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

[Kees Cook]

On Tue, Feb 23, 2016 at 12:38 PM, Jessica Yu <jeyu@xxxxxxxxxx> 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>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>
> 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.
>
> Patch based on linux-next-20160223.
>
> v3:
>  - Fix memory leak in error path (kfree() before returning)
>  - Remove redundant condition in while loop
>  - Style fix (*op)() -> op()
>
> v2:
>  - Use kstrndup() to copy the character set from fmt instead of using a
>    statically allocated array
>
>  lib/vsprintf.c | 41 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 525c8e1..983358a 100644
> --- 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);
> +                       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;
> --
> 2.4.3
>



-- 
Kees Cook
Chrome OS & Brillo Security