qrexec-client: Filter terminal output much more strictly

qrexec-client -t/-T (and therefore, qvm-run --pass-io) only handled the
escape character, \033. Everything else, such as Unicode and obscure
control characters, was passed through from the VM to the dom0 terminal.

Instead, replace all bytes except for a benign subset of ASCII. That's
still enough to allow progress bars to be drawn (tested using
"wget --progress=bar:force" and "pv --force").

qrexec/qrexec-client.c

@@ -34,9 +34,9 @@
 #include "qrexec.h"
 #include "libqrexec-utils.h"
 
-// whether qrexec-client should replace ESC with _ before printing the output
-int replace_esc_stdout = 0;
-int replace_esc_stderr = 0;
+// whether qrexec-client should replace problematic bytes with _ before printing the output
+int replace_chars_stdout = 0;
+int replace_chars_stderr = 0;
 
 #define VCHAN_BUFFER_SIZE 65536
 
@@ -332,12 +332,19 @@ static void handle_input(libvchan_t *vchan)
     }
 }
 
-void do_replace_esc(char *buf, int len) {
+void do_replace_chars(char *buf, int len) {
 	int i;
-
-	for (i = 0; i < len; i++)
-		if (buf[i] == '\033')
+	unsigned char c;
+
+	for (i = 0; i < len; i++) {
+		c = buf[i];
+		if ((c < '\040' || c > '\176') &&  /* not printable ASCII */
+		    (c != '\t') &&                 /* not tab */
+		    (c != '\n') &&                 /* not newline */
+		    (c != '\r') &&                 /* not return */
+		    (c != '\b'))                   /* not backspace */
 			buf[i] = '_';
+	}
 }
 
 static int handle_vchan_data(libvchan_t *vchan, struct buffer *stdin_buf)
@@ -378,8 +385,8 @@ static int handle_vchan_data(libvchan_t *vchan, struct buffer *stdin_buf)
         case MSG_DATA_STDOUT:
             if (local_stdin_fd == -1)
                 break;
-            if (replace_esc_stdout)
-                do_replace_esc(buf, hdr.len);
+            if (replace_chars_stdout)
+                do_replace_chars(buf, hdr.len);
             if (hdr.len == 0) {
                 /* restore flags, as we may have not the only copy of this file descriptor
                 */
@@ -408,8 +415,8 @@ static int handle_vchan_data(libvchan_t *vchan, struct buffer *stdin_buf)
             }
             break;
         case MSG_DATA_STDERR:
-            if (replace_esc_stderr)
-                do_replace_esc(buf, hdr.len);
+            if (replace_chars_stderr)
+                do_replace_chars(buf, hdr.len);
             write_all(2, buf, hdr.len);
             break;
         case MSG_DATA_EXIT_CODE:
@@ -542,7 +549,7 @@ static void usage(char *name)
             "-c request_id,src_domain_name,src_domain_id|"
             "-e] remote_cmdline\n"
             "-e means exit after sending cmd,\n"
-            "-t enables replacing ESC character with '_' in command output, -T is the same for stderr\n"
+            "-t enables replacing problematic bytes with '_' in command output, -T is the same for stderr\n"
             "-c: connect to existing process (response to trigger service call)\n"
             "-w timeout: override default connection timeout of 5s (set 0 for no timeout)\n",
             name);
@@ -666,10 +673,10 @@ int main(int argc, char **argv)
                 is_service = 1;
                 break;
             case 't':
-                replace_esc_stdout = 1;
+                replace_chars_stdout = 1;
                 break;
             case 'T':
-                replace_esc_stderr = 1;
+                replace_chars_stderr = 1;
                 break;
             case 'w':
                 connection_timeout = atoi(optarg);