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Tailcall VM interrupt crash #21922

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7c26c21
Add VM interrupt tailcall helper reproducer
morrisonlevi Apr 30, 2026
914efeb
Fix tailcall helper dispatch after VM interrupt
morrisonlevi Apr 30, 2026
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3 changes: 3 additions & 0 deletions Zend/zend_vm_execute.h
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3 changes: 3 additions & 0 deletions Zend/zend_vm_gen.php
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Original file line number Diff line number Diff line change
Expand Up @@ -2136,6 +2136,9 @@ function gen_executor($f, $skl, $spec, $kind, $executor_name, $initializer_name)
out($f,"# define ZEND_VM_DISPATCH_TO_HELPER(call) \\\n");
out($f," do { \\\n");
out($f," opline = call; \\\n");
out($f," if (UNEXPECTED(((uintptr_t)opline & ZEND_VM_ENTER_BIT))) { \\\n");
out($f," return opline; \\\n");
out($f," } \\\n");
out($f," ZEND_VM_TAIL_CALL(opline->handler(ZEND_OPCODE_HANDLER_ARGS_PASSTHRU)); \\\n");
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@bwoebi bwoebi Apr 30, 2026

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the alternative would be returning instead of tail-calling here. But that might be slower.

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@bwoebi bwoebi Apr 30, 2026

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Any thoughts @arnaud-lb ?

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@arnaud-lb arnaud-lb May 2, 2026 •
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Yes this would work. Possibly this may result in worse branch prediction as we revert to the main loop's central dispatch point.

Another alternative would be to make ZEND_VM_INTERRUPT() return an opline whose handler is zend_interrupt_helper, like we do for halt_op in the HYBRID and TAILCALL VMs: https://github.com/php/php-src/compare/arnaud-lb:vm-interrupt-tailcall-repro-3. This probably requires special handling in zend_jit_trace_execute(), I haven't investigated.

I've benchmarked the 3 approaches:

  1. check ENTER_BIT
  2. return opline
  3. return interrupt_op

Results:

Symfony:

base:  mean:  0.4459;  stddev:  0.0004;  diff:  -0.00%                       
1   :  mean:  0.4439;  stddev:  0.0004;  diff:  -0.45%;  p-value:  0.001000  (strong)
2   :  mean:  0.4471;  stddev:  0.0003;  diff:  +0.25%;  p-value:  0.001000  (strong)
3   :  mean:  0.4436;  stddev:  0.0003;  diff:  -0.52%;  p-value:  0.001000  (strong)

Symfony (valgrind):

base:  diff:  +0.00%
1   :  diff:  +0.12%
2   :  diff:  +0.01%
3   :  diff:  -0.00%

bench.php:

base:  mean:  0.8044;  stddev:  0.0004;  diff:  -0.00%                       
1   :  mean:  0.8161;  stddev:  0.0005;  diff:  +1.45%;  p-value:  0.001000  (strong)
2   :  mean:  0.8142;  stddev:  0.0016;  diff:  +1.22%;  p-value:  0.001000  (strong)
3   :  mean:  0.8043;  stddev:  0.0006;  diff:  -0.01%;  p-value:  0.690382  (weak)

bench.php (valgrind):

base:  diff:  +0.00%
1   :  diff:  +1.03%
2   :  diff:  -0.50%
3   :  diff:  -0.00%

The Symfony results do not make sense to me, but these are stable.

Approach 3 seems better overall, speed-wise, assuming that we don't find issues with it.

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@bwoebi bwoebi May 2, 2026 •
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Uh, that's a nice approach! Love it.

I don't see any issues, that approach will never have &call_interrupt_op show up in EX(opline) or the VM IP register so that's perfect.
I have no idea about the JIT code, so, I'll leave that to you :-D

out($f," } while (0)\n");
out($f,"# define ZEND_VM_DISPATCH_TO_LEAVE_HELPER(helper) opline = &call_leave_op; SAVE_OPLINE(); ZEND_VM_CONTINUE()\n");
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43 changes: 43 additions & 0 deletions ext/zend_test/object_handlers.c
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Expand Up @@ -232,6 +232,44 @@ ZEND_METHOD(NumericCastableNoOperations, __construct)
ZVAL_COPY(OBJ_PROP_NUM(Z_OBJ_P(ZEND_THIS), 0), n);
}

