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bpo-37851: faulthandler allocates its stack on demand #15358

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Merged
vstinner merged 1 commit into from
Aug 21, 2019
Merged

bpo-37851: faulthandler allocates its stack on demand #15358

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2019-08-21-13-43-04.bpo-37851.mIIfD_.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
The :mod:`faulthandler` module no longer allocates its alternative stack at
Python startup. Now the stack is only allocated at the first faulthandler
usage.
146 changes: 98 additions & 48 deletions Modules/faulthandler.c
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Original file line number Diff line number Diff line change
Expand Up @@ -125,7 +125,13 @@ static fault_handler_t faulthandler_handlers[] = {
static const size_t faulthandler_nsignals = \
Py_ARRAY_LENGTH(faulthandler_handlers);

#ifdef HAVE_SIGALTSTACK
/* Using an alternative stack requires sigaltstack()
and sigaction() SA_ONSTACK */
#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
# define FAULTHANDLER_USE_ALT_STACK
#endif

#ifdef FAULTHANDLER_USE_ALT_STACK
static stack_t stack;
static stack_t old_stack;
#endif
Expand Down Expand Up @@ -427,6 +433,36 @@ faulthandler_exc_handler(struct _EXCEPTION_POINTERS *exc_info)
}
#endif


#ifdef FAULTHANDLER_USE_ALT_STACK
static int
faulthandler_allocate_stack(void)
{
if (stack.ss_sp != NULL) {
return 0;
}
/* Allocate an alternate stack for faulthandler() signal handler
to be able to execute a signal handler on a stack overflow error */
stack.ss_sp = PyMem_Malloc(stack.ss_size);
if (stack.ss_sp == NULL) {
PyErr_NoMemory();
return -1;
}

int err = sigaltstack(&stack, &old_stack);
if (err) {
/* Release the stack to retry sigaltstack() next time */
PyMem_Free(stack.ss_sp);
stack.ss_sp = NULL;

PyErr_SetFromErrno(PyExc_OSError);
return -1;
}
return 0;
}
#endif


/* Install the handler for fatal signals, faulthandler_fatal_error(). */

static int
Expand All @@ -437,32 +473,35 @@ faulthandler_enable(void)
}
fatal_error.enabled = 1;

#ifdef FAULTHANDLER_USE_ALT_STACK
if (faulthandler_allocate_stack() < 0) {
return -1;
}
#endif

for (size_t i=0; i < faulthandler_nsignals; i++) {
fault_handler_t *handler;
#ifdef HAVE_SIGACTION
struct sigaction action;
#endif
int err;

handler = &faulthandler_handlers[i];
assert(!handler->enabled);
#ifdef HAVE_SIGACTION
struct sigaction action;
action.sa_handler = faulthandler_fatal_error;
sigemptyset(&action.sa_mask);
/* Do not prevent the signal from being received from within
its own signal handler */
action.sa_flags = SA_NODEFER;
#ifdef HAVE_SIGALTSTACK
if (stack.ss_sp != NULL) {
/* Call the signal handler on an alternate signal stack
provided by sigaltstack() */
action.sa_flags |= SA_ONSTACK;
}
#ifdef FAULTHANDLER_USE_ALT_STACK
assert(stack.ss_sp != NULL);
/* Call the signal handler on an alternate signal stack
provided by sigaltstack() */
action.sa_flags |= SA_ONSTACK;
#endif
err = sigaction(handler->signum, &action, &handler->previous);
#else
handler->previous = signal(handler->signum,
faulthandler_fatal_error);
faulthandler_fatal_error);
err = (handler->previous == SIG_ERR);
#endif
if (err) {
Expand Down Expand Up @@ -676,17 +715,37 @@ faulthandler_dump_traceback_later(PyObject *self,
}

tstate = get_thread_state();
if (tstate == NULL)
if (tstate == NULL) {
return NULL;
}

fd = faulthandler_get_fileno(&file);
if (fd < 0)
if (fd < 0) {
return NULL;
}

if (!thread.running) {
thread.running = PyThread_allocate_lock();
if (!thread.running) {
return PyErr_NoMemory();
}
}
if (!thread.cancel_event) {
thread.cancel_event = PyThread_allocate_lock();
if (!thread.cancel_event || !thread.running) {
return PyErr_NoMemory();
}

/* cancel_event starts to be acquired: it's only released to cancel
the thread. */
PyThread_acquire_lock(thread.cancel_event, 1);
}

/* format the timeout */
header = format_timeout(timeout_us);
if (header == NULL)
if (header == NULL) {
return PyErr_NoMemory();
}
header_len = strlen(header);

