fix sscanf %n match at end of input string

I was playing with some code that sometimes got a string where a %n
match should have been done where the input string ended, for example
like this:

  sscanf("abc123", "abc%d%n", &a, &n);  /* doesn't work */
  sscanf("abc123a", "abc%d%n", &a, &n); /* works */

However, the scanf function in the kernel doesn't convert the %n in that
case because it has already matched the complete input after %d and just
completely stops matching then. This patch fixes that.

[akpm@linux-foundation.org: cleanups]
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index cbab1df150cf..017290241261 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -825,6 +825,17 @@ int vsscanf(const char * buf, const char * fmt, va_list args)
                         break;
                 str = next;
         }
+
+        /*
+         * Now we've come all the way through so either the input string or the
+         * format ended. In the former case, there can be a %n at the current
+         * position in the format that needs to be filled.
+         */
+        if (*fmt == '%' && *(fmt + 1) == 'n') {
+                int *p = (int *)va_arg(args, int *);
+                *p = str - buf;
+        }
+
         return num;
 }