================ @@ -2700,19 +2701,18 @@ bool UnwindCursor<A, R>::setInfoForSigReturn(Registers_arm64 &) { // [1] https://github.com/torvalds/linux/blob/master/arch/arm64/kernel/vdso/sigreturn.S const pint_t pc = static_cast<pint_t>(this->getReg(UNW_REG_IP)); // The PC might contain an invalid address if the unwind info is bad, so - // directly accessing it could cause a segfault. Use process_vm_readv to read - // the memory safely instead. process_vm_readv was added in Linux 3.2, and - // AArch64 supported was added in Linux 3.7, so the syscall is guaranteed to - // be present. Unfortunately, there are Linux AArch64 environments where the - // libc wrapper for the syscall might not be present (e.g. Android 5), so call - // the syscall directly instead. + // directly accessing it could cause a segfault. Use pipe/write/read to read + // the memory safely instead. + int pipefd[2]; + if (pipe2(pipefd, O_CLOEXEC | O_NONBLOCK) == -1) + return false; uint32_t instructions[2]; - struct iovec local_iov = {&instructions, sizeof instructions}; - struct iovec remote_iov = {reinterpret_cast<void *>(pc), sizeof instructions}; - long bytesRead = - syscall(SYS_process_vm_readv, getpid(), &local_iov, 1, &remote_iov, 1, 0); + const auto bufferSize = sizeof instructions; + if (write(pipefd[1], reinterpret_cast<void *>(pc), bufferSize) != bufferSize) + return false; + const auto bytesRead = read(pipefd[0], instructions, bufferSize); ---------------- ajordanr-google wrote:
This is interesting, thanks. I definitely could, though I'm mildly hesitant for two reasons. The first is because of the [comment from address_is_readable.cc](https://chromium.googlesource.com/external/github.com/abseil/abseil-cpp/+/HEAD/absl/debugging/internal/address_is_readable.cc#74) ```c++ // This strategy depends on Linux implementation details, // so we rely on the test to alert us if it stops working. ``` The other is whether the check will span the full set of instructions we want to read. The Abseil implimentation checks 8 bytes, which I think should just about fit everything we need if it weren't for that 8 byte alignment: ```c++ // Align address on 8-byte boundary. On aarch64, checking last // byte before inaccessible page returned unexpected EFAULT. const uintptr_t u_addr = reinterpret_cast<uintptr_t>(addr) & ~uintptr_t{7}; ``` I'm not familiar with the alignment requirements for a (potentially broken) PC. You'd have more context than I do as to whether we'd need to check for both instructions with `rt_sigprocmask`? I notice in the comments that pipe was a discarded method, but it doesn't really explain why. Performance reasons perhaps? https://github.com/llvm/llvm-project/pull/74791 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits