Implement /proc/net/udp.

PiperOrigin-RevId: 266229756

8 files changed, 394 insertions, 46 deletions

diff --git a/test/syscalls/BUILD b/test/syscalls/BUILD
index ccae4925f..6947ddc25 100644
--- a/test/syscalls/BUILD
+++ b/test/syscalls/BUILD
@@ -691,6 +691,10 @@ syscall_test(
 
 syscall_test(test = "//test/syscalls/linux:proc_net_unix_test")
 
+syscall_test(test = "//test/syscalls/linux:proc_net_tcp_test")
+
+syscall_test(test = "//test/syscalls/linux:proc_net_udp_test")
+
 go_binary(
     name = "syscall_test_runner",
     srcs = ["syscall_test_runner.go"],
diff --git a/test/syscalls/linux/BUILD b/test/syscalls/linux/BUILD
index 88f3bfcb3..1ce38c929 100644
--- a/test/syscalls/linux/BUILD
+++ b/test/syscalls/linux/BUILD
@@ -3472,3 +3472,18 @@ cc_binary(
         "@com_google_googletest//:gtest",
     ],
 )
+
+cc_binary(
+    name = "proc_net_udp_test",
+    testonly = 1,
+    srcs = ["proc_net_udp.cc"],
+    linkstatic = 1,
+    deps = [
+        ":ip_socket_test_util",
+        "//test/util:file_descriptor",
+        "//test/util:test_main",
+        "//test/util:test_util",
+        "@com_google_absl//absl/strings",
+        "@com_google_googletest//:gtest",
+    ],
+)
diff --git a/test/syscalls/linux/ip_socket_test_util.cc b/test/syscalls/linux/ip_socket_test_util.cc
index c73262e72..410b42a47 100644
--- a/test/syscalls/linux/ip_socket_test_util.cc
+++ b/test/syscalls/linux/ip_socket_test_util.cc
@@ -23,6 +23,16 @@
 namespace gvisor {
 namespace testing {
 
+uint32_t IPFromInetSockaddr(const struct sockaddr* addr) {
+  auto* in_addr = reinterpret_cast<const struct sockaddr_in*>(addr);
+  return in_addr->sin_addr.s_addr;
+}
+
+uint16_t PortFromInetSockaddr(const struct sockaddr* addr) {
+  auto* in_addr = reinterpret_cast<const struct sockaddr_in*>(addr);
+  return ntohs(in_addr->sin_port);
+}
+
 PosixErrorOr<int> InterfaceIndex(std::string name) {
   // TODO(igudger): Consider using netlink.
   ifreq req = {};
diff --git a/test/syscalls/linux/ip_socket_test_util.h b/test/syscalls/linux/ip_socket_test_util.h
index b498a053d..3d36b9620 100644
--- a/test/syscalls/linux/ip_socket_test_util.h
+++ b/test/syscalls/linux/ip_socket_test_util.h
@@ -26,6 +26,31 @@
 namespace gvisor {
 namespace testing {
 
+// Possible values of the "st" field in a /proc/net/{tcp,udp} entry. Source:
+// Linux kernel, include/net/tcp_states.h.
+enum {
+  TCP_ESTABLISHED = 1,
+  TCP_SYN_SENT,
+  TCP_SYN_RECV,
+  TCP_FIN_WAIT1,
+  TCP_FIN_WAIT2,
+  TCP_TIME_WAIT,
+  TCP_CLOSE,
+  TCP_CLOSE_WAIT,
+  TCP_LAST_ACK,
+  TCP_LISTEN,
+  TCP_CLOSING,
+  TCP_NEW_SYN_RECV,
+
+  TCP_MAX_STATES
+};
+
+// Extracts the IP address from an inet sockaddr in network byte order.
+uint32_t IPFromInetSockaddr(const struct sockaddr* addr);
+
+// Extracts the port from an inet sockaddr in host byte order.
+uint16_t PortFromInetSockaddr(const struct sockaddr* addr);
+
 // InterfaceIndex returns the index of the named interface.
 PosixErrorOr<int> InterfaceIndex(std::string name);
 
diff --git a/test/syscalls/linux/proc_net_tcp.cc b/test/syscalls/linux/proc_net_tcp.cc
index 498f62d9c..f6d7ad0bb 100644
--- a/test/syscalls/linux/proc_net_tcp.cc
+++ b/test/syscalls/linux/proc_net_tcp.cc
@@ -38,25 +38,6 @@ constexpr char kProcNetTCPHeader[] =
     "retrnsmt   uid  timeout inode                                             "
     "        ";
 
