slirp: Ensure smbd and shared directory exist when enable smb

Users may pass the following parameters to qemu:
    $ qemu-kvm -net nic -net user,smb= ...
    $ qemu-kvm -net nic -net user,smb ...
    $ qemu-kvm -net nic -net user,smb=bad_directory ...

In these cases, qemu started successfully while samba server
failed to start. Users will confuse since samba server
failed silently without any indication of what it did wrong.

To avoid it, we check whether the shared directory exist and
if users have permission to access this directory when QEMU's
"built-in" SMB server is enabled.
Signed-off-by: NDunrong Huang <riegamaths@gmail.com>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>

net/slirp.c

@@ -497,6 +497,18 @@ static int slirp_smb(SlirpState* s, const char *exported_dir,
         return -1;
     }
 
+    if (access(CONFIG_SMBD_COMMAND, F_OK)) {
+        error_report("could not find '%s', please install it",
+                     CONFIG_SMBD_COMMAND);
+        return -1;
+    }
+
+    if (access(exported_dir, R_OK | X_OK)) {
+        error_report("no such directory '%s', or you do not have permission "
+                     "to access it, please check it", exported_dir);
+        return -1;
+    }
+
     snprintf(s->smb_dir, sizeof(s->smb_dir), "/tmp/qemu-smb.%ld-%d",
              (long)getpid(), instance++);
     if (mkdir(s->smb_dir, 0700) < 0) {