diff options
| -rw-r--r-- | build/prelude.mk | 2 | ||||
| -rw-r--r-- | src/main.c | 1 | ||||
| -rw-r--r-- | src/sighook.c | 600 | ||||
| -rw-r--r-- | src/sighook.h | 73 | ||||
| -rw-r--r-- | tests/main.c | 1 | ||||
| -rw-r--r-- | tests/sighook.c | 97 | ||||
| -rw-r--r-- | tests/tests.h | 3 |
7 files changed, 777 insertions, 0 deletions
diff --git a/build/prelude.mk b/build/prelude.mk index 5be26cb..d0968ea 100644 --- a/build/prelude.mk +++ b/build/prelude.mk @@ -92,6 +92,7 @@ LIBBFS := \ obj/src/parse.o \ obj/src/printf.o \ obj/src/pwcache.o \ + obj/src/sighook.o \ obj/src/stat.o \ obj/src/thread.o \ obj/src/trie.o \ @@ -108,6 +109,7 @@ UNIT_OBJS := \ obj/tests/bit.o \ obj/tests/ioq.o \ obj/tests/main.o \ + obj/tests/sighook.o \ obj/tests/trie.o \ obj/tests/xspawn.o \ obj/tests/xtime.o @@ -36,6 +36,7 @@ * - mtab.[ch] (parses the system's mount table) * - pwcache.[ch] (a cache for the user/group tables) * - sanity.h (sanitizer interfaces) + * - sighook.[ch] (signal hooks) * - stat.[ch] (wraps stat(), or statx() on Linux) * - thread.h (multi-threading) * - trie.[ch] (a trie set/map implementation) diff --git a/src/sighook.c b/src/sighook.c new file mode 100644 index 0000000..745ccb9 --- /dev/null +++ b/src/sighook.c @@ -0,0 +1,600 @@ +// Copyright © Tavian Barnes <tavianator@tavianator.com> +// SPDX-License-Identifier: 0BSD + +/** + * Dynamic (un)registration of signal handlers. + * + * Because signal handlers can interrupt any thread at an arbitrary point, they + * must be lock-free or risk deadlock. Therefore, we implement the global table + * of signal "hooks" with a simple read-copy-update (RCU) scheme. Readers get a + * reference-counted pointer (struct arc) to the table in a lock-free way, and + * release the reference count when finished. + * + * Updates are managed by struct rcu, which has two slots: one active and one + * inactive. Readers acquire a reference to the active slot. A single writer + * can safely update it by initializing the inactive slot, atomically swapping + * the slots, and waiting for the reference count of the newly inactive slot to + * drop to zero. Once it does, the old pointer can be safely freed. + */ + +#include "prelude.h" +#include "sighook.h" +#include "alloc.h" +#include "atomic.h" +#include "bfstd.h" +#include "diag.h" +#include "thread.h" +#include <errno.h> +#include <signal.h> +#include <stdlib.h> +#include <unistd.h> + +#if _POSIX_SEMAPHORES > 0 +# include <semaphore.h> +#endif + +/** + * An atomically reference-counted pointer. + */ +struct arc { + /** The current reference count (0 means empty). */ + atomic size_t refs; + /** The reference itself. */ + void *ptr; + +#if _POSIX_SEMAPHORES > 0 + /** A semaphore for arc_wake(). */ + sem_t sem; + /** sem_init() result. */ + int sem_status; +#endif +}; + +/** Initialize an arc. */ +static void arc_init(struct arc *arc) { + atomic_init(&arc->refs, 0); + arc->ptr = NULL; + +#if _POSIX_SEMAPHORES > 0 + arc->sem_status = sem_init(&arc->sem, false, 0); +#endif +} + +/** Get the current refcount. */ +static size_t arc_refs(const struct arc *arc) { + return load(&arc->refs, relaxed); +} + +/** Set the pointer in an empty arc. */ +static void arc_set(struct arc *arc, void *ptr) { + bfs_assert(arc_refs(arc) == 0); + bfs_assert(ptr); + + arc->ptr = ptr; + store(&arc->refs, 1, release); +} + +/** Acquire a reference. */ +static void *arc_get(struct arc *arc) { + size_t refs = arc_refs(arc); + do { + if (refs < 1) { + return NULL; + } + } while (!compare_exchange_weak(&arc->refs, &refs, refs + 1, acquire, relaxed)); + + return arc->ptr; +} + +/** Release a reference. */ +static void arc_put(struct arc *arc) { + size_t refs = fetch_sub(&arc->refs, 1, release); + + if (refs == 1) { +#if _POSIX_SEMAPHORES > 0 + if (arc->sem_status == 0 && sem_post(&arc->sem) != 0) { + abort(); + } +#endif + } +} + +/** Wait on the semaphore. */ +static int arc_sem_wait(struct arc *arc) { +#if _POSIX_SEMAPHORES > 0 + if (arc->sem_status == 0) { + while (sem_wait(&arc->sem) != 0) { + bfs_verify(errno == EINTR, "sem_wait(): %s", xstrerror(errno)); + } + return 