Unlike LL_FS_RO and LL_FS_RW, LL_TCP_* are currently optional: either
don't specify them and these access rights won't be in handled_accesses,
or specify them and only the values passed are allowed.

If you want to specify that no port can be bind()ed, you would think
(looking at the code quickly) that setting LL_TCP_BIND="" would do it.
Due to a quirk in the parsing logic and the use of atoi() returning 0 with
no error checking for empty strings, you end up allowing bind(0) (which
means bind to any ephemeral port) without realising it. The same occurred
when leaving a trailing/leading colon (e.g. "80:").

To reproduce:
export LL_FS_RO="/" LL_FS_RW="" LL_TCP_BIND=""

---8<----- Before this patch:
./sandboxer strace -e bind nc -n -vvv -l -p 0
Executing the sandboxed command...
bind(3, {sa_family=AF_INET, sin_port=htons(0),
      sin_addr=inet_addr("0.0.0.0")}, 16) = 0
Listening on 0.0.0.0 37629

---8<----- Expected:
./sandboxer strace -e bind nc -n -vvv -l -p 0
Executing the sandboxed command...
bind(3, {sa_family=AF_INET, sin_port=htons(0),
      sin_addr=inet_addr("0.0.0.0")}, 16) = -1 EACCES (Permission denied)
nc: Permission denied

Signed-off-by: Matthieu Buffet <redacted>
---
  samples/landlock/sandboxer.c | 13 ++++++++++++-
  1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
index e8223c3e781a..a84ae3a15482 100644
--- a/samples/landlock/sandboxer.c
+++ b/samples/landlock/sandboxer.c

@@ -168,7 +168,18 @@ static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
  
  	env_port_name_next = env_port_name;
  	while ((strport = strsep(&env_port_name_next, ENV_DELIMITER))) {
-		net_port.port = atoi(strport);
+		char *strport_num_end = NULL;
+
+		if (strcmp(strport, "") == 0)
+			continue;
+
+		errno = 0;
+		net_port.port = strtol(strport, &strport_num_end, 0);
+		if (errno != 0 || strport_num_end == strport) {
+			fprintf(stderr,
+				"Failed to parse port at \"%s\"\n", strport);
+			goto out_free_name;
+		}