Re: [PATCH v3] sscanf: implement basic character sets

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 23 Feb 2016 15:38:22 -0500 Jessica Yu <jeyu@redhat.com> 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@redhat.com>
> ---
> 
> This patch adds support for the '%[' conversion specifier for sscanf().
> This is useful in cases where we'd like to match substrings delimited by
> something other than spaces. The original motivation for this patch
> actually came from a livepatch discussion (See: https://lkml.org/lkml/2016/2/8/790),
> where we were trying to come up with a clean way to parse symbol names with
> substrings delimited by periods and commas.

It would be better to include the justification right here in the
changelog please.  Not via some link-to-discussion and definitely not
below the ^--- marker!  It's very important.

The deviation from the glibc behaviour is a bit of a worry,
particularly as it is done in a non-back-compat manner: code which
assumes "-" is non-magic might break if someone later adds range
support.

Presumably we can live with that - there won't be many callsites and
they can be grepped for.  But please, let's get a description of all
these considerations into the code as a comment.  Probably it would be
helpful to include a little usage example in that comment.

> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2714,6 +2714,47 @@ 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;
> +
> +			op = negate ? &strcspn : &strspn;
> +			if (negate)
> +				fmt++;
> +
> +			len = strcspn(fmt, "]");
> +			/* invalid format; stop here */
> +			if (!len)
> +				return num;
> +
> +			set = kstrndup(fmt, len, GFP_KERNEL);

Embedding a GFP_KERNEL allocation into vsscanf is problematic - it
limits the situations in which this functionality can be used.

afaict the allocation is there merely so we can null-terminate the
string so we can use existing library functions (strcspn, strspn).  Is
that compromise really worth it?  We could pretty easily convert
strcspn() into

	strcnspn(const char *s, const char *reject, size_t len)

and convert strcspn() to call that (ifndef __HAVE_ARCH_STRCSPN)

In fact I think we could still use strspn() and strcspn() on `fmt'
directly?  We just need to check for the return value exceeding `len'
and if so, treat that as a no-match?


> +			if (!set)
> +				return num;
> +
> +			/* advance fmt past ']' */
> +			fmt += len + 1;
> +
> +			len = op(str, set);
> +			/* no matches */
> +			if (!len) {
> +				kfree(set);
> +				return num;
> +			}
> +
> +			while (len-- && field_width--)
> +				*s++ = *str++;
> +			*s = '\0';
> +			kfree(set);
> +			num++;
> +		}
> +		continue;
>  		case 'o':
>  			base = 8;
>  			break;