/* Cancel previous thread, if running */
Expand Down Expand Up @@ -728,9 +787,10 @@ faulthandler_cancel_dump_traceback_later_py(PyObject *self,
}
#endif /* FAULTHANDLER_LATER */


#ifdef FAULTHANDLER_USER
static int
faulthandler_register(int signum, int chain, _Py_sighandler_t *p_previous)
faulthandler_register(int signum, int chain, _Py_sighandler_t *previous_p)
{
#ifdef HAVE_SIGACTION
struct sigaction action;
Expand All @@ -745,19 +805,19 @@ faulthandler_register(int signum, int chain, _Py_sighandler_t *p_previous)
own signal handler */
action.sa_flags = SA_NODEFER;
}
#ifdef HAVE_SIGALTSTACK
if (stack.ss_sp != NULL) {
/* Call the signal handler on an alternate signal stack
provided by sigaltstack() */
action.sa_flags |= SA_ONSTACK;
}
#ifdef FAULTHANDLER_USE_ALT_STACK
assert(stack.ss_sp != NULL);
/* Call the signal handler on an alternate signal stack
provided by sigaltstack() */
action.sa_flags |= SA_ONSTACK;
#endif
return sigaction(signum, &action, p_previous);
return sigaction(signum, &action, previous_p);
#else
_Py_sighandler_t previous;
previous = signal(signum, faulthandler_user);
if (p_previous != NULL)
*p_previous = previous;
if (previous_p != NULL) {
*previous_p = previous;
}
return (previous == SIG_ERR);
#endif
}
Expand Down Expand Up @@ -861,6 +921,12 @@ faulthandler_register_py(PyObject *self,
user = &user_signals[signum];

if (!user->enabled) {
#ifdef FAULTHANDLER_USE_ALT_STACK
if (faulthandler_allocate_stack() < 0) {
return NULL;
}
#endif

err = faulthandler_register(signum, chain, &previous);
if (err) {
PyErr_SetFromErrno(PyExc_OSError);
Expand Down Expand Up @@ -1094,7 +1160,7 @@ faulthandler_fatal_error_py(PyObject *self, PyObject *args)
Py_RETURN_NONE;
}

#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
#if defined(FAULTHANDLER_USE_ALT_STACK)
#define FAULTHANDLER_STACK_OVERFLOW

#ifdef __INTEL_COMPILER
Expand Down Expand Up @@ -1153,7 +1219,7 @@ faulthandler_stack_overflow(PyObject *self, PyObject *Py_UNUSED(ignored))
size, depth);
return NULL;
}
#endif /* defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION) */
#endif /* defined(FAULTHANDLER_USE_ALT_STACK) && defined(HAVE_SIGACTION) */


static int
Expand Down Expand Up @@ -1318,35 +1384,18 @@ faulthandler_init_enable(void)
PyStatus
_PyFaulthandler_Init(int enable)
{
#ifdef HAVE_SIGALTSTACK
int err;

/* Try to allocate an alternate stack for faulthandler() signal handler to
* be able to allocate memory on the stack, even on a stack overflow. If it
* fails, ignore the error. */
#ifdef FAULTHANDLER_USE_ALT_STACK
memset(&stack, 0, sizeof(stack));
stack.ss_flags = 0;
/* bpo-21131: allocate dedicated stack of SIGSTKSZ*2 bytes, instead of just
SIGSTKSZ bytes. Calling the previous signal handler in faulthandler
signal handler uses more than SIGSTKSZ bytes of stack memory on some
platforms. */
stack.ss_size = SIGSTKSZ * 2;
stack.ss_sp = PyMem_Malloc(stack.ss_size);
if (stack.ss_sp != NULL) {
err = sigaltstack(&stack, &old_stack);
if (err) {
PyMem_Free(stack.ss_sp);
stack.ss_sp = NULL;
}
}
#endif

#ifdef FAULTHANDLER_LATER
thread.file = NULL;
thread.cancel_event = PyThread_allocate_lock();
thread.running = PyThread_allocate_lock();
if (!thread.cancel_event || !thread.running) {
return _PyStatus_ERR("failed to allocate locks for faulthandler");
}
PyThread_acquire_lock(thread.cancel_event, 1);
memset(&thread, 0, sizeof(thread));
#endif

if (enable) {
Expand Down Expand Up @@ -1386,7 +1435,8 @@ void _PyFaulthandler_Fini(void)

/* fatal */
faulthandler_disable();
#ifdef HAVE_SIGALTSTACK

#ifdef FAULTHANDLER_USE_ALT_STACK
if (stack.ss_sp != NULL) {
/* Fetch the current alt stack */
stack_t current_stack;
Expand Down