lib/vsprintf.c: improve standard conformance of sscanf()

Xen's pciback points out a couple of deficiencies with vsscanf()'s
standard conformance:

- Trailing character matching cannot be checked by the caller: With a
  format string of "(%x:%x.%x) %n" absence of the closing parenthesis
  cannot be checked, as input of "(00:00.0)" doesn't cause the %n to be
  evaluated (because of the code not skipping white space before the
  trailing %n).

- The parameter corresponding to a trailing %n could get filled even if
  there was a matching error: With a format string of "(%x:%x.%x)%n",
  input of "(00:00.0]" would still fill the respective variable pointed to
  (and hence again make the mismatch non-detectable by the caller).

This patch aims at fixing those, but leaves other non-conforming aspects
of it untouched, among them these possibly relevant ones:

- improper handling of the assignment suppression character '*' (blindly
  discarding all succeeding non-white space from the format and input
  strings),

- not honoring conversion specifiers for %n, - not recognizing the C99
  conversion specifier 't' (recognized by vsprintf()).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 19 deletions

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 9287e2549936..39c99fea7c03 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2017,7 +2017,7 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
         s16 field_width;
         bool is_sign;
 
-        while (*fmt && *str) {
+        while (*fmt) {
                 /* skip any white space in format */
                 /* white space in format matchs any amount of
                  * white space, including none, in the input.
@@ -2042,6 +2042,8 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
                  * advance both strings to next white space
                  */
                 if (*fmt == '*') {
+                        if (!*str)
+                                break;
                         while (!isspace(*fmt) && *fmt != '%' && *fmt)
                                 fmt++;
                         while (!isspace(*str) && *str)
@@ -2070,7 +2072,17 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
                         }
                 }
 
-                if (!*fmt || !*str)
+                if (!*fmt)
+                        break;
+
+                if (*fmt == 'n') {
+                        /* return number of characters read so far */
+                        *va_arg(args, int *) = str - buf;
+                        ++fmt;
+                        continue;
+                }
+
+                if (!*str)
                         break;
 
                 base = 10;
@@ -2103,13 +2115,6 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
                         num++;
                 }
                 continue;
-                case 'n':
-                        /* return number of characters read so far */
-                {
-                        int *i = (int *)va_arg(args, int*);
-                        *i = str - buf;
-                }
-                continue;
                 case 'o':
                         base = 8;
                         break;
@@ -2210,16 +2215,6 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
                 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;
 }
 EXPORT_SYMBOL(vsscanf);