console/vsprintf: Implement snprintf

snprintf is a safe variant of sprintf. To avoid buffer overflows
we shouldn't use sprintf at all. But for now let's start by
implementing snprintf in first place.

Change-Id: Ic17d94b8cd91b72f66b84b0589a06b8abef5e5c9
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/4278
Tested-by: build bot (Jenkins)
Reviewed-by: Peter Stuge <peter@stuge.se>

src/console/vsprintf.c

@@ -23,28 +23,34 @@
 #include <console/vtxprintf.h>
 #include <trace.h>
 
-struct vsprintf_context
+struct vsnprintf_context
 {
 	char *str_buf;
+	size_t buf_limit;
 };
 
 static void str_tx_byte(unsigned char byte, void *data)
 {
-	struct vsprintf_context *ctx = data;
-	*ctx->str_buf = byte;
-	ctx->str_buf++;
+	struct vsnprintf_context *ctx = data;
+	if (ctx->buf_limit) {
+		*ctx->str_buf = byte;
+		ctx->str_buf++;
+		ctx->buf_limit--;
+	}
 }
 
-static int vsprintf(char *buf, const char *fmt, va_list args)
+static int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 {
 	int i;
-	struct vsprintf_context ctx;
+	struct vsnprintf_context ctx;
 
 	DISABLE_TRACE;
 
 	ctx.str_buf = buf;
+	ctx.buf_limit = size ? size - 1 : 0;
 	i = vtxdprintf(str_tx_byte, fmt, args, &ctx);
-	*ctx.str_buf = '\0';
+	if (size)
+		*ctx.str_buf = '\0';
 
 	ENABLE_TRACE;
 
@@ -57,7 +63,21 @@ int sprintf(char *buf, const char *fmt, ...)
 	int i;
 
 	va_start(args, fmt);
-	i = vsprintf(buf, fmt, args);
+	/* A trick: we have at most (size_t)-1 adressable space anyway, so
+	   if we output so much we'll crash anyway.  */
+	i = vsnprintf(buf, -1, fmt, args);
+	va_end(args);
+
+	return i;
+}
+
+int snprintf(char *buf, size_t size, const char *fmt, ...)
+{
+	va_list args;
+	int i;
+
+	va_start(args, fmt);
+	i = vsnprintf(buf, size, fmt, args);
 	va_end(args);
 
 	return i;

src/include/string.h

@@ -17,6 +17,7 @@ int memcmp(const void *s1, const void *s2, size_t n);
 void *memchr(const void *s, int c, size_t n);
 #if !defined(__PRE_RAM__)
 int sprintf(char * buf, const char *fmt, ...);
+int snprintf(char * buf, size_t size, const char *fmt, ...);
 #endif
 
 // simple string functions