[lldb] [Process] Watch for fork/vfork notifications

Watch for fork(2)/vfork(2) (also fork/vfork-style clone(2) on Linux)
notifications and explicitly detach the forked child process, and add
initial tests for these cases.  The code covers FreeBSD, Linux
and NetBSD process plugins.  There is no new user-visible functionality
provided -- this change lays foundations over subsequent work on fork
support.

Differential Revision: https://reviews.llvm.org/D98822

Author: mgorny

lldb/include/lldb/Host/linux/Host.h

@@ -0,0 +1,22 @@
+//===-- Host.h --------------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_HOST_LINUX_HOST_H
+#define LLDB_HOST_LINUX_HOST_H
+
+#include "lldb/lldb-types.h"
+#include "llvm/ADT/Optional.h"
+
+namespace lldb_private {
+
+// Get PID (i.e. the primary thread ID) corresponding to the specified TID.
+llvm::Optional<lldb::pid_t> getPIDForTID(lldb::pid_t tid);
+
+} // namespace lldb_private
+
+#endif // #ifndef LLDB_HOST_LINUX_HOST_H

lldb/source/Host/linux/Host.cpp

@@ -27,6 +27,7 @@
 #include "lldb/Host/FileSystem.h"
 #include "lldb/Host/Host.h"
 #include "lldb/Host/HostInfo.h"
+#include "lldb/Host/linux/Host.h"
 #include "lldb/Host/linux/Support.h"
 #include "lldb/Utility/DataExtractor.h"
 
@@ -53,7 +54,8 @@ class ProcessLaunchInfo;
 }
 
 static bool GetStatusInfo(::pid_t Pid, ProcessInstanceInfo &ProcessInfo,
-                          ProcessState &State, ::pid_t &TracerPid) {
+                          ProcessState &State, ::pid_t &TracerPid,
+                          ::pid_t &Tgid) {
   Log *log = GetLogIfAllCategoriesSet(LIBLLDB_LOG_HOST);
 
   auto BufferOrError = getProcFile(Pid, "status");
@@ -107,6 +109,9 @@ static bool GetStatusInfo(::pid_t Pid, ProcessInstanceInfo &ProcessInfo,
     } else if (Line.consume_front("TracerPid:")) {
       Line = Line.ltrim();
       Line.consumeInteger(10, TracerPid);
+    } else if (Line.consume_front("Tgid:")) {
+      Line = Line.ltrim();
+      Line.consumeInteger(10, Tgid);
     }
   }
   return true;
@@ -204,6 +209,7 @@ static void GetProcessEnviron(::pid_t pid, ProcessInstanceInfo &process_info) {
 static bool GetProcessAndStatInfo(::pid_t pid,
                                   ProcessInstanceInfo &process_info,
                                   ProcessState &State, ::pid_t &tracerpid) {
+  ::pid_t tgid;
   tracerpid = 0;
   process_info.Clear();
 
@@ -214,7 +220,7 @@ static bool GetProcessAndStatInfo(::pid_t pid,
   GetProcessEnviron(pid, process_info);
 
   // Get User and Group IDs and get tracer pid.
-  if (!GetStatusInfo(pid, process_info, State, tracerpid))
+  if (!GetStatusInfo(pid, process_info, State, tracerpid, tgid))
     return false;
 
   return true;
@@ -308,3 +314,14 @@ Environment Host::GetEnvironment() { return Environment(environ); }
 Status Host::ShellExpandArguments(ProcessLaunchInfo &launch_info) {
   return Status("unimplemented");
 }
+
+llvm::Optional<lldb::pid_t> lldb_private::getPIDForTID(lldb::pid_t tid) {
+  ::pid_t tracerpid, tgid = LLDB_INVALID_PROCESS_ID;
+  ProcessInstanceInfo process_info;
+  ProcessState state;
+
+  if (!GetStatusInfo(tid, process_info, state, tracerpid, tgid) ||
+      tgid == LLDB_INVALID_PROCESS_ID)
+    return llvm::None;
+  return tgid;
+}

lldb/source/Plugins/Process/FreeBSD/NativeProcessFreeBSD.cpp

@@ -247,6 +247,19 @@ void NativeProcessFreeBSD::MonitorSIGTRAP(lldb::pid_t pid) {
     return;
   }
 
