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/* SPDX-License-Identifier: LGPL-2.1-or-later */
#include <errno.h>
#include <signal.h>
#include <stddef.h>
#include <sys/mman.h>
#include "macro.h"
#include "memory-util.h"
#include "missing_syscall.h"
#include "process-util.h"
#include "sigbus.h"
#include "signal-util.h"
#define SIGBUS_QUEUE_MAX 64
static struct sigaction old_sigaction;
static unsigned n_installed = 0;
/* We maintain a fixed size list of page addresses that triggered a
SIGBUS. We access with list with atomic operations, so that we
don't have to deal with locks between signal handler and main
programs in possibly multiple threads. */
static void* volatile sigbus_queue[SIGBUS_QUEUE_MAX];
static volatile sig_atomic_t n_sigbus_queue = 0;
static void sigbus_push(void *addr) {
assert(addr);
/* Find a free place, increase the number of entries and leave, if we can */
for (size_t u = 0; u < SIGBUS_QUEUE_MAX; u++) {
/* OK to initialize this here since we haven't started the atomic ops yet */
void *tmp = NULL;
if (__atomic_compare_exchange_n(&sigbus_queue[u], &tmp, addr, false,
__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)) {
__atomic_fetch_add(&n_sigbus_queue, 1, __ATOMIC_SEQ_CST);
return;
}
}
/* If we can't, make sure the queue size is out of bounds, to
* mark it as overflowed */
for (;;) {
sig_atomic_t c;
__atomic_thread_fence(__ATOMIC_SEQ_CST);
c = n_sigbus_queue;
if (c > SIGBUS_QUEUE_MAX) /* already overflowed */
return;
/* OK if we clobber c here, since we either immediately return
* or it will be immediately reinitialized on next loop */
if (__atomic_compare_exchange_n(&n_sigbus_queue, &c, c + SIGBUS_QUEUE_MAX, false,
__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))
return;
}
}
int sigbus_pop(void **ret) {
assert(ret);
for (;;) {
unsigned u, c;
__atomic_thread_fence(__ATOMIC_SEQ_CST);
c = n_sigbus_queue;
if (_likely_(c == 0))
return 0;
if (_unlikely_(c > SIGBUS_QUEUE_MAX))
return -EOVERFLOW;
for (u = 0; u < SIGBUS_QUEUE_MAX; u++) {
void *addr;
addr = sigbus_queue[u];
if (!addr)
continue;
/* OK if we clobber addr here, since we either immediately return
* or it will be immediately reinitialized on next loop */
if (__atomic_compare_exchange_n(&sigbus_queue[u], &addr, NULL, false,
__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)) {
__atomic_fetch_sub(&n_sigbus_queue, 1, __ATOMIC_SEQ_CST);
/* If we successfully entered this if condition, addr won't
* have been modified since its assignment, so safe to use it */
*ret = addr;
return 1;
}
}
}
}
static void sigbus_handler(int sn, siginfo_t *si, void *data) {
unsigned long ul;
void *aligned;
assert(sn == SIGBUS);
assert(si);
if (si->si_code != BUS_ADRERR || !si->si_addr) {
assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
propagate_signal(sn, si);
return;
}
ul = (unsigned long) si->si_addr;
ul = ul / page_size();
ul = ul * page_size();
aligned = (void*) ul;
/* Let's remember which address failed */
sigbus_push(aligned);
/* Replace mapping with an anonymous page, so that the
* execution can continue, however with a zeroed out page */
assert_se(mmap(aligned, page_size(), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED, -1, 0) == aligned);
}
void sigbus_install(void) {
struct sigaction sa = {
.sa_sigaction = sigbus_handler,
.sa_flags = SA_SIGINFO,
};
/* make sure that sysconf() is not called from a signal handler because
* it is not guaranteed to be async-signal-safe since POSIX.1-2008 */
(void) page_size();
n_installed++;
if (n_installed == 1)
assert_se(sigaction(SIGBUS, &sa, &old_sigaction) == 0);
return;
}
void sigbus_reset(void) {
if (n_installed <= 0)
return;
n_installed--;
if (n_installed == 0)
assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
return;
}
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