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[release/6.0] Fix exception propagation over HW exception frame on ma…
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…cOS arm64 (#64262)

* Fix exception propagation over HW exception frame on macOS arm64

There is a problem unwinding over the PAL_DispatchExceptionWrapper
to the actual hardware exception location. The unwinder is unable
to get distinct LR and PC in that frame and sets both of them to
the same value. This is caused by the fact that the
PAL_DispatchExceptionWrapper is just an injected fake frame and
there was no real call. Calls always return with LR and PC set
to the same value.

The fix unifies the hardware exception frame unwinding with Linux
where we had problems unwinding over signal handler trampoline, so
PAL_VirtualUnwind skips the trampoline and now also the
PAL_DispatchExceptionWrapper frame by copying the context of
the exception as the unwound context.

* Fix GC stress C - wrong context being restored

The context that is restored in the PAL_DispatchException needs to be
the one from the exception, not the original saved one. That ensures
that the registers updated by the GC in GC stress C in the context
are properly restored after the execution is resumed.

* Fix exception context leak in GC stress C

The PAL_SEHException had the records allocated on stack, so the
direct context restoration after the EH for GC stress C completed
leaked those.

Co-authored-by: Jan Vorlicek <janvorli@microsoft.com>
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github-actions[bot] and janvorli authored Feb 7, 2022
1 parent 3b0b34c commit 2caf6fb
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Showing 6 changed files with 90 additions and 25 deletions.
18 changes: 6 additions & 12 deletions src/coreclr/pal/src/exception/machexception.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -369,28 +369,22 @@ void PAL_DispatchException(PCONTEXT pContext, PEXCEPTION_RECORD pExRecord, MachE
{
CPalThread *pThread = InternalGetCurrentThread();

CONTEXT *contextRecord;
EXCEPTION_RECORD *exceptionRecord;
AllocateExceptionRecords(&exceptionRecord, &contextRecord);
CONTEXT *contextRecord = pContext;
g_hardware_exception_context_locvar_offset = (int)((char*)&contextRecord - (char*)__builtin_frame_address(0));

*contextRecord = *pContext;
*exceptionRecord = *pExRecord;

contextRecord->ContextFlags |= CONTEXT_EXCEPTION_ACTIVE;
pContext->ContextFlags |= CONTEXT_EXCEPTION_ACTIVE;
bool continueExecution;

{
// The exception object takes ownership of the exceptionRecord and contextRecord
PAL_SEHException exception(exceptionRecord, contextRecord);
PAL_SEHException exception(pExRecord, pContext, true);

TRACE("PAL_DispatchException(EC %08x EA %p)\n", pExRecord->ExceptionCode, pExRecord->ExceptionAddress);

continueExecution = SEHProcessException(&exception);
if (continueExecution)
{
// Make a copy of the exception records so that we can free them before restoring the context
*pContext = *contextRecord;
*pExRecord = *exceptionRecord;
*pContext = *exception.ExceptionPointers.ContextRecord;
*pExRecord = *exception.ExceptionPointers.ExceptionRecord;
}

// The exception records are destroyed by the PAL_SEHException destructor now.
Expand Down
16 changes: 8 additions & 8 deletions src/coreclr/pal/src/exception/seh-unwind.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -476,7 +476,9 @@ void GetContextPointers(unw_cursor_t *cursor, unw_context_t *unwContext, KNONVOL

#ifndef HOST_WINDOWS

extern int g_common_signal_handler_context_locvar_offset;
// Frame pointer relative offset of a local containing a pointer to the windows style context of a location
// where a hardware exception occured.
int g_hardware_exception_context_locvar_offset = 0;

BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextPointers)
{
Expand All @@ -486,19 +488,17 @@ BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextP

DWORD64 curPc = CONTEXTGetPC(context);

