[RFC 3/3] lib/vsprintf.c: eliminate potential race in string()

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

If the string corresponding to a %s specifier can change under us, we
might end up copying a \0 byte to the output buffer. There might be
callers who expect the output buffer to contain a genuine C string
whose length is exactly the snprintf return value (assuming truncation
hasn't happened or has been checked for).

We can avoid this by only passing over the source string once,
stopping the first time we meet a nul byte (or when we reach the given
precision), and then letting widen_string() handle left/right space
padding.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 lib/vsprintf.c | 28 +++++++++-------------------
 1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index acead77594b5..d518849b6b75 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -557,32 +557,22 @@ char *widen_string(char *buf, int n, char *end, struct printf_spec spec)
 static noinline_for_stack
 char *string(char *buf, char *end, const char *s, struct printf_spec spec)
 {
-	int len, i;
+	int len = 0;
+	size_t lim = spec.precision;
 
 	if ((unsigned long)s < PAGE_SIZE)
 		s = "(null)";
 
-	len = strnlen(s, spec.precision);
-
-	if (!(spec.flags & LEFT)) {
-		while (len < spec.field_width--) {
-			if (buf < end)
-				*buf = ' ';
-			++buf;
-		}
-	}
-	for (i = 0; i < len; ++i) {
-		if (buf < end)
-			*buf = *s;
-		++buf; ++s;
-	}
-	while (len < spec.field_width--) {
+	while (lim--) {
+		char c = *s++;
+		if (!c)
+			break;
 		if (buf < end)
-			*buf = ' ';
+			*buf = c;
 		++buf;
+		++len;
 	}
-
-	return buf;
+	return widen_string(buf, len, end, spec);
 }
 
 static noinline_for_stack
-- 
2.1.3