0; + } +#endif + + return -1; +} + +/** Wait for all references to be released. */ +static void *arc_wait(struct arc *arc) { + size_t refs = fetch_sub(&arc->refs, 1, relaxed); + bfs_assert(refs > 0); + + --refs; + while (refs > 0) { + if (arc_sem_wait(arc) == 0) { + bfs_assert(arc_refs(arc) == 0); + // sem_wait() provides enough ordering, so we can skip the fence + goto done; + } + + // Some platforms (like macOS) don't support unnamed semaphores, + // but we can always busy-wait + spin_loop(); + refs = arc_refs(arc); + } + + thread_fence(&arc->refs, acquire); + +done:; + void *ptr = arc->ptr; + arc->ptr = NULL; + return ptr; +} + +/** Destroy an arc. */ +static void arc_destroy(struct arc *arc) { + bfs_assert(arc_refs(arc) <= 1); + +#if _POSIX_SEMAPHORES > 0 + if (arc->sem_status == 0) { + bfs_verify(sem_destroy(&arc->sem) == 0, "sem_destroy(): %s", xstrerror(errno)); + } +#endif +} + +/** + * A simple read-copy-update memory reclamation scheme. + */ +struct rcu { + /** The currently active slot. */ + atomic size_t active; + /** The two slots. */ + struct arc slots[2]; +}; + +/** Sentinel value for RCU, since arc uses NULL already. */ +static void *RCU_NULL = &RCU_NULL; + +/** Initialize an RCU block. */ +static void rcu_init(struct rcu *rcu) { + atomic_init(&rcu->active, 0); + arc_init(&rcu->slots[0]); + arc_init(&rcu->slots[1]); + arc_set(&rcu->slots[0], RCU_NULL); +} + +/** Get the active slot. */ +static struct arc *rcu_active(struct rcu *rcu) { + size_t i = load(&rcu->active, relaxed); + return &rcu->slots[i]; +} + +/** Read an RCU-protected pointer. */ +static void *rcu_read(struct rcu *rcu, struct arc **slot) { + while (true) { + *slot = rcu_active(rcu); + void *ptr = arc_get(*slot); + if (ptr == RCU_NULL) { + return NULL; + } else if (ptr) { + return ptr; + } + // Otherwise, the other slot became active; retry + } +} + +/** Get the RCU-protected pointer without acquiring a reference. */ +static void *rcu_peek(struct rcu *rcu) { + struct arc *arc = rcu_active(rcu); + void *ptr = arc->ptr; + if (ptr == RCU_NULL) { + return NULL; + } else { + return ptr; + } +} + +/** Update an RCU-protected pointer, and return the old one. */ +static void *rcu_update(struct rcu *rcu, void *ptr) { + size_t i = load(&rcu->active, relaxed); + struct arc *prev = &rcu->slots[i]; + + size_t j = i ^ 1; + struct arc *next = &rcu->slots[j]; + + arc_set(next, ptr ? ptr : RCU_NULL); + store(&rcu->active, j, relaxed); + return arc_wait(prev); +} + +struct sighook { + int sig; + sighook_fn *fn; + void *arg; + enum sigflags flags; +}; + +/** + * A table of signal hooks. + */ +struct sigtable { + /** The number of filled slots. */ + size_t filled; + /** The length of the array. */ + size_t size; + /** An array of signal hooks. */ + struct arc hooks[]; +}; + +/** Add a hook to a table. */ +static int sigtable_add(struct sigtable *table, struct sighook *hook) { + if (!table || table->filled == table->size) { + return -1; + } + + for (size_t i = 0; i < table->size; ++i) { + struct arc *arc = &table->hooks[i]; + if (arc_refs(arc) == 0) { + arc_set(arc, hook); + ++table->filled; + return 0; + } + } + + return -1; +} + +/** Delete a hook from a table. */ +static int sigtable_del(struct sigtable *table, struct sighook *hook) { + for (size_t i = 0; i < table->size; ++i) { + struct arc *arc = &table->hooks[i]; + if (arc->ptr == hook) { + arc_wait(arc); + --table->filled; + return 0; + } + } + + return -1; +} + +/** Create a bigger copy of a signal table. */ +static struct sigtable *sigtable_grow(struct sigtable *prev) { + size_t old_size = prev ? prev->size : 0; + size_t new_size = old_size ? 