Initial Commit

absl/debugging/internal/address_is_readable.cc

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+// Copyright 2017 The Abseil Authors.
+//
+// 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.
+
+// base::AddressIsReadable() probes an address to see whether it is readable,
+// without faulting.
+
+#include "absl/debugging/internal/address_is_readable.h"
+
+#if !defined(__linux__) || defined(__ANDROID__)
+
+namespace absl {
+namespace debug_internal {
+
+// On platforms other than Linux, just return true.
+bool AddressIsReadable(const void* /* addr */) { return true; }
+
+}  // namespace debug_internal
+}  // namespace absl
+
+#else
+
+#include <fcntl.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include <atomic>
+#include <cerrno>
+#include <cstdint>
+
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+namespace debug_internal {
+
+// Pack a pid and two file descriptors into a 64-bit word,
+// using 16, 24, and 24 bits for each respectively.
+static uint64_t Pack(uint64_t pid, uint64_t read_fd, uint64_t write_fd) {
+  ABSL_RAW_CHECK((read_fd >> 24) == 0 && (write_fd >> 24) == 0,
+                 "fd out of range");
+  return (pid << 48) | ((read_fd & 0xffffff) << 24) | (write_fd & 0xffffff);
+}
+
+// Unpack x into a pid and two file descriptors, where x was created with
+// Pack().
+static void Unpack(uint64_t x, int *pid, int *read_fd, int *write_fd) {
+  *pid = x >> 48;
+  *read_fd = (x >> 24) & 0xffffff;
+  *write_fd = x & 0xffffff;
+}
+
+// Return whether the byte at *addr is readable, without faulting.
+// Save and restores errno.   Returns true on systems where
+// unimplemented.
+// This is a namespace-scoped variable for correct zero-initialization.
+static std::atomic<uint64_t> pid_and_fds;  // initially 0, an invalid pid.
+bool AddressIsReadable(const void *addr) {
+  int save_errno = errno;
+  // We test whether a byte is readable by using write().  Normally, this would
+  // be done via a cached file descriptor to /dev/null, but linux fails to
+  // check whether the byte is readable when the destination is /dev/null, so
+  // we use a cached pipe.  We store the pid of the process that created the
+  // pipe to handle the case where a process forks, and the child closes all
+  // the file descriptors and then calls this routine.  This is not perfect:
+  // the child could use the routine, then close all file descriptors and then
+  // use this routine again.  But the likely use of this routine is when
+  // crashing, to test the validity of pages when dumping the stack.  Beware
+  // that we may leak file descriptors, but we're unlikely to leak many.
+  int bytes_written;
+  int current_pid = getpid() & 0xffff;   // we use only the low order 16 bits
+  do {  // until we do not get EBADF trying to use file descriptors
+    int pid;
+    int read_fd;
+    int write_fd;
+    uint64_t local_pid_and_fds = pid_and_fds.load(std::memory_order_relaxed);
+    Unpack(local_pid_and_fds, &pid, &read_fd, &write_fd);
+    while (current_pid != pid) {
+      int p[2];
+      // new pipe
+      if (pipe(p) != 0) {
+        ABSL_RAW_LOG(FATAL, "Failed to create pipe, errno=%d", errno);
+      }
+      fcntl(p[0], F_SETFD, FD_CLOEXEC);
+      fcntl(p[1], F_SETFD, FD_CLOEXEC);
+      uint64_t new_pid_and_fds = Pack(current_pid, p[0], p[1]);
+      if (pid_and_fds.compare_exchange_strong(
+              local_pid_and_fds, new_pid_and_fds, std::memory_order_relaxed,
+              std::memory_order_relaxed)) {
+        local_pid_and_fds = new_pid_and_fds;  // fds exposed to other threads
+      } else {  // fds not exposed to other threads; we can close them.
+        close(p[0]);
+        close(p[1]);
+        local_pid_and_fds = pid_and_fds.load(std::memory_order_relaxed);
+      }
+      Unpack(local_pid_and_fds, &pid, &read_fd, &write_fd);
+    }
+    errno = 0;
+    // Use syscall(SYS_write, ...) instead of write() to prevent ASAN
+    // and other checkers from complaining about accesses to arbitrary
+    // memory.
+    do {
+      bytes_written = syscall(SYS_write, write_fd, addr, 1);
+    } while (bytes_written == -1 && errno == EINTR);
+    if (bytes_written == 1) {   // remove the byte from the pipe
+      char c;
+      while (read(read_fd, &c, 1) == -1 && errno == EINTR) {
+      }
+    }
+    if (errno == EBADF) {  // Descriptors invalid.
+      // If pid_and_fds contains the problematic file descriptors we just used,
+      // this call will forget them, and the loop will try again.
+      pid_and_fds.compare_exchange_strong(local_pid_and_fds, 0,
+                                          std::memory_order_relaxed,
+                                          std::memory_order_relaxed);
+    }
+  } while (errno == EBADF);
+  errno = save_errno;
+  return bytes_written == 1;
+}
+
+}  // namespace debug_internal
+}  // namespace absl
+
+#endif