Re: sscanf: implement basic character sets

[Jessica Yu]

+++ Andy Shevchenko [23/02/16 12:56 +0200]:
> On Mon, 2016-02-22 at 16:24 -0500, Jessica Yu wrote:
> > Implement basic character sets for the '%[]' conversion specifier.
> >
> > The '%[]' conversion specifier matches a nonempty sequence of
> > characters
> > from the specified set of accepted (or with '^', rejected) characters
> > between the brackets. The substring matched is to be made up of
> > characters
> > in (or not in) the set. This implementation differs from its glibc
> > counterpart in that it does not support character ranges (e.g., 'a-z'
> > or
> > '0-9'), the hyphen '-' is *not* a special character, and the brackets
> > themselves cannot be matched.
> >
> > Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
> > ---
> > Patch based on linux-next-20160222.
> >
> > v2:
> >  - Use kstrndup() to copy the character set from fmt instead of using
> > a
> >    statically allocated array
> >  
> >  lib/vsprintf.c | 39 +++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 39 insertions(+)
> >
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index 525c8e1..93a6f52 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -2714,6 +2714,45 @@ int vsscanf(const char *buf, const char *fmt,
> > va_list args)
> >  			num++;
> >  		}
> >  		continue;
> > +		case '[':
> > +		{
> > +			char *s = (char *)va_arg(args, char *);
> > +			char *set;
> > +			size_t (*op)(const char *str, const char
> > *set);
> > +			size_t len = 0;
> > +			bool negate = (*(fmt) == '^');
> > +
> > +			if (field_width == -1)
> > +				field_width = SHRT_MAX;
>
> I'm not sure if it's needed here. It will count down till 0 in any
> case.

I think it might be good to be consistent with the '%s' specifier code
and have some sort of upper bound set, even if it is much more likely
that len will get to 0 before field_width does.

> > +
> > +			op = negate ? &strcspn : &strspn;
> > +			if (negate)
> > +				fmt++;
>
> > +
> > +			len = strcspn(fmt, "]");
> > +			/* invalid format; stop here */
> > +			if (!len)
> > +				return num;
> > +
> > +			set = kstrndup(fmt, len, GFP_KERNEL);
> > +			if (!set)
> > +				return num;
> > +
> > +			/* advance fmt past ']' */
> > +			fmt += len + 1;
> > +
> > +			len = (*op)(str, set);
>
> Can we use just normal form:
>  op();
> ?
>
> > +			/* no matches */
> > +			if (!len)
>
> Memory leak here.
>
> > +				return num;
> > +
> > +			while (*str && len-- && field_width--)
> > +				*s++ = *str++;
>
> Looks like strcpy() variant. First of all, is it possible to have *str
> == '\0' when len != 0?

Good point. The *str check is redundant, since after the call to
strspn/strcspn we know there are at least len bytes in str, so that
check can be removed.

> > +			*s = '\0';
> > +			kfree(set);
> > +			num++;
> > +		}
> > +		continue;
> >  		case 'o':
> >  			base = 8;
> >  			break;
>
> --
> Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Intel Finland Oy