-// Possible values of the "st" field in a /proc/net/tcp entry. Source: Linux
-// kernel, include/net/tcp_states.h.
-enum {
-  TCP_ESTABLISHED = 1,
-  TCP_SYN_SENT,
-  TCP_SYN_RECV,
-  TCP_FIN_WAIT1,
-  TCP_FIN_WAIT2,
-  TCP_TIME_WAIT,
-  TCP_CLOSE,
-  TCP_CLOSE_WAIT,
-  TCP_LAST_ACK,
-  TCP_LISTEN,
-  TCP_CLOSING,
-  TCP_NEW_SYN_RECV,
-
-  TCP_MAX_STATES
-};
-
 // TCPEntry represents a single entry from /proc/net/tcp.
 struct TCPEntry {
   uint32_t local_addr;
@@ -70,42 +51,35 @@ struct TCPEntry {
   uint64_t inode;
 };
 
-uint32_t IP(const struct sockaddr* addr) {
-  auto* in_addr = reinterpret_cast<const struct sockaddr_in*>(addr);
-  return in_addr->sin_addr.s_addr;
-}
-
-uint16_t Port(const struct sockaddr* addr) {
-  auto* in_addr = reinterpret_cast<const struct sockaddr_in*>(addr);
-  return ntohs(in_addr->sin_port);
-}
-
 // Finds the first entry in 'entries' for which 'predicate' returns true.
-// Returns true on match, and sets 'match' to point to the matching entry.
-bool FindBy(std::vector<TCPEntry> entries, TCPEntry* match,
+// Returns true on match, and sets 'match' to a copy of the matching entry. If
+// 'match' is null, it's ignored.
+bool FindBy(const std::vector<TCPEntry>& entries, TCPEntry* match,
             std::function<bool(const TCPEntry&)> predicate) {
-  for (int i = 0; i < entries.size(); ++i) {
-    if (predicate(entries[i])) {
-      *match = entries[i];
+  for (const TCPEntry& entry : entries) {
+    if (predicate(entry)) {
+      if (match != nullptr) {
+        *match = entry;
+      }
       return true;
     }
   }
   return false;
 }
 
-bool FindByLocalAddr(std::vector<TCPEntry> entries, TCPEntry* match,
+bool FindByLocalAddr(const std::vector<TCPEntry>& entries, TCPEntry* match,
                      const struct sockaddr* addr) {
-  uint32_t host = IP(addr);
-  uint16_t port = Port(addr);
+  uint32_t host = IPFromInetSockaddr(addr);
+  uint16_t port = PortFromInetSockaddr(addr);
   return FindBy(entries, match, [host, port](const TCPEntry& e) {
     return (e.local_addr == host && e.local_port == port);
   });
 }
 
-bool FindByRemoteAddr(std::vector<TCPEntry> entries, TCPEntry* match,
+bool FindByRemoteAddr(const std::vector<TCPEntry>& entries, TCPEntry* match,
                       const struct sockaddr* addr) {
-  uint32_t host = IP(addr);
-  uint16_t port = Port(addr);
+  uint32_t host = IPFromInetSockaddr(addr);
+  uint16_t port = PortFromInetSockaddr(addr);
   return FindBy(entries, match, [host, port](const TCPEntry& e) {
     return (e.remote_addr == host && e.remote_port == port);
   });
@@ -120,7 +94,7 @@ PosixErrorOr<std::vector<TCPEntry>> ProcNetTCPEntries() {
   std::vector<TCPEntry> entries;
   std::vector<std::string> lines = StrSplit(content, '\n');
   std::cerr << "<contents of /proc/net/tcp>" << std::endl;
-  for (std::string line : lines) {
+  for (const std::string& line : lines) {
     std::cerr << line << std::endl;
 
     if (!found_header) {
@@ -204,9 +178,8 @@ TEST(ProcNetTCP, BindAcceptConnect) {
     EXPECT_EQ(entries.size(), 2);
   }
 