+  if (info.pl_flags & PL_FLAG_FORKED) {
+    MonitorClone(info.pl_child_pid);
+    return;
+  }
+
+  if (info.pl_flags & PL_FLAG_VFORK_DONE) {
+    Status error =
+        PtraceWrapper(PT_CONTINUE, pid, reinterpret_cast<void *>(1), 0);
+    if (error.Fail())
+      SetState(StateType::eStateInvalid);
+    return;
+  }
+
   if (info.pl_lwpid > 0) {
     for (const auto &t : m_threads) {
       if (t->GetID() == static_cast<lldb::tid_t>(info.pl_lwpid))
@@ -705,17 +718,17 @@ NativeProcessFreeBSD::GetFileLoadAddress(const llvm::StringRef &file_name,
 
 void NativeProcessFreeBSD::SigchldHandler() {
   Log *log(ProcessPOSIXLog::GetLogIfAllCategoriesSet(POSIX_LOG_PROCESS));
-  // Process all pending waitpid notifications.
   int status;
   ::pid_t wait_pid =
       llvm::sys::RetryAfterSignal(-1, waitpid, GetID(), &status, WNOHANG);
 
   if (wait_pid == 0)
-    return; // We are done.
+    return;
 
   if (wait_pid == -1) {
     Status error(errno, eErrorTypePOSIX);
     LLDB_LOG(log, "waitpid ({0}, &status, _) failed: {1}", GetID(), error);
+    return;
   }
 
   WaitStatus wait_status = WaitStatus::Decode(status);
@@ -885,7 +898,7 @@ Status NativeProcessFreeBSD::SetupTrace() {
       PtraceWrapper(PT_GET_EVENT_MASK, GetID(), &events, sizeof(events));
   if (status.Fail())
     return status;
-  events |= PTRACE_LWP;
+  events |= PTRACE_LWP | PTRACE_FORK | PTRACE_VFORK;
   status = PtraceWrapper(PT_SET_EVENT_MASK, GetID(), &events, sizeof(events));
   if (status.Fail())
     return status;
@@ -919,3 +932,53 @@ Status NativeProcessFreeBSD::ReinitializeThreads() {
 bool NativeProcessFreeBSD::SupportHardwareSingleStepping() const {
   return !m_arch.IsMIPS();
 }
+
+void NativeProcessFreeBSD::MonitorClone(::pid_t child_pid) {
+  Log *log(ProcessPOSIXLog::GetLogIfAllCategoriesSet(POSIX_LOG_PROCESS));
+  LLDB_LOG(log, "fork, child_pid={0}", child_pid);
+
+  int status;
+  ::pid_t wait_pid =
+      llvm::sys::RetryAfterSignal(-1, ::waitpid, child_pid, &status, 0);
+  if (wait_pid != child_pid) {
+    LLDB_LOG(log,
+             "waiting for pid {0} failed. Assuming the pid has "
+             "disappeared in the meantime",
+             child_pid);
+    return;
+  }
+  if (WIFEXITED(status)) {
+    LLDB_LOG(log,
+             "waiting for pid {0} returned an 'exited' event. Not "
+             "tracking it.",
+             child_pid);
+    return;
+  }
+
+  MainLoop unused_loop;
+  NativeProcessFreeBSD child_process{static_cast<::pid_t>(child_pid),
+                                     m_terminal_fd, *m_delegates[0], m_arch,
+                                     unused_loop};
+  child_process.ReinitializeThreads();
+  auto *child_thread =
+      static_cast<NativeThreadFreeBSD *>(child_process.GetCurrentThread());
+  assert(child_thread);
+  // new processes inherit dbregs, so we need to clear them
+  llvm::Error error = child_thread->GetRegisterContext().ClearDBRegs();
+  if (error) {
+    LLDB_LOG_ERROR(log, std::move(error),
+                   "failed to clear dbregs in forked process {1}: {0}",
+                   child_pid);
+    SetState(StateType::eStateInvalid);
+    return;
+  }
+
+  child_process.Detach();
+  Status pt_error =
+      PtraceWrapper(PT_CONTINUE, GetID(), reinterpret_cast<void *>(1), 0);
+  if (pt_error.Fail()) {
+    LLDB_LOG_ERROR(log, pt_error.ToError(),
+                   "unable to resume parent process {1}: {0}", GetID());
+    SetState(StateType::eStateInvalid);
+  }
+}

