journald: process SIGBUS for the memory maps we set up

[elogind.git] / src / shared / sigbus.c

diff --git a/src/shared/sigbus.c b/src/shared/sigbus.c
new file mode 100644 (file)
index 0000000..0108603
--- /dev/null
+++ b/src/shared/sigbus.c
@@ -0,0 +1,152 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <signal.h>
+#include <sys/mman.h>
+
+#include "macro.h"
+#include "util.h"
+#include "sigbus.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) {
+        unsigned u;
+
+        assert(addr);
+
+        /* Find a free place, increase the number of entries and leave, if we can */
+        for (u = 0; u < SIGBUS_QUEUE_MAX; u++)
+                if (__sync_bool_compare_and_swap(&sigbus_queue[u], NULL, addr)) {
+                        __sync_fetch_and_add(&n_sigbus_queue, 1);
+                        return;
+                }
+
+        /* If we can't, make sure the queue size is out of bounds, to
+         * mark it as overflow */
+        for (;;) {
+                unsigned c;
+
+                __sync_synchronize();
+                c = n_sigbus_queue;
+
+                if (c > SIGBUS_QUEUE_MAX) /* already overflow */
+                        return;
+
+                if (__sync_bool_compare_and_swap(&n_sigbus_queue, c, c + SIGBUS_QUEUE_MAX))
+                        return;
+        }
+}
+
+int sigbus_pop(void **ret) {
+        assert(ret);
+
+        for (;;) {
+                unsigned u, c;
+
+                __sync_synchronize();
+                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;
+
+                        if (__sync_bool_compare_and_swap(&sigbus_queue[u], addr, NULL)) {
+                                __sync_fetch_and_sub(&n_sigbus_queue, 1);
+                                *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);
+                raise(SIGBUS);
+                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,
+        };
+
+        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;
+}