From cec7140211ad6cbb5915bf5035c08c3a78051856 Mon Sep 17 00:00:00 2001
From: Babneet Singh <sbabneet@ca.ibm.com>
Date: Tue, 19 Dec 2023 00:20:55 -0500
Subject: [PATCH] Initialize recycled continuations in createContinuation
 before usage

Currently, recycled continuations are reset and initialized in
freeContinuation at the end of their use. There are also gaps where
the recycled continuation might retain old values and not be properly
reset.

The above approach leads to corruption and segfaults as seen in
the runJavaThread failure reported in #16728.

To fix the above crash, all continuations (new and recycled) are
initialized and reset in createContinuation just before the
continuation begins execution.

Related: #16728

Signed-off-by: Babneet Singh <sbabneet@ca.ibm.com>
---
 runtime/vm/ContinuationHelpers.cpp | 57 ++++++++++++++----------------
 1 file changed, 26 insertions(+), 31 deletions(-)

diff --git a/runtime/vm/ContinuationHelpers.cpp b/runtime/vm/ContinuationHelpers.cpp
index 69c2200897a..8479fcffed7 100644
--- a/runtime/vm/ContinuationHelpers.cpp
+++ b/runtime/vm/ContinuationHelpers.cpp
@@ -81,13 +81,13 @@ createContinuation(J9VMThread *currentThread, j9object_t continuationObject)
 		vm->avgCacheLookupTime += (I_64)j9time_hires_delta(start, j9time_hires_clock(), OMRPORT_TIME_DELTA_IN_NANOSECONDS);
 #endif /* defined(J9VM_PROF_CONTINUATION_ALLOCATION) */
 
+	J9JavaStack *stack = NULL;
+	J9SFJNINativeMethodFrame *frame = NULL;
 	/* No cache found, allocate new continuation structure. */
 	if (NULL == continuation) {
 #if defined(J9VM_PROF_CONTINUATION_ALLOCATION)
 		start = j9time_hires_clock();
 #endif /* defined(J9VM_PROF_CONTINUATION_ALLOCATION) */
-		J9JavaStack *stack = NULL;
-		J9SFJNINativeMethodFrame *frame = NULL;
 		continuation = (J9VMContinuation*)j9mem_allocate_memory(sizeof(J9VMContinuation), OMRMEM_CATEGORY_THREADS);
 		if (NULL == continuation) {
 			vm->internalVMFunctions->setNativeOutOfMemoryError(currentThread, 0, 0);
@@ -95,8 +95,6 @@ createContinuation(J9VMThread *currentThread, j9object_t continuationObject)
 			goto end;
 		}
 
-		memset(continuation, 0, sizeof(J9VMContinuation));
-
 #ifdef J9VM_INTERP_GROWABLE_STACKS
 #define VMTHR_INITIAL_STACK_SIZE ((vm->initialStackSize > (UDATA) vm->stackSize) ? vm->stackSize : vm->initialStackSize)
 #else
@@ -112,22 +110,6 @@ createContinuation(J9VMThread *currentThread, j9object_t continuationObject)
 
 #undef VMTHR_INITIAL_STACK_SIZE
 
-		continuation->stackObject = stack;
-		continuation->stackOverflowMark2 = J9JAVASTACK_STACKOVERFLOWMARK(stack);
-		continuation->stackOverflowMark = continuation->stackOverflowMark2;
-
-		frame = ((J9SFJNINativeMethodFrame*)stack->end) - 1;
-		frame->method = NULL;
-		frame->specialFrameFlags = 0;
-		frame->savedCP = NULL;
-		frame->savedPC = (U_8*)(UDATA)J9SF_FRAME_TYPE_END_OF_STACK;
-		frame->savedA0 = (UDATA*)(UDATA)J9SF_A0_INVISIBLE_TAG;
-		continuation->sp = (UDATA*)frame;
-		continuation->literals = NULL;
-		continuation->pc = (U_8*)J9SF_FRAME_TYPE_JNI_NATIVE_METHOD;
-		continuation->arg0EA = (UDATA*)&frame->savedA0;
-		continuation->stackObject->isVirtual = TRUE;
-
 #if defined(J9VM_PROF_CONTINUATION_ALLOCATION)
 		I_64 totalTime = (I_64)j9time_hires_delta(start, j9time_hires_clock(), OMRPORT_TIME_DELTA_IN_NANOSECONDS);
 		if (totalTime > 10000) {
@@ -138,8 +120,32 @@ createContinuation(J9VMThread *currentThread, j9object_t continuationObject)
 			vm->fastAllocAvgTime += totalTime;
 		}
 #endif /* defined(J9VM_PROF_CONTINUATION_ALLOCATION) */
+	} else {
+		/* Reset and reuse the stack in the recycled continuation. */
+		stack = continuation->stackObject;
+		stack->previous = NULL;
+		stack->firstReferenceFrame = 0;
 	}
 
+	/* Reset all fields in the new or recycled continuation. */
+	memset(continuation, 0, sizeof(J9VMContinuation));
+
+	continuation->stackObject = stack;
+	continuation->stackOverflowMark2 = J9JAVASTACK_STACKOVERFLOWMARK(stack);
+	continuation->stackOverflowMark = continuation->stackOverflowMark2;
+
+	frame = ((J9SFJNINativeMethodFrame*)stack->end) - 1;
+	frame->method = NULL;
+	frame->specialFrameFlags = 0;
+	frame->savedCP = NULL;
+	frame->savedPC = (U_8*)(UDATA)J9SF_FRAME_TYPE_END_OF_STACK;
+	frame->savedA0 = (UDATA*)(UDATA)J9SF_A0_INVISIBLE_TAG;
+	continuation->sp = (UDATA*)frame;
+	continuation->literals = NULL;
+	continuation->pc = (U_8*)J9SF_FRAME_TYPE_JNI_NATIVE_METHOD;
+	continuation->arg0EA = (UDATA*)&frame->savedA0;
+	continuation->stackObject->isVirtual = TRUE;
+
 	J9VMJDKINTERNALVMCONTINUATION_SET_VMREF(currentThread, continuationObject, continuation);
 
 	/* GC Hook to register Continuation object. */
@@ -357,17 +363,6 @@ recycleContinuation(J9JavaVM *vm, J9VMThread *vmThread, J9VMContinuation* contin
 {
 	PORT_ACCESS_FROM_JAVAVM(vm);
 	bool cached = false;
-	/* Prepare continuationState for recycle, and reset stack initial state. */
-	J9SFJNINativeMethodFrame *frame = ((J9SFJNINativeMethodFrame*)continuation->stackObject->end) - 1;
-	frame->method = NULL;
-	frame->specialFrameFlags = 0;
-	frame->savedCP = NULL;
-	frame->savedPC = (U_8*)(UDATA)J9SF_FRAME_TYPE_END_OF_STACK;
-	frame->savedA0 = (UDATA*)(UDATA)J9SF_A0_INVISIBLE_TAG;
-	continuation->sp = (UDATA*)frame;
-	continuation->literals = NULL;
-	continuation->pc = (U_8*)J9SF_FRAME_TYPE_JNI_NATIVE_METHOD;
-	continuation->arg0EA = (UDATA*)&frame->savedA0;
 
 	if (!skipLocalCache && (0 < vm->continuationT1Size)) {
 		/* If called by carrier thread (not global), try to store in local cache first.