journald: process SIGBUS for the memory maps we set up

Even though we use fallocate() it appears that file systems like btrfs
will trigger SIGBUS on certain low-disk-space situation. We should
handle that, hence catch the signal, add it to a list of invalidated
pages, and replace the page with an empty memory area. After each write
check if SIGBUS was triggered, and consider the write invalid if it was.

This should make journald a lot more robust with file systems where
fallocate() is not reliable, for example all CoW file systems
(btrfs...), where changing written data can fail with disk full errors.

https://bugzilla.redhat.com/show_bug.cgi?id=1045810

2 files changed, 177 insertions, 0 deletions

diff --git a/src/shared/sigbus.c b/src/shared/sigbus.c
new file mode 100644
index 0000000000..0108603fe8
--- /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;
+}
diff --git a/src/shared/sigbus.h b/src/shared/sigbus.h
new file mode 100644
index 0000000000..25593af2d4
--- /dev/null
+++ b/src/shared/sigbus.h
@@ -0,0 +1,25 @@
+/*-*- 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/>.
+***/
+
+void sigbus_install(void);
+void sigbus_reset(void);
+
+int sigbus_pop(void **ret);