#ifndef __APPLE__
// Check if the PC is the return address from the SEHProcessException in the common_signal_handler.
// If that's the case, extract its local variable containing the windows style context of the hardware
// Check if the PC is the return address from the SEHProcessException.
// If that's the case, extract its local variable containing a pointer to the windows style context of the hardware
// exception and return that. This skips the hardware signal handler trampoline that the libunwind
// cannot cross on some systems.
// cannot cross on some systems. On macOS, it skips a similar trampoline we create in HijackFaultingThread.
if ((void*)curPc == g_SEHProcessExceptionReturnAddress)
{
CONTEXT* signalContext = (CONTEXT*)(CONTEXTGetFP(context) + g_common_signal_handler_context_locvar_offset);
memcpy_s(context, sizeof(CONTEXT), signalContext, sizeof(CONTEXT));
CONTEXT* exceptionContext = *(CONTEXT**)(CONTEXTGetFP(context) + g_hardware_exception_context_locvar_offset);
memcpy_s(context, sizeof(CONTEXT), exceptionContext, sizeof(CONTEXT));

return TRUE;
}
#endif

if ((context->ContextFlags & CONTEXT_EXCEPTION_ACTIVE) != 0)
{
Expand Down
7 changes: 2 additions & 5 deletions src/coreclr/pal/src/exception/signal.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -114,10 +114,6 @@ struct sigaction g_previous_sigabrt;

#if !HAVE_MACH_EXCEPTIONS

// Offset of the local variable containing pointer to windows style context in the common_signal_handler function.
// This offset is relative to the frame pointer.
int g_common_signal_handler_context_locvar_offset = 0;

// TOP of special stack for handling stack overflow
volatile void* g_stackOverflowHandlerStack = NULL;

Expand Down Expand Up @@ -931,11 +927,12 @@ static bool common_signal_handler(int code, siginfo_t *siginfo, void *sigcontext
#if !HAVE_MACH_EXCEPTIONS
sigset_t signal_set;
CONTEXT signalContextRecord;
CONTEXT* signalContextRecordPtr = &signalContextRecord;
EXCEPTION_RECORD exceptionRecord;
native_context_t *ucontext;

ucontext = (native_context_t *)sigcontext;
g_common_signal_handler_context_locvar_offset = (int)((char*)&signalContextRecord - (char*)__builtin_frame_address(0));
g_hardware_exception_context_locvar_offset = (int)((char*)&signalContextRecordPtr - (char*)__builtin_frame_address(0));

if (code == (SIGSEGV | StackOverflowFlag))
{
Expand Down
5 changes: 5 additions & 0 deletions src/coreclr/pal/src/include/pal/seh.hpp
Original file line number Diff line number Diff line change
Expand Up @@ -145,5 +145,10 @@ CorUnix::PAL_ERROR SEHDisable(CorUnix::CPalThread *pthrCurrent);

}

// Offset of the local variable containing pointer to windows style context in the common_signal_handler / PAL_DispatchException function.
// This offset is relative to the frame pointer.
extern int g_hardware_exception_context_locvar_offset;


#endif /* _PAL_SEH_HPP_ */

60 changes: 60 additions & 0 deletions src/tests/Regressions/coreclr/GitHub_62058/test62058.cs
Original file line number Diff line number Diff line change
@@ -0,0 +1,60 @@
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;

public class Program
{
private interface IFoo
{
bool IsValid { get; }
}

private class Foo : IFoo
{
public bool IsValid { get; set; }
}

public static int Main(string[] args)
{
bool warmup = new Foo().IsValid;
CatchIgnore(() =>
CatchRethrow(() =>
{
IFoo[] foos = {new Foo(), null};
foreach (var foo in foos)
{
bool check = foo.IsValid;
}
}));

return 100;
}

public static void CatchRethrow(Action action)
{
try
{
action.Invoke();
}
catch (Exception e)
{
Console.Out.WriteLine("catch");
Console.Out.Flush();
throw new Exception("catch", e);
}
}

public static void CatchIgnore(Action action)
{
try
{
action.Invoke();
}
catch (Exception)
{
Console.Out.WriteLine("ignore");
Console.Out.Flush();
}
}
}
9 changes: 9 additions & 0 deletions src/tests/Regressions/coreclr/GitHub_62058/test62058.csproj
Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<CLRTestPriority>1</CLRTestPriority>
</PropertyGroup>
<ItemGroup>
<Compile Include="test62058.cs" />
</ItemGroup>
</Project>

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