qrexec: provide common function for handling service call

Reduce code duplication by moving parsing of "QUBESRPC" magic command to
one place.
Call qubes-rpc-multiplexer directly with execve(), to avoid string
expansions in its parameters.

qrexec-lib/exec.c

@@ -24,6 +24,8 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <stdio.h>
+#include <string.h>
+#include "qrexec.h"
 #include "libqrexec-utils.h"
 
 static do_exec_t *exec_func = NULL;
@@ -31,6 +33,29 @@ void register_exec_func(do_exec_t *func) {
     exec_func = func;
 }
 
+void exec_qubes_rpc_if_requested(char *prog, char *const envp[]) {
+    /* avoid calling qubes-rpc-multiplexer through shell */
+    if (strncmp(prog, RPC_REQUEST_COMMAND, RPC_REQUEST_COMMAND_LEN) == 0) {
+        char *tok;
+        char *argv[16]; // right now 6 are used, but allow future extensions
+        size_t i = 0;
+
+        tok=strtok(prog, " ");
+        do {
+            if (i >= sizeof(argv)/sizeof(argv[0])-1) {
+                fprintf(stderr, "To many arguments to %s\n", RPC_REQUEST_COMMAND);
+                exit(1);
+            }
+            argv[i++] = tok;
+        } while ((tok=strtok(NULL, " ")));
+        argv[i] = NULL;
+        argv[0] = QUBES_RPC_MULTIPLEXER_PATH;
+        execve(QUBES_RPC_MULTIPLEXER_PATH, argv, envp);
+        perror("exec qubes-rpc-multiplexer");
+        exit(1);
+    }
+}
+
 void fix_fds(int fdin, int fdout, int fderr)
 {
     int i;
@@ -68,7 +93,7 @@ void do_fork_exec(const char *cmdline, int *pid, int *stdin_fd, int *stdout_fd,
                 fix_fds(inpipe[0], outpipe[1], 2);
 
             if (exec_func != NULL)
-                exec_func(cmdline);
+                exec_func((char*)cmdline);
             exit(-1);
         default:;
     }

qrexec-lib/libqrexec-utils.h

@@ -33,8 +33,14 @@ struct buffer {
 #define WRITE_STDIN_BUFFERED  1 /* something still in the buffer */
 #define WRITE_STDIN_ERROR     2 /* write error, errno set */
 
-typedef void (do_exec_t)(const char *);
+typedef void (do_exec_t)(char *);
 void register_exec_func(do_exec_t *func);
+/*
+ * exec() qubes-rpc-multiplexer if *prog* starts with magic "QUBESRPC" keyword,
+ * do not return in that case; pass *envp* to execve() as en environment
+ * otherwise, return false without any action
+ */
+void exec_qubes_rpc_if_requested(char *prog, char *const envp[]);
 
 void buffer_init(struct buffer *b);
 void buffer_free(struct buffer *b);