virt-login-shell: honour the -c option to launch commands

The virt-login-shell program is supposed to look like a
regular shell to clients. Login services like sshd
expect the shell to accept a '-c cmdstring' argument to
specify a command to launch instead of presenting an
interactive prompt.

We can implement this by simply passing the '-c cmdstring'
data straight through to the real shell we use. This does
not open any security holes, since the command is not run
until we're inside the container namespaces. This allows
scp to work for users with virt-login-shell.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

tools/virt-login-shell.c

@@ -100,20 +100,19 @@ static int virLoginShellAllowedUser(virConfPtr conf,
     return ret;
 }
 
-static char **virLoginShellGetShellArgv(virConfPtr conf)
+static int virLoginShellGetShellArgv(virConfPtr conf,
+                                     char ***retshargv,
+                                     size_t *retshargvlen)
 {
     size_t i;
+    size_t len;
     char **shargv = NULL;
-    virConfValuePtr p;
+    virConfValuePtr p, pp;
 
     p = virConfGetValue(conf, "shell");
-    if (!p)
-        return virStringSplit("/bin/sh -l", " ", 3);
-
-    if (p->type == VIR_CONF_LIST) {
-        size_t len;
-        virConfValuePtr pp;
-
+    if (!p) {
+        len = 2; /* /bin/sh -l */
+    } else if (p->type == VIR_CONF_LIST) {
         /* Calc length and check items */
         for (len = 0, pp = p->list; pp; len++, pp = pp->next) {
             if (pp->type != VIR_CONF_STRING) {
@@ -122,18 +121,41 @@ static char **virLoginShellGetShellArgv(virConfPtr conf)
                 goto error;
             }
         }
+    } else {
+        virReportSystemError(EINVAL, "%s",
+                             _("shell must be a list of strings"));
+        goto error;
+    }
 
-        if (VIR_ALLOC_N(shargv, len + 1) < 0)
+    len++; /* NULL terminator */
+
+    if (VIR_ALLOC_N(shargv, len) < 0)
+        goto error;
+
+    i = 0;
+    if (!p) {
+        if (VIR_STRDUP(shargv[i++], "/bin/sh") < 0)
             goto error;
-        for (i = 0, pp = p->list; pp; i++, pp = pp->next) {
-            if (VIR_STRDUP(shargv[i], pp->str) < 0)
+        if (VIR_STRDUP(shargv[i++], "-l") < 0)
+            goto error;
+    } else if (p->type == VIR_CONF_LIST) {
+        for (pp = p->list; pp; pp = pp->next) {
+            if (VIR_STRDUP(shargv[i++], pp->str) < 0)
                 goto error;
         }
     }
-    return shargv;
+
+    shargv[i] = NULL;
+
+    *retshargvlen = i;
+    *retshargv = shargv;
+
+    return 0;
  error:
+    *retshargv = NULL;
+    *retshargvlen = 0;
     virStringFreeList(shargv);
-    return NULL;
+    return -1;
 }
 
 static char *progname;
@@ -151,6 +173,7 @@ usage(void)
               "Options:\n"
               "  -h | --help            Display program help\n"
               "  -V | --version         Display program version\n"
+              "  -c CMD                 Run CMD via shell\n"
               "\n"
               "libvirt login shell\n"),
             progname);
@@ -177,6 +200,7 @@ main(int argc, char **argv)
     gid_t gid = getgid();
     char *name = NULL;
     char **shargv = NULL;
+    size_t shargvlen = 0;
     virSecurityModelPtr secmodel = NULL;
     virSecurityLabelPtr seclabel = NULL;
     virDomainPtr dom = NULL;
@@ -190,6 +214,7 @@ main(int argc, char **argv)
     int *fdlist = NULL;
     int openmax;
     size_t i;
+    const char *cmdstr = NULL;
 
     struct option opt[] = {
         {"help", no_argument, NULL, 'h'},
@@ -210,7 +235,7 @@ main(int argc, char **argv)
     if (virGettextInitialize() < 0)
         return ret;
 
-    while ((arg = getopt_long(argc, argv, "hV", opt, &longindex)) != -1) {
+    while ((arg = getopt_long(argc, argv, "hVc:", opt, &longindex)) != -1) {
         switch (arg) {
         case 'h':
             usage();
@@ -220,6 +245,10 @@ main(int argc, char **argv)
             show_version();
             exit(EXIT_SUCCESS);
 
+        case 'c':
+            cmdstr = optarg;
+            break;
+
         case '?':
         default:
             usage();
@@ -255,7 +284,7 @@ main(int argc, char **argv)
     if (virLoginShellAllowedUser(conf, name, groups) < 0)
         goto cleanup;
 
-    if (!(shargv = virLoginShellGetShellArgv(conf)))
+    if (virLoginShellGetShellArgv(conf, &shargv, &shargvlen) < 0)
         goto cleanup;
 
     conn = virConnectOpen("lxc:///");
@@ -308,6 +337,16 @@ main(int argc, char **argv)
         goto cleanup;
     }
 
+    if (cmdstr) {
+        if (VIR_REALLOC_N(shargv, shargvlen + 3) < 0)
+            goto cleanup;
+        if (VIR_STRDUP(shargv[shargvlen++], "-c") < 0)
+            goto cleanup;
+        if (VIR_STRDUP(shargv[shargvlen++], cmdstr) < 0)
+            goto cleanup;
+        shargv[shargvlen] = NULL;
+    }
+
     /* A fork is required to create new process in correct pid namespace.  */
     if ((cpid = virFork()) < 0)
         goto cleanup;

tools/virt-login-shell.pod

@@ -23,6 +23,11 @@ The basic structure of most virt-login-shell usage is:
 
 =over
 
+=item B<-c CMD>
+
+Instruct the shell to run CMD instead of presenting an
+interactive shell prompt.
+
 =item B<-h, --help>
 
 Display command line help usage then exit.