static zend_class_entry *vm_interrupt_comparable_ce;
static zend_object_handlers vm_interrupt_comparable_object_handlers;

static zend_object* vm_interrupt_comparable_object_create_ex(zend_class_entry* ce, zend_long l) {
zend_object *obj = zend_objects_new(ce);
object_properties_init(obj, ce);
obj->handlers = &vm_interrupt_comparable_object_handlers;
ZVAL_LONG(OBJ_PROP_NUM(obj, 0), l);
return obj;
}

static zend_object *vm_interrupt_comparable_object_create(zend_class_entry *ce)
{
return vm_interrupt_comparable_object_create_ex(ce, 0);
}

static int vm_interrupt_comparable_compare(zval *op1, zval *op2)
{
ZEND_COMPARE_OBJECTS_FALLBACK(op1, op2);

zend_atomic_bool_store_ex(&EG(vm_interrupt), true);

return ZEND_THREEWAY_COMPARE(
Z_LVAL_P(OBJ_PROP_NUM(Z_OBJ_P(op1), 0)),
Z_LVAL_P(OBJ_PROP_NUM(Z_OBJ_P(op2), 0)));
}

ZEND_METHOD(VmInterruptComparable, __construct)
{
zend_long l;

ZEND_PARSE_PARAMETERS_START(1, 1)
Z_PARAM_LONG(l)
ZEND_PARSE_PARAMETERS_END();

ZVAL_LONG(OBJ_PROP_NUM(Z_OBJ_P(ZEND_THIS), 0), l);
}

static zend_class_entry *dimension_handlers_no_ArrayAccess_ce;
static zend_object_handlers dimension_handlers_no_ArrayAccess_object_handlers;

Expand Down Expand Up @@ -302,6 +340,11 @@ void zend_test_object_handlers_init(void)
memcpy(&numeric_castable_no_operation_object_handlers, &std_object_handlers, sizeof(zend_object_handlers));
numeric_castable_no_operation_object_handlers.cast_object = numeric_castable_no_operation_cast_object;

vm_interrupt_comparable_ce = register_class_VmInterruptComparable();
vm_interrupt_comparable_ce->create_object = vm_interrupt_comparable_object_create;
memcpy(&vm_interrupt_comparable_object_handlers, &std_object_handlers, sizeof(zend_object_handlers));
vm_interrupt_comparable_object_handlers.compare = vm_interrupt_comparable_compare;

dimension_handlers_no_ArrayAccess_ce = register_class_DimensionHandlersNoArrayAccess();
dimension_handlers_no_ArrayAccess_ce->create_object = dimension_handlers_no_ArrayAccess_object_create;
memcpy(&dimension_handlers_no_ArrayAccess_object_handlers, &std_object_handlers, sizeof(zend_object_handlers));
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5 changes: 5 additions & 0 deletions ext/zend_test/object_handlers.stub.php
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Expand Up @@ -23,6 +23,11 @@ final class NumericCastableNoOperations {
public function __construct(int|float $val) {}
}

final class VmInterruptComparable {
private int $val;
public function __construct(int $val) {}
}

class DimensionHandlersNoArrayAccess {
public bool $read = false;
public bool $write = false;
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26 changes: 25 additions & 1 deletion ext/zend_test/object_handlers_arginfo.h
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26 changes: 26 additions & 0 deletions ext/zend_test/tests/observer_vm_interrupt_tailcall_helper.phpt
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@@ -0,0 +1,26 @@
--TEST--
Observer: VM interrupt during tailcall helper dispatch
--DESCRIPTION--
This exercises a VM interrupt raised while an opcode handler dispatches to an
extra-argument helper. On the tailcall VM, the helper may return an opline
tagged with ZEND_VM_ENTER_BIT; treating that tagged value as a zend_op * before
tailcalling the next handler can crash.
--EXTENSIONS--
zend_test
--INI--
opcache.jit=0
zend_test.observer.set_vm_interrupt_on_begin=1
--FILE--
<?php
function trigger(VmInterruptComparable $left, VmInterruptComparable $right): object
{
if ($left < $right) {
return new Exception();
}
return new stdClass();
}

echo get_class(trigger(new VmInterruptComparable(2), new VmInterruptComparable(1))), "\n";
?>
--EXPECT--
stdClass
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