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Make the SpanStack happier with cloning operations #3976

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38 changes: 38 additions & 0 deletions tests/ext/span_stack/span_stack_clone_dropped_root.phpt
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Original file line number Diff line number Diff line change
@@ -0,0 +1,38 @@
--TEST--
Cloning a span stack must not leave a dangling root_span alias when the root span is dropped (#3943)
--ENV--
DD_TRACE_AUTO_FLUSH_ENABLED=0
DD_TRACE_GENERATE_ROOT_SPAN=1
DD_CODE_ORIGIN_FOR_SPANS_ENABLED=0
--FILE--
<?php

// A populated baggage array on the root span is what turned the dangling root_span read into the
// observed GC_ADDREF crash; keep it so this exercises the exact path.
\DDTrace\root_span()->baggage["k"] = "v";

// Open a sub-trace, capture its stack, then close that sub-trace's own root span. The sub-stack now
// has no active span and no root span of its own.
\DDTrace\start_trace_span();
$sub = \DDTrace\active_stack();
\DDTrace\close_span();

// Cloning must not copy the parent stack's root_span verbatim as a non-owning alias: the clone holds
// no reference to that root span, so the pointer would dangle once it is dropped below.
$clone = clone $sub;

// Drop the (auto) root span via the runtime config change.
ini_set("datadog.trace.generate_root_span", "0");

// Before the fix, starting a new trace whose parent stack is the clone dereferenced the freed root
// span in ddtrace_set_root_span_properties -> ddtrace_inherit_span_properties (heap-use-after-free).
\DDTrace\switch_stack($clone);
\DDTrace\start_trace_span()->name = "after clone";

echo "completed without use-after-free\n";
var_dump(\DDTrace\active_span()->name);

?>
--EXPECT--
completed without use-after-free
string(11) "after clone"
11 changes: 7 additions & 4 deletions tracer/functions.c
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Original file line number Diff line number Diff line change
Expand Up @@ -394,7 +394,6 @@ static zend_object *ddtrace_span_stack_clone_obj(zend_object *old_obj) {
ddtrace_span_stack *oldstack = (ddtrace_span_stack *)old_obj;
if (oldstack->parent_stack) { // if this is false, we're copying an initial stack
stack->root_stack = stack->parent_stack->root_stack;
stack->root_span = stack->parent_stack->root_span;
}
if (oldstack->root_stack == oldstack) {
stack->root_stack = stack;
Expand All @@ -407,10 +406,9 @@ static zend_object *ddtrace_span_stack_clone_obj(zend_object *old_obj) {
}
if (pspan) {
ZVAL_OBJ_COPY(&stack->property_active, &pspan->std);
stack->root_span = SPANDATA(pspan)->root;
} else {
if (oldstack->root_span && oldstack->root_span->stack == oldstack) {
stack->root_span = NULL;
}
stack->root_span = NULL;
stack->active = NULL;
ZVAL_NULL(&stack->property_active);
}
Expand Down Expand Up @@ -2398,6 +2396,11 @@ PHP_FUNCTION(DDTrace_try_drop_span) {
RETURN_FALSE;
}

// Assert span stack for manual instantiations of SpanStack/clones.
if (!span->stack || span->stack->active != &span->props) {
RETURN_FALSE;
}

bool on_active_stack = active_stack == span->stack;
if (!on_active_stack) {
GC_ADDREF(&active_stack->std);
Expand Down
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