virsh: Clarify escape sequence

Currently, we put no strains on escape sequence possibly leaving users
with console that cannot be terminated. However, not all ASCII
characters can be used as escape sequence. Only those falling in
@ - _ can be; implement and document this constraint.

tools/console.c

@@ -34,6 +34,7 @@
 # include <errno.h>
 # include <unistd.h>
 # include <signal.h>
+# include <c-ctype.h>
 
 # include "internal.h"
 # include "console.h"
@@ -292,7 +293,7 @@ static char
 vshGetEscapeChar(const char *s)
 {
     if (*s == '^')
-        return CONTROL(s[1]);
+        return CONTROL(c_toupper(s[1]));
 
     return *s;
 }

tools/virsh.c

@@ -19879,6 +19879,16 @@ vshShowVersion(vshControl *ctl ATTRIBUTE_UNUSED)
     vshPrint(ctl, "\n");
 }
 
+static bool
+vshAllowedEscapeChar(char c)
+{
+    /* Allowed escape characters:
+     * a-z A-Z @ [ \ ] ^ _
+     */
+    return ('a' <= c && c <= 'z') ||
+        ('@' <= c && c <= '_');
+}
+
 /*
  * argv[]:  virsh [options] [command]
  *
@@ -19942,7 +19952,8 @@ vshParseArgv(vshControl *ctl, int argc, char **argv)
         case 'e':
             len = strlen(optarg);
 
-            if ((len == 2 && *optarg == '^') ||
+            if ((len == 2 && *optarg == '^' &&
+                 vshAllowedEscapeChar(optarg[1])) ||
                 (len == 1 && *optarg != '^')) {
                 ctl->escapeChar = optarg;
             } else {

tools/virsh.pod

@@ -95,7 +95,8 @@ Output elapsed time information for each command.
 =item B<-e>, B<--escape> I<string>
 
 Set alternative escape sequence for I<console> command. By default,
-telnet's B<^]> is used.
+telnet's B<^]> is used. Allowed characters when using hat notation are:
+alphabetic character, @, [, ], \, ^, _.
 
 =back