97 lines
3.3 KiB
C++
97 lines
3.3 KiB
C++
// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// base::AddressIsReadable() probes an address to see whether it is readable,
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// without faulting.
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#include "absl/debugging/internal/address_is_readable.h"
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#if !defined(__linux__) || defined(__ANDROID__)
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace debugging_internal {
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// On platforms other than Linux, just return true.
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bool AddressIsReadable(const void* /* addr */) { return true; }
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} // namespace debugging_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#else // __linux__ && !__ANDROID__
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#include <stdint.h>
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#include <syscall.h>
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#include <unistd.h>
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#include "absl/base/internal/errno_saver.h"
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#include "absl/base/internal/raw_logging.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace debugging_internal {
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// NOTE: be extra careful about adding any interposable function calls here
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// (such as open(), read(), etc.). These symbols may be interposed and will get
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// invoked in contexts they don't expect.
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//
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// NOTE: any new system calls here may also require sandbox reconfiguration.
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//
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bool AddressIsReadable(const void *addr) {
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// Align address on 8-byte boundary. On aarch64, checking last
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// byte before inaccessible page returned unexpected EFAULT.
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const uintptr_t u_addr = reinterpret_cast<uintptr_t>(addr) & ~7;
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addr = reinterpret_cast<const void *>(u_addr);
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// rt_sigprocmask below will succeed for this input.
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if (addr == nullptr) return false;
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absl::base_internal::ErrnoSaver errno_saver;
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// Here we probe with some syscall which
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// - accepts an 8-byte region of user memory as input
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// - tests for EFAULT before other validation
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// - has no problematic side-effects
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//
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// rt_sigprocmask(2) works for this. It copies sizeof(kernel_sigset_t)==8
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// bytes from the address into the kernel memory before any validation.
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//
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// The call can never succeed, since the `how` parameter is not one of
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// SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK.
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//
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// This strategy depends on Linux implementation details,
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// so we rely on the test to alert us if it stops working.
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//
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// Some discarded past approaches:
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// - msync() doesn't reject PROT_NONE regions
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// - write() on /dev/null doesn't return EFAULT
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// - write() on a pipe requires creating it and draining the writes
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// - connect() works but is problematic for sandboxes and needs a valid
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// file descriptor
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//
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// This can never succeed (invalid first argument to sigprocmask).
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ABSL_RAW_CHECK(syscall(SYS_rt_sigprocmask, ~0, addr, nullptr,
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/*sizeof(kernel_sigset_t)*/ 8) == -1,
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"unexpected success");
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ABSL_RAW_CHECK(errno == EFAULT || errno == EINVAL, "unexpected errno");
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return errno != EFAULT;
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}
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} // namespace debugging_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // __linux__ && !__ANDROID__
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