-  TCPEntry e;
-  EXPECT_TRUE(FindByLocalAddr(entries, &e, sockets->first_addr()));
-  EXPECT_TRUE(FindByRemoteAddr(entries, &e, sockets->first_addr()));
+  EXPECT_TRUE(FindByLocalAddr(entries, nullptr, sockets->first_addr()));
+  EXPECT_TRUE(FindByRemoteAddr(entries, nullptr, sockets->first_addr()));
 }
 
 TEST(ProcNetTCP, InodeReasonable) {
@@ -261,8 +234,8 @@ TEST(ProcNetTCP, State) {
   FileDescriptor accepted =
       ASSERT_NO_ERRNO_AND_VALUE(Accept(server->get(), nullptr, nullptr));
 
-  const uint32_t accepted_local_host = IP(&addr);
-  const uint16_t accepted_local_port = Port(&addr);
+  const uint32_t accepted_local_host = IPFromInetSockaddr(&addr);
+  const uint16_t accepted_local_port = PortFromInetSockaddr(&addr);
 
   entries = ASSERT_NO_ERRNO_AND_VALUE(ProcNetTCPEntries());
   TCPEntry accepted_entry;
diff --git a/test/syscalls/linux/proc_net_udp.cc b/test/syscalls/linux/proc_net_udp.cc
new file mode 100644
index 000000000..369df8e0e
--- /dev/null
+++ b/test/syscalls/linux/proc_net_udp.cc
@@ -0,0 +1,309 @@
+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "gtest/gtest.h"
+#include "gtest/gtest.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_join.h"
+#include "absl/strings/str_split.h"
+#include "test/syscalls/linux/ip_socket_test_util.h"
+#include "test/util/file_descriptor.h"
+#include "test/util/test_util.h"
+
+namespace gvisor {
+namespace testing {
+namespace {
+
+using absl::StrCat;
+using absl::StrFormat;
+using absl::StrSplit;
+
+constexpr char kProcNetUDPHeader[] =
+    "  sl  local_address rem_address   st tx_queue rx_queue tr tm->when "
+    "retrnsmt   uid  timeout inode ref pointer drops             ";
+
+// UDPEntry represents a single entry from /proc/net/udp.
+struct UDPEntry {
+  uint32_t local_addr;
+  uint16_t local_port;
+
+  uint32_t remote_addr;
+  uint16_t remote_port;
+
+  uint64_t state;
+  uint64_t uid;
+  uint64_t inode;
+};
+
+std::string DescribeFirstInetSocket(const SocketPair& sockets) {
+  const struct sockaddr* addr = sockets.first_addr();
+  return StrFormat("First test socket: fd:%d %8X:%4X", sockets.first_fd(),
+                   IPFromInetSockaddr(addr), PortFromInetSockaddr(addr));
+}
+
+std::string DescribeSecondInetSocket(const SocketPair& sockets) {
+  const struct sockaddr* addr = sockets.second_addr();
+  return StrFormat("Second test socket fd:%d %8X:%4X", sockets.second_fd(),
+                   IPFromInetSockaddr(addr), PortFromInetSockaddr(addr));
+}
+
+// Finds the first entry in 'entries' for which 'predicate' returns true.
+// Returns true on match, and set 'match' to a copy of the matching entry. If
+// 'match' is null, it's ignored.
+bool FindBy(const std::vector<UDPEntry>& entries, UDPEntry* match,
+            std::function<bool(const UDPEntry&)> predicate) {
+  for (const UDPEntry& entry : entries) {
+    if (predicate(entry)) {
+      if (match != nullptr) {
+        *match = entry;
+      }
+      return true;
+    }
+  }
+  return false;
+}
+
+bool FindByLocalAddr(const std::vector<UDPEntry>& entries, UDPEntry* match,
+                     const struct sockaddr* addr) {
+  uint32_t host = IPFromInetSockaddr(addr);
+  uint16_t port = PortFromInetSockaddr(addr);
+  return FindBy(entries, match, [host, port](const UDPEntry& e) {
