Only check for bindable ports if we are unsure if it will fail.

We currently assume that the only way for Tor to listen on ports in the
privileged port range (1 to 1023), on Linux, is if we are granted the
NET_BIND_SERVICE capability. Today on Linux, it's possible to specify
the beginning of the unprivileged port range using a sysctl
configuration option. Docker (and thus the CI service Tor uses) recently
changed this sysctl value to 0, which causes our tests to fail as they
assume that we should NOT be able to bind to a privileged port *without*
the NET_BIND_SERVICE capability.

In this patch, we read the value of the sysctl value via the /proc/sys/
filesystem iff it's present, otherwise we assume the default
unprivileged port range begins at port 1024.

See: tor#40275

1 files changed, 57 insertions, 3 deletions

diff --git a/src/test/test_switch_id.c b/src/test/test_switch_id.c
index baddf8d66e..91277e374f 100644
--- a/src/test/test_switch_id.c
+++ b/src/test/test_switch_id.c
@@ -31,7 +31,47 @@ static const struct {
   { NULL, 0 }
 };
 
+/* Returns the first port that we think we can bind to without special
+ * permissions. Usually this function returns 1024. */
+static uint16_t
+unprivileged_port_range_start(void)
+{
+  uint16_t result = 1024;
+
+#if defined(__linux__)
+  char *content = NULL;
+
+  content = read_file_to_str(
+              "/proc/sys/net/ipv4/ip_unprivileged_port_start",
+              0,
+              NULL);
+
+  if (content != NULL) {
+    int ok = 1;
+    uint16_t tmp_result;
+
+    tmp_result = (uint16_t)tor_parse_long(content, 10, 0, 65535, &ok, NULL);
+
+    if (ok) {
+      result = tmp_result;
+    } else {
+      fprintf(stderr,
+              "Unable to convert ip_unprivileged_port_start to integer: %s\n",
+              content);
+    }
+  }
+
+  tor_free(content);
+#endif /* defined(__linux__) */
+
+  return result;
+}
+
 #if !defined(_WIN32)
+
+#define PORT_TEST_RANGE_START 600
+#define PORT_TEST_RANGE_END   1024
+
 /* 0 on no, 1 on yes, -1 on failure. */
 static int
 check_can_bind_low_ports(void)
@@ -41,7 +81,7 @@ check_can_bind_low_ports(void)
   memset(&sin, 0, sizeof(sin));
   sin.sin_family = AF_INET;
 
-  for (port = 600; port < 1024; ++port) {
+  for (port = PORT_TEST_RANGE_START; port < PORT_TEST_RANGE_END; ++port) {
     sin.sin_port = htons(port);
     tor_socket_t fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
     if (! SOCKET_OK(fd)) {
@@ -149,10 +189,24 @@ main(int argc, char **argv)
       /* Succeed if we can do a setuid with capability retention, and doing so
        * does not make us lose the ability to bind low ports */
     {
-      int keepcaps = (test_id == TEST_SETUID_KEEPCAPS);
+      const int keepcaps = (test_id == TEST_SETUID_KEEPCAPS);
       okay = switch_id(username, keepcaps ? SWITCH_ID_KEEP_BINDLOW : 0) == 0;
+
       if (okay) {
-        okay = check_can_bind_low_ports() == keepcaps;
+        /* Only run this check if there are ports we may not be able to bind
+         * to. */
+        const uint16_t min_port = unprivileged_port_range_start();
+
+        if (min_port >= PORT_TEST_RANGE_START &&
+            min_port < PORT_TEST_RANGE_END) {
+          okay = check_can_bind_low_ports() == keepcaps;
+        } else {
+          fprintf(stderr,
+                  "Skipping check for whether we can bind to any "
+                  "privileged ports as the user system seems to "
+                  "allow us to bind to ports even without any "
+                  "capabilities set.\n");
+        }
       }
       break;
     }