Re: [PATCH] NLS: improve UTF8 -> UTF16 string conversion routine

[Alan Stern]

On Fri, 18 Nov 2011, NamJae Jeon wrote:

> 2011/11/18 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:
> > The utf8s_to_utf16s conversion routine needs to be improved. ÂUnlike
> > its utf16s_to_utf8s sibling, it doesn't accept arguments specifying
> > the maximum length of the output buffer or the endianness of its
> > 16-bit output.
> >
> > This patch (as1501) adds the two missing arguments, and adjusts the
> > only two places in the kernel where the function is called. ÂA
> > follow-on patch will add a third caller that does utilize the new
> > capabilities.
> >
> > The two conversion routines are still annoyingly inconsistent in the
> > way they handle invalid byte combinations. ÂBut that's a subject for a
> > different patch.

> > Index: usb-3.2/fs/nls/nls_base.c
> > ===================================================================
> > --- usb-3.2.orig/fs/nls/nls_base.c
> > +++ usb-3.2/fs/nls/nls_base.c
> > @@ -114,34 +114,57 @@ int utf32_to_utf8(unicode_t u, u8 *s, in
> > Â}
> > ÂEXPORT_SYMBOL(utf32_to_utf8);
> >
> > -int utf8s_to_utf16s(const u8 *s, int len, wchar_t *pwcs)
> > +static inline void put_utf16(wchar_t *s, unsigned c, enum utf16_endian endian)
> > +{
> > + Â Â Â switch (endian) {
> > + Â Â Â default:
> > + Â Â Â Â Â Â Â *s = (wchar_t) c;
> > + Â Â Â Â Â Â Â break;
> > + Â Â Â case UTF16_LITTLE_ENDIAN:
> > + Â Â Â Â Â Â Â *s = __cpu_to_le16(c);
> > + Â Â Â Â Â Â Â break;
> > + Â Â Â case UTF16_BIG_ENDIAN:
> > + Â Â Â Â Â Â Â *s = __cpu_to_be16(c);
> > + Â Â Â Â Â Â Â break;
> > + Â Â Â }
> > +}
> > +
> > +int utf8s_to_utf16s(const u8 *s, int len, enum utf16_endian endian,
> > + Â Â Â Â Â Â Â wchar_t *pwcs, int maxlen)
> > Â{
> > Â Â Â Âu16 *op;
> > Â Â Â Âint size;
> > Â Â Â Âunicode_t u;
> >
> > Â Â Â Âop = pwcs;
> > - Â Â Â while (*s && len > 0) {
> > + Â Â Â while (len > 0 && maxlen > 0 && *s) {
> > Â Â Â Â Â Â Â Âif (*s & 0x80) {
> > Â Â Â Â Â Â Â Â Â Â Â Âsize = utf8_to_utf32(s, len, &u);
> > Â Â Â Â Â Â Â Â Â Â Â Âif (size < 0)
> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âreturn -EINVAL;
> > + Â Â Â Â Â Â Â Â Â Â Â s += size;
> > + Â Â Â Â Â Â Â Â Â Â Â len -= size;
> Why did you move this code to here ?

Mainly in order to keep the counter updates near the place where the
character is read.  Also, in an earlier version of the patch, I used a
"continue" instead of the "break" statement three lines below.  For
that to work, the updates to s and len had to be moved up here.

> > Â Â Â Â Â Â Â Â Â Â Â Âif (u >= PLANE_SIZE) {
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â if (maxlen < 2)
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â break;
> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âu -= PLANE_SIZE;
> > - Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â *op++ = (wchar_t) (SURROGATE_PAIR |
> > - Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ((u >> 10) & SURROGATE_BITS));
> > - Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â *op++ = (wchar_t) (SURROGATE_PAIR |
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â put_utf16(op++, SURROGATE_PAIR |
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ((u >> 10) & SURROGATE_BITS),
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â endian);
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â put_utf16(op++, SURROGATE_PAIR |
> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ÂSURROGATE_LOW |
> > - Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (u & SURROGATE_BITS));
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (u & SURROGATE_BITS),
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â endian);
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â maxlen -= 2;
> 
> Why did you use contants value(-2) instead of maxlen -= size; value ?

"maxlen -= size" would be completely wrong, because size is the length
of the utf8 input and maxlen is the number of 16-bit slots remaining
in the output buffer.  A surrogate pair uses two 16-bit values,
therefore maxlen has to be decreased by 2.

> > Â Â Â Â Â Â Â Â Â Â Â Â} else {
> > - Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â *op++ = (wchar_t) u;
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â put_utf16(op++, u, endian);
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â maxlen--;
> > Â Â Â Â Â Â Â Â Â Â Â Â}
> > - Â Â Â Â Â Â Â Â Â Â Â s += size;
> > - Â Â Â Â Â Â Â Â Â Â Â len -= size;
> > Â Â Â Â Â Â Â Â} else {
> > - Â Â Â Â Â Â Â Â Â Â Â *op++ = *s++;
> > + Â Â Â Â Â Â Â Â Â Â Â put_utf16(op++, *s++, endian);
> > Â Â Â Â Â Â Â Â Â Â Â Âlen--;
> > + Â Â Â Â Â Â Â Â Â Â Â maxlen--;
> > Â Â Â Â Â Â Â Â}
> > Â Â Â Â}
> > Â Â Â Âreturn op - pwcs;
> > Index: usb-3.2/fs/fat/namei_vfat.c
> > ===================================================================
> > --- usb-3.2.orig/fs/fat/namei_vfat.c
> > +++ usb-3.2/fs/fat/namei_vfat.c
> > @@ -512,7 +512,8 @@ xlate_to_uni(const unsigned char *name,
> > Â Â Â Âint charlen;
> >
> > Â Â Â Âif (utf8) {
> > - Â Â Â Â Â Â Â *outlen = utf8s_to_utf16s(name, len, (wchar_t *)outname);
> > + Â Â Â Â Â Â Â *outlen = utf8s_to_utf16s(name, len, UTF16_HOST_ENDIAN,
> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (wchar_t *) outname, FAT_LFN_LEN + 2);
> Is there the reason why you plus 2 to FAT_LFN_LEN ?

So that the "(*outlen > FAT_LFN_LEN)" test below will work correctly.
If the maximum length were set to FAT_LFN_LEN then the test would
always fail.  If the maximum length were set to FAT_LFN_LEN + 1 then
the test would fail when the next character to be stored was a
surrogate pair.

> > Â Â Â Â Â Â Â Âif (*outlen < 0)
> > Â Â Â Â Â Â Â Â Â Â Â Âreturn *outlen;
> > Â Â Â Â Â Â Â Âelse if (*outlen > FAT_LFN_LEN)
>                           return -ENAMETOOLONG;
> "else if (*outlen > FAT_LFN_LEN)" code  is needed ? Is there the case
> that *outlen is over FAT_LFN_LEN in your patch ?

I have no idea.  That test was already there, I didn't add or change it.

> Thanks.

Alan Stern

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