[Lldb-commits] [lldb] 87fadb3 - [lldb] Correctly annotate threads at a bp site as hitting it (#82709)

[via lldb-commits]

Author: Jason Molenda
Date: 2024-02-23T14:45:22-08:00
New Revision: 87fadb3929163752f650a1fc08d5fb13ee4c1a3f

URL: https://github.com/llvm/llvm-project/commit/87fadb3929163752f650a1fc08d5fb13ee4c1a3f
DIFF: https://github.com/llvm/llvm-project/commit/87fadb3929163752f650a1fc08d5fb13ee4c1a3f.diff

LOG: [lldb] Correctly annotate threads at a bp site as hitting it (#82709)

This is next in my series of "fix the racey tests that fail on
greendragon" addressing the failure of TestConcurrentManyBreakpoints.py
where we set a breakpoint in a function that 100 threads execute, and we
check that we hit the breakpoint 100 times. But sometimes it is only hit
99 times, and the test fails.

When we hit a software breakpoint, the pc value for the thread is the
address of the breakpoint instruction - as if it had not been hit yet.
And because a user might ADD a breakpoint for the current pc from the
commandline, when we go to resume execution, any thread that is sitting
at a breakpoint site will be silently advanced past the breakpoint
instruction (disable bp, instruction step that thread, re-enable bp)
before resuming -- whether that thread has hit its breakpoint or not.

What this test is exposing is that there is another corner case, a
thread that is sitting at a breakpoint site but has not yet executed the
breakpoint instruction. The thread will have no stop reason, no mach
exception, so it will not be recorded as having hit the breakpoint
(because it hasn't yet). But when we resume execution, because it is
sitting at a breakpoint site, we advance past it and miss the breakpoint
hit.

In 2016 Abhishek Aggarwal handled a similar issue with a patch in
`ProcessGDBRemote::SetThreadStopInfo()`, adding a breakpoint StopInfo
for a thread sitting at a breakpoint site that has no stop reason.
debugserver's `jThreadsInfo` would not correctly execute Abhishek's code
though because it would respond with `"reason":"none"` for a thread with
no stop reason, and `SetThreadStopInfo()` expected an empty reason here.
The first part of my patch is to clear the `reason` if it is `"none"` so
we flow through the code correctly.

On Darwin, though, our stop reply packet (Txx...) includes the
`threads`, `thread-pcs`, and `jstopinfo` keys, which give us the tids
for all current threads, the pc values for those threads, and
`jstopinfo` has a JSON dictionary with the mach exceptions for all
threads that have a mach exception. In
`ProcessGDBRemote::CalculateThreadStopInfo()` we set the StopInfo for
each thread for a private stop and if we have `jstopinfo` it is the
source of all the StopInfos. I have to add the same logic here, to give
the thread a breakpoint StopInfo even though it hasn't executed the
breakpoint yet. In this case we are very early in thread construction
and I only have the information in the Txx stop reply packet -- tids,
pcs, and jstopinfo, so I can't use the normal general mechanisms of
going through the RegisterContext to get the pc, it's a bit different.

If I hack debugserver to not issue `jstopinfo`,
`CalculateThreadStopInfo` will fall back to sending `qThreadStopInfo`
for each thread and going through
`ProcessGDBRemote::SetThreadStopInfo()` to set the stop infos (and with
the `reason:none` fix, use Abhishek's code).

rdar://110549165

Added: 
    

Modified: 
    lldb/include/lldb/Target/Thread.h
    lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

Removed: 
    


################################################################################
diff  --git a/lldb/include/lldb/Target/Thread.h b/lldb/include/lldb/Target/Thread.h
index 96ca95ad233ff7..1efef93b17ded8 100644
--- a/lldb/include/lldb/Target/Thread.h
+++ b/lldb/include/lldb/Target/Thread.h
@@ -1163,13 +1163,20 @@ class Thread : public std::enable_shared_from_this<Thread>,
 
   void CalculatePublicStopInfo();
 
