base: handle unsupported printf commands better

The commit consumes the argument of a unsupported printf command.

Without the commit - a subsequent command uses the argument of the preceding
command, which may cause memory corruption or page faults for sequences using
string commands, e.g.

Genode::printf("%#x %s\n", 0x20, "Test");

'#' is not supported by Genode::printf. In this scenario a pagefault at
address 0x20 is caused.

Fixes #1701

repos/base/src/base/console/console.cc

@@ -335,6 +335,8 @@ void Console::vprintf(const char *format, va_list list)
 			case Format_command::INVALID:
 
 				_out_string("<warning: unsupported format string argument>");
+				/* consume the argument of the unsupported command */
+				va_arg(list, long);
 				break;
 		}
 

repos/base/src/test/printf/main.cc

@@ -16,6 +16,10 @@
 
 int main(int argc, char **argv)
 {
+	/* test that unsupported commands don't crash the printf parser */
+	Genode::printf("%#x %s\n", 0x38, "test 1");
+	Genode::printf("%#lx %s\n", 0x38L, "test 2");
+
 	Genode::printf("-1 = %d = %ld\n", -1, -1L);
 
 	return 0;

tool/autopilot.list

@@ -55,3 +55,4 @@ stdcxx
 nic_loopback
 platform
 report_rom
+printf