os: Treat ssh as a non-local client (v4)

By the time we get to ComputeLocalClient, we've already done
NextAvailableClient → ReserveClientIds → DetermineClientCmd (assuming
we're built with #define CLIENTIDS), so we can look up the name of the
client process and refuse to treat ssh's X forwarding as if it were
local.

v2: (Michel Dänzer)
    * Only match "ssh" itself, not other executable names starting with
      that prefix.
    * Ignore executable path for the match.
v3: (Michel Dänzer)
    * Use GetClientCmdName (Mark Kettenis)
    * Perform check on Windows as well, but only ignore path on Cygwin
      (Martin Peres, Emil Velikov, Jon Turney)
v4: (Michel Dänzer)
    * Cut of any colon and whatever comes after it. (Adam Jackson)
    * Add bugzilla reference.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93261

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>

os/access.c

@@ -173,6 +173,10 @@ SOFTWARE.
 
 #endif                          /* WIN32 */
 
+#if !defined(WIN32) || defined(__CYGWIN__)
+#include <libgen.h>
+#endif
+
 #define X_INCLUDE_NETDB_H
 #include <X11/Xos_r.h>
 
@@ -1080,9 +1084,8 @@ ResetHosts(const char *display)
     }
 }
 
-/* Is client on the local host */
-Bool
-ComputeLocalClient(ClientPtr client)
+static Bool
+xtransLocalClient(ClientPtr client)
 {
     int alen, family, notused;
     Xtransaddr *from = NULL;
@@ -1115,6 +1118,40 @@ ComputeLocalClient(ClientPtr client)
     return FALSE;
 }
 
+/* Is client on the local host */
+Bool
+ComputeLocalClient(ClientPtr client)
+{
+    const char *cmdname = GetClientCmdName(client);
+
+    if (!xtransLocalClient(client))
+        return FALSE;
+
+    /* If the executable name is "ssh", assume that this client connection
+     * is forwarded from another host via SSH
+     */
+    if (cmdname) {
+        char **cmd;
+        Bool ret;
+
+        /* Cut off any colon and whatever comes after it, see
+         * https://lists.freedesktop.org/archives/xorg-devel/2015-December/048164.html
+         */
+        cmd = xstrtokenize(cmdname, ":");
+
+#if !defined(WIN32) || defined(__CYGWIN__)
+        cmd[0] = basename(cmd[0]);
+#endif
+
+        ret = strcmp(cmd[0], "ssh") != 0;
+        free(cmd);
+
+        return ret;
+    }
+
+    return TRUE;
+}
+
 /*
  * Return the uid and all gids of a connected local client
  * Allocates a LocalClientCredRec - caller must call FreeLocalClientCreds