[xfer] Avoid using stack-allocated memory in xfer_printf()

xfer_printf() occasionally has to deal with strings that are
potentially long, such as HTTP URIs with multiple query parameters.
Allocating these on the stack can lead to stack overruns and memory
corruption.

Fix by using vasprintf() instead of a stack allocation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/xfer.c

 FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <string.h>
+#include <stdlib.h>
 #include <stdio.h>
 #include <errno.h>
 #include <ipxe/iobuf.h>
  */
 int xfer_vprintf ( struct interface *intf, const char *format,
 		   va_list args ) {
-	size_t len;
 	va_list args_tmp;
+	char *buf;
+	int len;
+	int rc;
 
+	/* Create temporary string */
 	va_copy ( args_tmp, args );
-	len = vsnprintf ( NULL, 0, format, args );
-	{
-		char buf[len + 1];
-		vsnprintf ( buf, sizeof ( buf ), format, args_tmp );
-		va_end ( args_tmp );
-		return xfer_deliver_raw ( intf, buf, len );
+	len = vasprintf ( &buf, format, args );
+	if ( len < 0 ) {
+		rc = len;
+		goto err_asprintf;
 	}
+	va_end ( args_tmp );
+
+	/* Transmit string */
+	if ( ( rc = xfer_deliver_raw ( intf, buf, len ) ) != 0 )
+		goto err_deliver;
+
+ err_deliver:
+	free ( buf );
+ err_asprintf:
+	return rc;
 }
 
 /**