
> something functional in the end.
> 
> This exposes details of the encodings available in the unicode
> subsystem, to fill that gap.
> 
> Cc: Theodore Ts'o <tytso@....edu>
> Cc: Jaegeuk Kim <jaegeuk@...nel.org>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>
> 
> ---
> Changes since v1:
>   - Make init functions static. (lkp)
> 
>  Documentation/ABI/testing/sysfs-fs-unicode | 13 +++++
>  fs/unicode/utf8-core.c                     | 64 ++++++++++++++++++++++
>  fs/unicode/utf8-norm.c                     | 18 ++++++
>  fs/unicode/utf8n.h                         |  5 ++
>  4 files changed, 100 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-fs-unicode
> 
> diff --git a/Documentation/ABI/testing/sysfs-fs-unicode b/Documentation/ABI/testing/sysfs-fs-unicode
> new file mode 100644
> index 000000000000..15c63367bb8e
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-fs-unicode
> @@ -0,0 +1,13 @@
> +What:		/sys/fs/unicode/latest
> +Date:		April 2020
> +Contact:	Gabriel Krisman Bertazi <krisman@...labora.com>
> +Description:
> +		The latest version of the Unicode Standard supported by
> +		this kernel
> +

Missing stop at the end of the sentence?

> +What:		/sys/fs/unicode/encodings
> +Date:		April 2020
> +Contact:	Gabriel Krisman Bertazi <krisman@...labora.com>
> +Description:
> +		List of encodings and corresponding versions supported
> +		by this kernel

Ditto.

> diff --git a/fs/unicode/utf8-core.c b/fs/unicode/utf8-core.c
> index 2a878b739115..b48e13e823a5 100644
> --- a/fs/unicode/utf8-core.c
> +++ b/fs/unicode/utf8-core.c
> @@ -6,6 +6,7 @@
>  #include <linux/parser.h>
>  #include <linux/errno.h>
>  #include <linux/unicode.h>
> +#include <linux/fs.h>
>  
>  #include "utf8n.h"
>  
> @@ -212,4 +213,67 @@ void utf8_unload(struct unicode_map *um)
>  }
>  EXPORT_SYMBOL(utf8_unload);
>  
> +static ssize_t latest_show(struct kobject *kobj,
> +			   struct kobj_attribute *attr, char *buf)
> +{
> +	int l = utf8version_latest();
> +
> +	return snprintf(buf, PAGE_SIZE, "UTF-8 %d.%d.%d\n", UNICODE_AGE_MAJ(l),
> +			UNICODE_AGE_MIN(l), UNICODE_AGE_REV(l));
> +
> +}
> +static ssize_t encodings_show(struct kobject *kobj,
> +			      struct kobj_attribute *attr, char *buf)
> +{
> +	int n;
> +
> +	n = snprintf(buf, PAGE_SIZE, "UTF-8:");
> +	n += utf8version_list(buf + n, PAGE_SIZE - n);
> +	n += snprintf(buf+n, PAGE_SIZE-n, "\n");
> +

I was wondering how sysfs-compliant this was,
in terms of one value per attribute.

Although, we do seem to break this on a few cases.

> +	return n;
> +}
> +
> +#define UCD_ATTR(x) \
> +	static struct kobj_attribute x ## _attr = __ATTR_RO(x)
> +
> +UCD_ATTR(latest);
> +UCD_ATTR(encodings);
> +
> +static struct attribute *ucd_attrs[] = {
> +	&latest_attr.attr,
> +	&encodings_attr.attr,
> +	NULL,
> +};
> +static const struct attribute_group ucd_attr_group = {
> +	.attrs = ucd_attrs,
> +};
> +static struct kobject *ucd_root;
> +
> +static int __init ucd_init(void)
> +{
> +	int ret;
> +
> +	ucd_root = kobject_create_and_add("unicode", fs_kobj);
> +	if (!ucd_root)
> +		return -ENOMEM;
> +
> +	ret = sysfs_create_group(ucd_root, &ucd_attr_group);
> +	if (ret) {
> +		kobject_put(ucd_root);
> +		ucd_root = NULL;
> +		return ret;
> +	}
> +
> +	return 0;
> +}
> +
> +static void __exit ucd_exit(void)
> +{
> +	kobject_put(ucd_root);
> +}
> +
> +module_init(ucd_init);

This code is not a module, so how about fs_initcall?

> +module_exit(ucd_exit)
> +

I can be wrong, but I see no way to remove it :-)

Thanks,
Ezequiel