linux-kernel - NLS: utf8 conversions

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Date:	Fri, 24 Apr 2009 16:02:52 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Kernel development list <linux-kernel@...r.kernel.org>
cc:	USB list <linux-usb@...r.kernel.org>
Subject: NLS: utf8 conversions

Although nobody seems to have made a big deal about it, the conversions
between utf8 and utf16 done by fs/nls/nls_base.c are wrong in a couple
of important respects:

	They don't handle Unicode code points larger than U+FFFF.

	They don't detect invalid values, in particular, surrogate
	code points.

The problems stem from the fact the characters at issue can't be
represented by a single 16-bit wchar_t.  But that's no excuse for
performing an incorrect conversion to or from utf16.

Are there any definite thoughts on how this should be handled?  I don't 
see any way for the single-character conversion routines (utf8_mbtowc 
and utf8_wctomb) to come to grips with these issues, except perhaps for 
returning an error when a character would be invalid or too big to fit 
in 16 bits.

The string-oriented routines (utf8_mbstowcs and utf8_wcstombs) could be 
adapted to deal with these issues properly.

Any comments or suggestions for other approaches?

Alan Stern

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