+    return (e.local_addr == host && e.local_port == port);
+  });
+}
+
+bool FindByRemoteAddr(const std::vector<UDPEntry>& entries, UDPEntry* match,
+                      const struct sockaddr* addr) {
+  uint32_t host = IPFromInetSockaddr(addr);
+  uint16_t port = PortFromInetSockaddr(addr);
+  return FindBy(entries, match, [host, port](const UDPEntry& e) {
+    return (e.remote_addr == host && e.remote_port == port);
+  });
+}
+
+PosixErrorOr<uint64_t> InodeFromSocketFD(int fd) {
+  ASSIGN_OR_RETURN_ERRNO(struct stat s, Fstat(fd));
+  if (!S_ISSOCK(s.st_mode)) {
+    return PosixError(EINVAL, StrFormat("FD %d is not a socket", fd));
+  }
+  return s.st_ino;
+}
+
+PosixErrorOr<bool> FindByFD(const std::vector<UDPEntry>& entries,
+                            UDPEntry* match, int fd) {
+  ASSIGN_OR_RETURN_ERRNO(uint64_t inode, InodeFromSocketFD(fd));
+  return FindBy(entries, match,
+                [inode](const UDPEntry& e) { return (e.inode == inode); });
+}
+
+// Returns a parsed representation of /proc/net/udp entries.
+PosixErrorOr<std::vector<UDPEntry>> ProcNetUDPEntries() {
+  std::string content;
+  RETURN_IF_ERRNO(GetContents("/proc/net/udp", &content));
+
+  bool found_header = false;
+  std::vector<UDPEntry> entries;
+  std::vector<std::string> lines = StrSplit(content, '\n');
+  std::cerr << "<contents of /proc/net/udp>" << std::endl;
+  for (const std::string& line : lines) {
+    std::cerr << line << std::endl;
+
+    if (!found_header) {
+      EXPECT_EQ(line, kProcNetUDPHeader);
+      found_header = true;
+      continue;
+    }
+    if (line.empty()) {
+      continue;
+    }
+
+    // Parse a single entry from /proc/net/udp.
+    //
+    // Example entries:
+    //
+    // clang-format off
+    //
+    //  sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode ref pointer drops
+    // 3503: 0100007F:0035 00000000:0000 07 00000000:00000000 00:00000000 00000000     0        0 33317 2 0000000000000000 0
+    // 3518: 00000000:0044 00000000:0000 07 00000000:00000000 00:00000000 00000000     0        0 40394 2 0000000000000000 0
+    //   ^     ^       ^     ^       ^   ^     ^       ^      ^     ^        ^         ^        ^   ^   ^      ^           ^
+    //   0     1       2     3       4   5     6       7      8     9       10        11       12  13  14     15           16
+    //
+    // clang-format on
+
+    UDPEntry entry;
+    std::vector<std::string> fields =
+        StrSplit(line, absl::ByAnyChar(": "), absl::SkipEmpty());
+
+    ASSIGN_OR_RETURN_ERRNO(entry.local_addr, AtoiBase(fields[1], 16));
+    ASSIGN_OR_RETURN_ERRNO(entry.local_port, AtoiBase(fields[2], 16));
+
+    ASSIGN_OR_RETURN_ERRNO(entry.remote_addr, AtoiBase(fields[3], 16));
+    ASSIGN_OR_RETURN_ERRNO(entry.remote_port, AtoiBase(fields[4], 16));
+
+    ASSIGN_OR_RETURN_ERRNO(entry.state, AtoiBase(fields[5], 16));
+    ASSIGN_OR_RETURN_ERRNO(entry.uid, Atoi<uint64_t>(fields[11]));
+    ASSIGN_OR_RETURN_ERRNO(entry.inode, Atoi<uint64_t>(fields[13]));
+
+    // Linux shares internal data structures between TCP and UDP sockets. The
+    // proc entries for UDP sockets share some fields with TCP sockets, but
+    // these fields should always be zero as they're not meaningful for UDP
+    // sockets.
