Optimize TAP interface check.

Before this change, the TAP interface check would search through every
active process' file descriptors to find which tap interfaces were in
use.

Instead, now we attempt to open each TAP interface and report any
errors. This reduces the runtime of the TAP interface check from about
6 seconds to under 0.1 seconds (when tested on my workstation).

Bug: 401259067
Test: check launcher.log for total runtime improvement

Author: Brian Daniels

base/cvd/allocd/alloc_utils.cpp

@@ -30,6 +30,12 @@
 #include "cuttlefish/host/commands/cvd/utils/common.h"
 #include "cuttlefish/result/result.h"
 
+#ifdef __linux__
+#include <linux/if_tun.h>
+#endif
+
+#include <net/if.h>
+
 namespace cuttlefish {
 
 bool CreateEthernetIface(std::string_view name, std::string_view bridge_name) {
@@ -124,6 +130,27 @@ bool CreateTap(std::string_view name) {
   return true;
 }
 
+#ifdef __linux__
+Result<void> ValidateTapInterfaceIsUsable(const std::string& interface_name) {
+  constexpr auto kTunTapDev = "/dev/net/tun";
+
+  auto tap_fd = SharedFD::Open(kTunTapDev, O_RDWR | O_NONBLOCK);
+  CF_EXPECTF(tap_fd->IsOpen(), "Unable to open tun device: {}",
+             tap_fd->StrError());
+
+  struct ifreq ifr;
+  memset(&ifr, 0, sizeof(ifr));
+  ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_VNET_HDR;
+  strncpy(ifr.ifr_name, interface_name.c_str(), IFNAMSIZ);
+
+  int err = tap_fd->Ioctl(TUNSETIFF, &ifr);
+  CF_EXPECTF(err == 0, "Unable to connect to {} tap interface: {}",
+             interface_name, tap_fd->StrError());
+
+  return {};
+}
+#endif
+
 bool DestroyIface(std::string_view name) {
   if (!ShutdownIface(name).ok()) {
     LOG(WARNING) << "Failed to shutdown tap interface: " << name;

base/cvd/allocd/alloc_utils.h

@@ -23,6 +23,7 @@
 
 #include <optional>
 #include <string_view>
+#include "cuttlefish/result/result.h"
 
 namespace cuttlefish {
 
@@ -53,6 +54,10 @@ std::optional<std::string> GetUserName(uid_t uid);
 
 bool CreateTap(std::string_view name);
 
+#ifdef __linux__
+Result<void> ValidateTapInterfaceIsUsable(const std::string& interface_name);
+#endif
+
 
 bool DestroyIface(std::string_view name);
 

base/cvd/cuttlefish/common/libs/utils/network.cpp

@@ -16,20 +16,9 @@
 
 #include "cuttlefish/common/libs/utils/network.h"
 
-#ifdef __linux__
-#include <linux/if_ether.h>
-// Kernel headers don't mix well with userspace headers, but there is no
-// userspace header that provides the if_tun.h #defines.  Include the kernel
-// header, but move conflicting definitions out of the way using macros.
-#define ethhdr __kernel_ethhdr
-#include <linux/if_tun.h>
-#undef ethhdr
-#endif
-
 #include <arpa/inet.h>
 #include <fcntl.h>
 #include <ifaddrs.h>
-#include <net/if.h>
 #include <stdint.h>
 
 #include <cstring>
@@ -86,68 +75,6 @@ bool NetworkInterfaceExists(const std::string& interface_name) {
   return ret;
 }
 
