1 files changed, 152 insertions, 0 deletions
diff --git a/src/basic/sigbus.c b/src/basic/sigbus.c
new file mode 100644
index 0000000..47ab0b8
--- /dev/null
+++ b/src/basic/sigbus.c
@@ -0,0 +1,152 @@
+/* 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;
+}