+    EXPECT_EQ(fields[8], "00") << StrFormat("sl:%s, tr", fields[0]);
+    EXPECT_EQ(fields[9], "00000000") << StrFormat("sl:%s, tm->when", fields[0]);
+    EXPECT_EQ(fields[10], "00000000")
+        << StrFormat("sl:%s, retrnsmt", fields[0]);
+    EXPECT_EQ(fields[12], "0") << StrFormat("sl:%s, timeout", fields[0]);
+
+    entries.push_back(entry);
+  }
+  std::cerr << "<end of /proc/net/udp>" << std::endl;
+
+  return entries;
+}
+
+TEST(ProcNetUDP, Exists) {
+  const std::string content =
+      ASSERT_NO_ERRNO_AND_VALUE(GetContents("/proc/net/udp"));
+  const std::string header_line = StrCat(kProcNetUDPHeader, "\n");
+  EXPECT_THAT(content, ::testing::StartsWith(header_line));
+}
+
+TEST(ProcNetUDP, EntryUID) {
+  auto sockets =
+      ASSERT_NO_ERRNO_AND_VALUE(IPv4UDPBidirectionalBindSocketPair(0).Create());
+  std::vector<UDPEntry> entries =
+      ASSERT_NO_ERRNO_AND_VALUE(ProcNetUDPEntries());
+  UDPEntry e;
+  ASSERT_TRUE(FindByLocalAddr(entries, &e, sockets->first_addr()))
+      << DescribeFirstInetSocket(*sockets);
+  EXPECT_EQ(e.uid, geteuid());
+  ASSERT_TRUE(FindByRemoteAddr(entries, &e, sockets->first_addr()))
+      << DescribeSecondInetSocket(*sockets);
+  EXPECT_EQ(e.uid, geteuid());
+}
+
+TEST(ProcNetUDP, FindMutualEntries) {
+  auto sockets =
+      ASSERT_NO_ERRNO_AND_VALUE(IPv4UDPBidirectionalBindSocketPair(0).Create());
+  std::vector<UDPEntry> entries =
+      ASSERT_NO_ERRNO_AND_VALUE(ProcNetUDPEntries());
+
+  EXPECT_TRUE(FindByLocalAddr(entries, nullptr, sockets->first_addr()))
+      << DescribeFirstInetSocket(*sockets);
+  EXPECT_TRUE(FindByRemoteAddr(entries, nullptr, sockets->first_addr()))
+      << DescribeSecondInetSocket(*sockets);
+
+  EXPECT_TRUE(FindByLocalAddr(entries, nullptr, sockets->second_addr()))
+      << DescribeSecondInetSocket(*sockets);
+  EXPECT_TRUE(FindByRemoteAddr(entries, nullptr, sockets->second_addr()))
+      << DescribeFirstInetSocket(*sockets);
+}
+
+TEST(ProcNetUDP, EntriesRemovedOnClose) {
+  auto sockets =
+      ASSERT_NO_ERRNO_AND_VALUE(IPv4UDPBidirectionalBindSocketPair(0).Create());
+  std::vector<UDPEntry> entries =
+      ASSERT_NO_ERRNO_AND_VALUE(ProcNetUDPEntries());
+
+  EXPECT_TRUE(FindByLocalAddr(entries, nullptr, sockets->first_addr()))
+      << DescribeFirstInetSocket(*sockets);
+  EXPECT_TRUE(FindByLocalAddr(entries, nullptr, sockets->second_addr()))
+      << DescribeSecondInetSocket(*sockets);
+
+  EXPECT_THAT(close(sockets->release_first_fd()), SyscallSucceeds());
+  entries = ASSERT_NO_ERRNO_AND_VALUE(ProcNetUDPEntries());
+  // First socket's entry should be gone, but the second socket's entry should
+  // still exist.
+  EXPECT_FALSE(FindByLocalAddr(entries, nullptr, sockets->first_addr()))
+      << DescribeFirstInetSocket(*sockets);
+  EXPECT_TRUE(FindByLocalAddr(entries, nullptr, sockets->second_addr()))
+      << DescribeSecondInetSocket(*sockets);
+
+  EXPECT_THAT(close(sockets->release_second_fd()), SyscallSucceeds());
+  entries = ASSERT_NO_ERRNO_AND_VALUE(ProcNetUDPEntries());
+  // Both entries should be gone.
+  EXPECT_FALSE(FindByLocalAddr(entries, nullptr, sockets->first_addr()))
+      << DescribeFirstInetSocket(*sockets);
+  EXPECT_FALSE(FindByLocalAddr(entries, nullptr, sockets->second_addr()))
+      << DescribeSecondInetSocket(*sockets);
