linux-kernel - Re: [PATCH v8 02/13] exfat: add super block operations

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Message-ID: <20200102141332.ibdj7gfwyyrfhkhi@pali>
Date:   Thu, 2 Jan 2020 15:13:32 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Namjae Jeon <linkinjeon@...il.com>
Cc:     Namjae Jeon <namjae.jeon@...sung.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        gregkh@...uxfoundation.org, valdis.kletnieks@...edu, hch@....de,
        sj1557.seo@...sung.com
Subject: Re: [PATCH v8 02/13] exfat: add super block operations

On Thursday 02 January 2020 22:41:40 Namjae Jeon wrote:
> 2020-01-02 22:16 GMT+09:00, Pali Rohár <pali.rohar@...il.com>:
> > On Thursday 02 January 2020 09:30:29 Pali Rohár wrote:
> >> On Thursday 02 January 2020 15:06:16 Namjae Jeon wrote:
> >> > > > +static const struct fs_parameter_spec exfat_param_specs[] = {
> >> > > > +	fsparam_u32("uid",			Opt_uid),
> >> > > > +	fsparam_u32("gid",			Opt_gid),
> >> > > > +	fsparam_u32oct("umask",			Opt_umask),
> >> > > > +	fsparam_u32oct("dmask",			Opt_dmask),
> >> > > > +	fsparam_u32oct("fmask",			Opt_fmask),
> >> > > > +	fsparam_u32oct("allow_utime",		Opt_allow_utime),
> >> > > > +	fsparam_string("iocharset",		Opt_charset),
> >> > > > +	fsparam_flag("utf8",			Opt_utf8),
> >> > >
> >> > > Hello! What is the purpose of having extra special "utf8" mount
> >> > > option?
> >> > > Is not one "iocharset=utf8" option enough?
> >> > utf8 nls_table supports utf8<->utf32 conversion and does not support
> >> > surrogate character conversion.
> >>
> >> So in other words, this is just subset of UTF-8 just to 3 byte long
> >> sequences (for Unicode code points up to the U+FFFF).
> >
> > Anyway, this is limitation of kernel's NLS framework? Or limitation in
> > current exfat driver implementation?
> This is not exfat driver issue. Please check fatfs, cifs, etc..
> >
> > Because if it is in kernel's NLS framework then all kernel drivers would
> > be affected by this limitation, and not only exfat.
> Yes, FATfs also has two options and There seems to be
> CONFIG_FAT_DEFAULT_UTF8 option to avoid the issue you said.
> 
> config FAT_DEFAULT_UTF8
>         bool "Enable FAT UTF-8 option by default"
>         depends on VFAT_FS
>         default n
>         help
>           Set this if you would like to have "utf8" mount option set
>           by default when mounting FAT filesystems.
> 
>           Even if you say Y here can always disable UTF-8 for
>           particular mount by adding "utf8=0" to mount options.
> 
>           Say Y if you use UTF-8 encoding for file names, N otherwise.

I know that VFAT has two options for it, but I think this is historic
relict (backward compatibility). There are also other suspicious options
which today do nothing (e.g. cvf_format). So I would rather do not
compare it with 20 years old fat code...

But I have already looked at kernel's NLS implementation and it is
really limited to Unicode code points up to the U+FFFF. Kernel's wchar_t
type is is just for Plane-0 (u16) and all NLS encodings works with
wchar_t.

For "full Unicode" there is kernel type unicode_t.

And there is another thing, utf8 NLS encoding does not have defined
charset2upper/charset2lower tables.

So for iocharset=utf8 you really do not want to use utf8 NLS module.

But seem that above limitation for Plane-0 in kernel's NLS is not a
problem as all encodings (except utf8) fit into Plane-0. So only UTF-8
needs special handling, other encodings can use kernel's NLS tables.

> But the way you suggested looks better.
> 
> Thanks!

-- 
Pali Rohár
pali.rohar@...il.com