// 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

//

//      https://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 {

ABSL_NAMESPACE_BEGIN

namespace debugging_internal {

// On platforms other than Linux, just return true.

bool AddressIsReadable(const void* /* addr */) { return true; }

}  // namespace debugging_internal

ABSL_NAMESPACE_END

}  // namespace absl

#else  // __linux__ && !__ANDROID__

#include <stdint.h>

#include <syscall.h>

#include <unistd.h>

#include "absl/base/internal/errno_saver.h"

#include "absl/base/internal/raw_logging.h"

namespace absl {

ABSL_NAMESPACE_BEGIN

namespace debugging_internal {

// NOTE: be extra careful about adding any interposable function calls here

// (such as open(), read(), etc.). These symbols may be interposed and will get

// invoked in contexts they don't expect.

//

// NOTE: any new system calls here may also require sandbox reconfiguration.

//

bool AddressIsReadable(const void *addr) {

  // rt_sigprocmask below checks 8 contiguous bytes. If addr resides in the

  // last 7 bytes of a page (unaligned), rt_sigprocmask would additionally

  // check the readability of the next page, which is not desired. Align

  // address on 8-byte boundary to check only the current page.

  const uintptr_t u_addr = reinterpret_cast<uintptr_t>(addr) & ~uintptr_t{7};

  addr = reinterpret_cast<const void *>(u_addr);

  // rt_sigprocmask below will succeed for this input.

  if (addr == nullptr) return false;

  absl::base_internal::ErrnoSaver errno_saver;

  // Here we probe with some syscall which

  // - accepts an 8-byte region of user memory as input

  // - tests for EFAULT before other validation

  // - has no problematic side-effects

  //

  // rt_sigprocmask(2) works for this.  It copies sizeof(kernel_sigset_t)==8

  // bytes from the address into the kernel memory before any validation.

  //

  // The call can never succeed, since the `how` parameter is not one of

  // SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK.

  //

  // This strategy depends on Linux implementation details,

  // so we rely on the test to alert us if it stops working.

  //

  // Some discarded past approaches:

  // - msync() doesn't reject PROT_NONE regions

  // - write() on /dev/null doesn't return EFAULT

  // - write() on a pipe requires creating it and draining the writes

  // - connect() works but is problematic for sandboxes and needs a valid

  //   file descriptor

  //

  // This can never succeed (invalid first argument to sigprocmask).

  ABSL_RAW_CHECK(syscall(SYS_rt_sigprocmask, ~0, addr, nullptr,

                         /*sizeof(kernel_sigset_t)*/ 8) == -1,

                 "unexpected success");

  ABSL_RAW_CHECK(errno == EFAULT || errno == EINVAL, "unexpected errno");

  return errno != EFAULT;

}

}  // namespace debugging_internal

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // __linux__ && !__ANDROID__