[PATCH v2] sscanf: implement basic character sets

[Jessica Yu]

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;
+
+			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)
+				return num;
+
+			while (*str && len-- && field_width--)
+				*s++ = *str++;
+			*s = '\0';
+			kfree(set);
+			num++;
+		}
+		continue;
 		case 'o':
 			base = 8;
 			break;
-- 
2.4.3