-  // Ask the thread subclass to set its stop info.
-  //
-  // Thread subclasses should call Thread::SetStopInfo(...) with the reason the
-  // thread stopped.
-  //
-  // \return
-  //      True if Thread::SetStopInfo(...) was called, false otherwise.
+  /// Ask the thread subclass to set its stop info.
+  ///
+  /// Thread subclasses should call Thread::SetStopInfo(...) with the reason the
+  /// thread stopped.
+  ///
+  /// A thread that is sitting at a breakpoint site, but has not yet executed
+  /// the breakpoint instruction, should have a breakpoint-hit StopInfo set.
+  /// When execution is resumed, any thread sitting at a breakpoint site will
+  /// instruction-step over the breakpoint instruction silently, and we will
+  /// never record this breakpoint as being hit, updating the hit count,
+  /// possibly executing breakpoint commands or conditions.
+  ///
+  /// \return
+  ///      True if Thread::SetStopInfo(...) was called, false otherwise.
   virtual bool CalculateStopInfo() = 0;
 
   // Gets the temporary resume state for a thread.

diff  --git a/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp b/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
index 629b191f3117aa..3dc40ee6bb9ffc 100644
--- a/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
@@ -1600,6 +1600,26 @@ bool ProcessGDBRemote::CalculateThreadStopInfo(ThreadGDBRemote *thread) {
     // has no stop reason.
     thread->GetRegisterContext()->InvalidateIfNeeded(true);
     if (!GetThreadStopInfoFromJSON(thread, m_jstopinfo_sp)) {
+      // If a thread is stopped at a breakpoint site, set that as the stop
+      // reason even if it hasn't executed the breakpoint instruction yet.
+      // We will silently step over the breakpoint when we resume execution
+      // and miss the fact that this thread hit the breakpoint.
+      const size_t num_thread_ids = m_thread_ids.size();
+      for (size_t i = 0; i < num_thread_ids; i++) {
+        if (m_thread_ids[i] == thread->GetID() && m_thread_pcs.size() > i) {
+          addr_t pc = m_thread_pcs[i];
+          lldb::BreakpointSiteSP bp_site_sp =
+              thread->GetProcess()->GetBreakpointSiteList().FindByAddress(pc);
+          if (bp_site_sp) {
+            if (bp_site_sp->ValidForThisThread(*thread)) {
+              thread->SetStopInfo(
+                  StopInfo::CreateStopReasonWithBreakpointSiteID(
+                      *thread, bp_site_sp->GetID()));
+              return true;
+            }
+          }
+        }
+      }
       thread->SetStopInfo(StopInfoSP());
     }
     return true;
@@ -1722,7 +1742,9 @@ ThreadSP ProcessGDBRemote::SetThreadStopInfo(
     } else {
       bool handled = false;
       bool did_exec = false;
-      if (!reason.empty()) {
+      // debugserver can send reason = "none" which is equivalent
+      // to no reason.
+      if (!reason.empty() && reason != "none") {
         if (reason == "trace") {
           addr_t pc = thread_sp->GetRegisterContext()->GetPC();
           lldb::BreakpointSiteSP bp_site_sp =
@@ -1864,11 +1886,10 @@ ThreadSP ProcessGDBRemote::SetThreadStopInfo(
         lldb::BreakpointSiteSP bp_site_sp =
             thread_sp->GetProcess()->GetBreakpointSiteList().FindByAddress(pc);
 
-        // If the current pc is a breakpoint site then the StopInfo should be
-        // set to Breakpoint even though the remote stub did not set it as such.
-        // This can happen when the thread is involuntarily interrupted (e.g.
-        // due to stops on other threads) just as it is about to execute the
-        // breakpoint instruction.
+        // If a thread is stopped at a breakpoint site, set that as the stop
+        // reason even if it hasn't executed the breakpoint instruction yet.
+        // We will silently step over the breakpoint when we resume execution
+        // and miss the fact that this thread hit the breakpoint.
         if (bp_site_sp && bp_site_sp->ValidForThisThread(*thread_sp)) {
           thread_sp->SetStopInfo(StopInfo::CreateStopReasonWithBreakpointSiteID(
               *thread_sp, bp_site_sp->GetID()));