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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
//
// 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__
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