lldb/source/Plugins/Process/FreeBSD/NativeProcessFreeBSD.h

@@ -113,6 +113,7 @@ class NativeProcessFreeBSD : public NativeProcessELF,
   void MonitorSIGSTOP(lldb::pid_t pid);
   void MonitorSIGTRAP(lldb::pid_t pid);
   void MonitorSignal(lldb::pid_t pid, int signal);
+  void MonitorClone(::pid_t child_pid);
 
   Status PopulateMemoryRegionCache();
   void SigchldHandler();

lldb/source/Plugins/Process/FreeBSD/NativeRegisterContextFreeBSD.h

@@ -32,6 +32,8 @@ class NativeRegisterContextFreeBSD
   virtual llvm::Error
   CopyHardwareWatchpointsFrom(NativeRegisterContextFreeBSD &source) = 0;
 
+  virtual llvm::Error ClearDBRegs() { return llvm::Error::success(); }
+
 protected:
   virtual NativeProcessFreeBSD &GetProcess();
   virtual ::pid_t GetProcessPid();

lldb/source/Plugins/Process/FreeBSD/NativeRegisterContextFreeBSD_arm64.cpp

@@ -285,4 +285,19 @@ NativeRegisterContextFreeBSD_arm64::WriteHardwareDebugRegs(DREGType) {
 #endif
 }
 
+llvm::Error NativeRegisterContextFreeBSD_arm64::ClearDBRegs() {
+#ifdef LLDB_HAS_FREEBSD_WATCHPOINT
+  if (llvm::Error error = ReadHardwareDebugInfo())
+    return error;
+
+  for (uint32_t i = 0; i < m_max_hbp_supported; i++)
+    m_hbp_regs[i].control = 0;
+  for (uint32_t i = 0; i < m_max_hwp_supported; i++)
+    m_hwp_regs[i].control = 0;
+  return WriteHardwareDebugRegs(eDREGTypeWATCH);
+#else
+  return llvm::error::success();
+#endif
+}
+
 #endif // defined (__aarch64__)

lldb/source/Plugins/Process/FreeBSD/NativeRegisterContextFreeBSD_arm64.h

@@ -58,6 +58,8 @@ class NativeRegisterContextFreeBSD_arm64
   llvm::Error
   CopyHardwareWatchpointsFrom(NativeRegisterContextFreeBSD &source) override;
 
+  llvm::Error ClearDBRegs() override;
+
 private:
   // Due to alignment, FreeBSD reg/fpreg are a few bytes larger than
   // LLDB's GPR/FPU structs.  However, all fields have matching offsets

lldb/source/Plugins/Process/FreeBSD/NativeRegisterContextFreeBSD_x86_64.cpp

@@ -653,4 +653,10 @@ NativeRegisterContextFreeBSD_x86_64::GetYMMSplitReg(uint32_t reg) {
   return YMMSplitPtr{&fpreg->sv_xmm[reg_index], &ymmreg[reg_index]};
 }
 
+llvm::Error NativeRegisterContextFreeBSD_x86_64::ClearDBRegs() {
+  uint64_t zero = 0;
+  RegisterValue dr7{zero};
+  return WriteRegister(GetDR(7), dr7).ToError();
+}
+
 #endif // defined(__x86_64__)

lldb/source/Plugins/Process/FreeBSD/NativeRegisterContextFreeBSD_x86_64.h

@@ -55,6 +55,8 @@ class NativeRegisterContextFreeBSD_x86_64
   llvm::Error
   CopyHardwareWatchpointsFrom(NativeRegisterContextFreeBSD &source) override;
 
+  llvm::Error ClearDBRegs() override;
+
 private:
   // Private member types.
   enum RegSetKind {