+}
+
+PosixErrorOr<std::unique_ptr<FileDescriptor>> BoundUDPSocket() {
+  ASSIGN_OR_RETURN_ERRNO(std::unique_ptr<FileDescriptor> socket,
+                         IPv4UDPUnboundSocket(0).Create());
+  struct sockaddr_in addr;
+  addr.sin_family = AF_INET;
+  addr.sin_addr.s_addr = htonl(INADDR_ANY);
+  addr.sin_port = 0;
+
+  int res = bind(socket->get(), reinterpret_cast<const struct sockaddr*>(&addr),
+                 sizeof(addr));
+  if (res) {
+    return PosixError(errno, "bind()");
+  }
+  return socket;
+}
+
+TEST(ProcNetUDP, BoundEntry) {
+  std::unique_ptr<FileDescriptor> socket =
+      ASSERT_NO_ERRNO_AND_VALUE(BoundUDPSocket());
+  struct sockaddr addr;
+  socklen_t len = sizeof(addr);
+  ASSERT_THAT(getsockname(socket->get(), &addr, &len), SyscallSucceeds());
+  uint16_t port = PortFromInetSockaddr(&addr);
+
+  std::vector<UDPEntry> entries =
+      ASSERT_NO_ERRNO_AND_VALUE(ProcNetUDPEntries());
+  UDPEntry e;
+  ASSERT_TRUE(ASSERT_NO_ERRNO_AND_VALUE(FindByFD(entries, &e, socket->get())));
+  EXPECT_EQ(e.local_port, port);
+  EXPECT_EQ(e.remote_addr, 0);
+  EXPECT_EQ(e.remote_port, 0);
+}
+
+TEST(ProcNetUDP, BoundSocketStateClosed) {
+  std::unique_ptr<FileDescriptor> socket =
+      ASSERT_NO_ERRNO_AND_VALUE(BoundUDPSocket());
+  std::vector<UDPEntry> entries =
+      ASSERT_NO_ERRNO_AND_VALUE(ProcNetUDPEntries());
+  UDPEntry e;
+  ASSERT_TRUE(ASSERT_NO_ERRNO_AND_VALUE(FindByFD(entries, &e, socket->get())));
+  EXPECT_EQ(e.state, TCP_CLOSE);
+}
+
+TEST(ProcNetUDP, ConnectedSocketStateEstablished) {
+  auto sockets =
+      ASSERT_NO_ERRNO_AND_VALUE(IPv4UDPBidirectionalBindSocketPair(0).Create());
+  std::vector<UDPEntry> entries =
+      ASSERT_NO_ERRNO_AND_VALUE(ProcNetUDPEntries());
+
+  UDPEntry e;
+  ASSERT_TRUE(FindByLocalAddr(entries, &e, sockets->first_addr()))
+      << DescribeFirstInetSocket(*sockets);
+  EXPECT_EQ(e.state, TCP_ESTABLISHED);
+
+  ASSERT_TRUE(FindByLocalAddr(entries, &e, sockets->second_addr()))
+      << DescribeSecondInetSocket(*sockets);
+  EXPECT_EQ(e.state, TCP_ESTABLISHED);
+}
+
+}  // namespace
+}  // namespace testing
+}  // namespace gvisor
diff --git a/test/util/fs_util.cc b/test/util/fs_util.cc
index ae49725a0..f7d231b14 100644
--- a/test/util/fs_util.cc
+++ b/test/util/fs_util.cc
@@ -105,6 +105,15 @@ PosixErrorOr<struct stat> Stat(absl::string_view path) {
   return stat_buf;
 }
 
+PosixErrorOr<struct stat> Fstat(int fd) {
+  struct stat stat_buf;
+  int res = fstat(fd, &stat_buf);
+  if (res < 0) {
+    return PosixError(errno, absl::StrCat("fstat ", fd));
+  }
+  return stat_buf;
+}
+
 PosixErrorOr<bool> Exists(absl::string_view path) {
   struct stat stat_buf;
   int res = stat(std::string(path).c_str(), &stat_buf);
diff --git a/test/util/fs_util.h b/test/util/fs_util.h
index 3969f8309..e5b555891 100644
--- a/test/util/fs_util.h
+++ b/test/util/fs_util.h
@@ -35,6 +35,9 @@ PosixErrorOr<bool> Exists(absl::string_view path);
 // Returns a stat structure for the given path or an error.
 PosixErrorOr<struct stat> Stat(absl::string_view path);
 
+// Returns a stat struct for the given fd.
+PosixErrorOr<struct stat> Fstat(int fd);
+
 // Deletes the file or directory at path or returns an error.
 PosixError Delete(absl::string_view path);