Fix bug with TCP bind w/ SO_REUSEADDR.

In gVisor today its possible that when trying to bind a TCP socket
w/ SO_REUSEADDR specified and requesting the kernel pick a port by
setting port to zero can result in a previously bound port being
returned. This behaviour is incorrect as the user is clearly requesting
a free port. The behaviour is fine when the user explicity specifies
a port.

This change now checks if the user specified a port when making a port
reservation for a TCP port and only returns unbound ports even if
SO_REUSEADDR was specified.

Fixes #6209

PiperOrigin-RevId: 382607638

1 files changed, 39 insertions, 0 deletions

diff --git a/test/syscalls/linux/socket_test_util.cc b/test/syscalls/linux/socket_test_util.cc
index 5e36472b4..1afb1ab50 100644
--- a/test/syscalls/linux/socket_test_util.cc
+++ b/test/syscalls/linux/socket_test_util.cc
@@ -24,6 +24,7 @@
 #include "gtest/gtest.h"
 #include "absl/memory/memory.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/str_split.h"
 #include "absl/time/clock.h"
 #include "absl/types/optional.h"
 #include "test/util/file_descriptor.h"
@@ -1067,5 +1068,43 @@ void SetupTimeWaitClose(const TestAddress* listener,
   absl::SleepFor(absl::Seconds(1));
 }
 
+constexpr char kRangeFile[] = "/proc/sys/net/ipv4/ip_local_port_range";
+
+PosixErrorOr<int> MaybeLimitEphemeralPorts() {
+  int min = 0;
+  int max = 1 << 16;
+
+  // Read the ephemeral range from /proc.
+  ASSIGN_OR_RETURN_ERRNO(std::string rangefile, GetContents(kRangeFile));
+  const std::string err_msg =
+      absl::StrFormat("%s has invalid content: %s", kRangeFile, rangefile);
+  if (rangefile.back() != '\n') {
+    return PosixError(EINVAL, err_msg);
+  }
+  rangefile.pop_back();
+  std::vector<std::string> range =
+      absl::StrSplit(rangefile, absl::ByAnyChar("\t "));
+  if (range.size() < 2 || !absl::SimpleAtoi(range.front(), &min) ||
+      !absl::SimpleAtoi(range.back(), &max)) {
+    return PosixError(EINVAL, err_msg);
+  }
+
+  // If we can open as writable, limit the range.
+  if (!access(kRangeFile, W_OK)) {
+    ASSIGN_OR_RETURN_ERRNO(FileDescriptor fd,
+                           Open(kRangeFile, O_WRONLY | O_TRUNC, 0));
+    max = min + 50;
+    const std::string small_range = absl::StrFormat("%d %d", min, max);
+    int n = write(fd.get(), small_range.c_str(), small_range.size());
+    if (n < 0) {
+      return PosixError(
+          errno,
+          absl::StrFormat("write(%d [%s], \"%s\", %d)", fd.get(), kRangeFile,
+                          small_range.c_str(), small_range.size()));
+    }
+  }
+  return max - min;
+}
+
 }  // namespace testing
 }  // namespace gvisor