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experiment: detect re-entrancy in ddtrace_serialize_closed_spans #3562

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experiment: detect re-entrancy in ddtrace_serialize_closed_spans #3562

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experiment: detect re-entrancy in ddtrace_serialize_closed_spans
morrisonlevi Jan 11, 2026
cbafd8a
Merge branch 'master' into levi/reentrancy
morrisonlevi Jan 12, 2026
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20 changes: 20 additions & 0 deletions ext/span.c
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@
#include "telemetry.h"
#include "ext/standard/php_string.h"
#include <hook/hook.h>
#include <signal.h>
#include "user_request.h"
#include "zend_types.h"
#include "sidecar.h"
Expand All @@ -37,6 +38,14 @@ static void dd_reset_span_counters(void) {
DDTRACE_G(closed_spans_count) = 0;
}

static zend_never_inline ZEND_COLD void ddtrace_crash_serialization_reentrancy(const char *where) {
LOG(ERROR, "Detected ddtrace serialization re-entrancy at %s; crashing intentionally for diagnosis", where);
/* Crash in a way that triggers crashtracking (SIGSEGV handler), not SIGABRT. */
raise(SIGSEGV);
/* If SIGSEGV is blocked or no handler is installed for some reason, hard-exit. */
_Exit(128 + SIGSEGV);
}

void ddtrace_init_span_stacks(void) {
DDTRACE_G(top_closed_stack) = NULL;
dd_reset_span_counters();
Expand Down Expand Up @@ -1091,6 +1100,15 @@ void ddtrace_drop_span(ddtrace_span_data *span) {
}

void ddtrace_serialize_closed_spans(ddog_TracesBytes *traces, bool fast_shutdown) {
// Prevent re-entrancy: serialization can be triggered from userland (explicit flush),
// from close paths (auto-flush), and indirectly via destructors/GC.
// Nested serialization risks use-after-free via iterator invalidation across many call sites.
static ZEND_TLS bool ddtrace_in_serialization = false;
if (ddtrace_in_serialization) {
ddtrace_crash_serialization_reentrancy("ddtrace_serialize_closed_spans");
}
ddtrace_in_serialization = true;

if (DDTRACE_G(top_closed_stack)) {
ddtrace_span_stack *rootstack = DDTRACE_G(top_closed_stack);
DDTRACE_G(top_closed_stack) = NULL;
Expand Down Expand Up @@ -1138,6 +1156,8 @@ void ddtrace_serialize_closed_spans(ddog_TracesBytes *traces, bool fast_shutdown
// Reset closed span counter for limit-refresh, don't touch open spans
DDTRACE_G(closed_spans_count) = 0;
DDTRACE_G(dropped_spans_count) = 0;

ddtrace_in_serialization = false;
}

void ddtrace_serialize_closed_spans_with_cycle(ddog_TracesBytes *traces, bool fast_shutdown) {
Expand Down
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