-#ifdef __linux__
-static std::optional<Command> GrepCommand() {
-  if (FileExists("/usr/bin/grep")) {
-    return Command("/usr/bin/grep");
-  } else if (FileExists("/bin/grep")) {
-    return Command("/bin/grep");
-  } else {
-    return {};
-  }
-}
-
-std::set<std::string> TapInterfacesInUse() {
-  std::vector<std::string> fdinfo_list;
-
-  Result<std::vector<std::string>> processes = DirectoryContents("/proc");
-  if (!processes.ok()) {
-    LOG(ERROR) << "Failed to get contents of `/proc/`";
-    return {};
-  }
-  for (const std::string& process : *processes) {
-    std::string fdinfo_path = fmt::format("/proc/{}/fdinfo", process);
-    Result<std::vector<std::string>> fdinfos = DirectoryContents(fdinfo_path);
-    if (!fdinfos.ok()) {
-      VLOG(1) << "Failed to get contents of '" << fdinfo_path << "'";
-      continue;
-    }
-    for (const std::string& fdinfo : *fdinfos) {
-      std::string path = fmt::format("/proc/{}/fdinfo/{}", process, fdinfo);
-      fdinfo_list.emplace_back(std::move(path));
-    }
-  }
-
-  std::optional<Command> cmd = GrepCommand();
-  if (!cmd) {
-    LOG(WARNING) << "Unable to test TAP interface usage";
-    return {};
-  }
-  cmd->AddParameter("-E").AddParameter("-h").AddParameter("-e").AddParameter(
-      "^iff:.*");
-
-  for (const std::string& fdinfo : fdinfo_list) {
-    cmd->AddParameter(fdinfo);
-  }
-
-  std::string stdout_str = RunAndCaptureStdout(std::move(*cmd)).value_or("");
-
-  std::vector<std::string_view> lines = absl::StrSplit(stdout_str, '\n');
-  std::set<std::string> tap_interfaces;
-  for (const auto& line : lines) {
-    if (line.empty()) {
-      continue;
-    }
-    if (!absl::StartsWith(line, "iff:\t")) {
-      LOG(ERROR) << "Unexpected line \"" << line << "\"";
-      continue;
-    }
-    tap_interfaces.emplace(line.substr(std::string("iff:\t").size()));
-  }
-  return tap_interfaces;
-}
-#endif
-
 std::string MacAddressToString(const uint8_t mac[6]) {
   std::vector<uint8_t> mac_vec(mac, mac + 6);
   return fmt::format("{:0>2x}", fmt::join(mac_vec, ":"));

base/cvd/cuttlefish/common/libs/utils/network.h

@@ -22,12 +22,6 @@
 namespace cuttlefish {
 // Check network interface with given name exists, such as cvd-ebr.
 bool NetworkInterfaceExists(const std::string& interface_name);
-
-#ifdef __linux__
-// Returns a list of TAP devices that have open file descriptors
-std::set<std::string> TapInterfacesInUse();
-#endif
-
 void GenerateCorrespondingIpv6ForMac(const uint8_t mac[6], uint8_t out[16]);
 void GenerateMobileMacForInstance(int index, uint8_t out[6]);
 void GenerateEthMacForInstance(int index, uint8_t out[6]);

base/cvd/cuttlefish/host/commands/run_cvd/BUILD.bazel

@@ -162,6 +162,7 @@ cf_cc_library(
     srcs = ["validate.cpp"],
     hdrs = ["validate.h"],
     deps = [
+        "//allocd:alloc_utils",
         "//cuttlefish/common/libs/utils:in_sandbox",
         "//cuttlefish/common/libs/utils:network",
         "//cuttlefish/host/libs/config:cuttlefish_config",

base/cvd/cuttlefish/host/commands/run_cvd/validate.cpp

@@ -21,8 +21,8 @@
 
 #include "absl/log/log.h"
 
+#include "allocd/alloc_utils.h"
 #include "cuttlefish/common/libs/utils/in_sandbox.h"
-#include "cuttlefish/common/libs/utils/network.h"
 #include "cuttlefish/host/libs/config/cuttlefish_config.h"
 #include "cuttlefish/posix/strerror.h"
 #include "cuttlefish/result/result.h"
@@ -35,13 +35,13 @@ static Result<void> TestTapDevices(
   if (InSandbox()) {
     return {};
   }
-  auto taps = TapInterfacesInUse();
   auto wifi = instance.wifi_tap_name();
-  CF_EXPECTF(taps.count(wifi) == 0, "Device \"{}\" in use", wifi);
+  CF_EXPECTF(ValidateTapInterfaceIsUsable(wifi), "Device \"{}\" in use", wifi);
   auto mobile = instance.mobile_tap_name();
-  CF_EXPECTF(taps.count(mobile) == 0, "Device \"{}\" in use", mobile);
+  CF_EXPECTF(ValidateTapInterfaceIsUsable(mobile), "Device \"{}\" in use",
+             mobile);
   auto eth = instance.ethernet_tap_name();
-  CF_EXPECTF(taps.count(eth) == 0, "Device \"{}\" in use", eth);
+  CF_EXPECTF(ValidateTapInterfaceIsUsable(eth), "Device \"{}\" in use", eth);
 #else
   (void)instance;
 #endif