[libunwind] [libunwind] Replace process_vm_readv with pipe (PR #74791)

[Fangrui Song via cfe-commits]

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@@ -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);
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MaskRay wrote:

Consider SYS_rt_sigprocmask like absl/debugging/internal/address_is_readable.cc

https://github.com/llvm/llvm-project/pull/74791