2 * old_size : 1; + struct sigtable *table = ALLOC_FLEX(struct sigtable, hooks, new_size); + if (!table) { + return NULL; + } + + table->filled = 0; + table->size = new_size; + for (size_t i = 0; i < new_size; ++i) { + arc_init(&table->hooks[i]); + } + + for (size_t i = 0; i < old_size; ++i) { + struct sighook *hook = prev->hooks[i].ptr; + if (hook) { + bfs_verify(sigtable_add(table, hook) == 0); + } + } + + return table; +} + +/** Free a signal table. */ +static void sigtable_free(struct sigtable *table) { + if (!table) { + return; + } + + for (size_t i = 0; i < table->size; ++i) { + struct arc *arc = &table->hooks[i]; + arc_destroy(arc); + } + free(table); +} + +/** Add a hook to a signal table, growing it if necessary. */ +static int rcu_sigtable_add(struct rcu *rcu, struct sighook *hook) { + struct sigtable *prev = rcu_peek(rcu); + if (sigtable_add(prev, hook) == 0) { + return 0; + } + + struct sigtable *next = sigtable_grow(prev); + if (!next) { + return -1; + } + + bfs_verify(sigtable_add(next, hook) == 0); + rcu_update(rcu, next); + sigtable_free(prev); + return 0; +} + +/** The global table of signal hooks. */ +static struct rcu rcu_sighooks; +/** The global table of atsigexit() hooks. */ +static struct rcu rcu_exithooks; + +/** Mutex for initialization and RCU writer exclusion. */ +static pthread_mutex_t sigmutex = PTHREAD_MUTEX_INITIALIZER; + +/** Check if a signal was generated by userspace. */ +static bool is_user_generated(const siginfo_t *info) { + // https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03 + // + // If si_code is SI_USER or SI_QUEUE, or any value less than or + // equal to 0, then the signal was generated by a process ... + int code = info->si_code; + return code == SI_USER || code == SI_QUEUE || code <= 0; +} + +/** Check if a signal is caused by a fault. */ +static bool is_fault(const siginfo_t *info) { + int sig = info->si_signo; + if (sig == SIGBUS || sig == SIGFPE || sig == SIGILL || sig == SIGSEGV) { + return !is_user_generated(info); + } else { + return false; + } +} + +// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html +static const int FATAL_SIGNALS[] = { + SIGABRT, + SIGALRM, + SIGBUS, + SIGFPE, + SIGHUP, + SIGILL, + SIGINT, + SIGPIPE, + SIGQUIT, + SIGSEGV, + SIGTERM, + SIGUSR1, + SIGUSR2, +#ifdef SIGPOLL + SIGPOLL, +#endif +#ifdef SIGPROF + SIGPROF, +#endif +#ifdef SIGSYS + SIGSYS, +#endif + SIGTRAP, +#ifdef SIGVTALRM + SIGVTALRM, +#endif + SIGXCPU, + SIGXFSZ, +}; + +/** Check if a signal's default action is to terminate the process. */ +static bool is_fatal(int sig) { + for (size_t i = 0; i < countof(FATAL_SIGNALS); ++i) { + if (sig == FATAL_SIGNALS[i]) { + return true; + } + } + +#ifdef SIGRTMIN + // https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03 + // + // The default actions for the realtime signals in the range + // SIGRTMIN to SIGRTMAX shall be to terminate the process + // abnormally. + if (sig >= SIGRTMIN && sig <= SIGRTMAX) { + return true; + } +#endif + + return false; +} + +/** Reraise a fatal signal. */ +static noreturn void reraise(int sig) { + // Restore the default signal action + if (signal(sig, SIG_DFL) == SIG_ERR) { + goto fail; + } + + // Unblock the signal, since we didn't set SA_NODEFER + sigset_t mask; + if (sigemptyset(&mask) != 0 + || sigaddset(&mask, sig) != 0 + || pthread_sigmask(SIG_UNBLOCK, &mask, NULL) != 0) { + goto fail; + } + + raise(sig); +fail: + abort(); +} + +/** Find any matching hooks and run them. */ +static enum sigflags run_hooks(struct rcu *rcu, int sig, siginfo_t *info) { + enum sigflags ret = 0; + struct arc *slot; + struct sigtable *table = rcu_read(rcu, &slot); + if (!table) { + goto done; + } + + for (size_t i = 0; i < table->size; ++i) { + struct arc *arc = &table->hooks[i]; + struct sighook *hook = arc_get(arc); + if (!hook) { + continue; + } + + if (hook->sig == sig || hook->sig == 0) { + hook->fn(sig, info, hook->arg); + ret |= hook->flags; + } + arc_put(arc); + } + +done: + arc_put(slot); + return ret; +} + +/** Dispatches a signal to the registered handlers. */ +static void sigdispatch(int sig, siginfo_t *info, void *context) { + // https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03 + // + // The behavior of a process is undefined after it returns normally + // from a signal-catching function for a SIGBUS, SIGFPE, SIGILL, or + // SIGSEGV signal that was not generated by kill(), sigqueue(), or + // raise(). + if (is_fault(info)) { + reraise(sig); + } + + // https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03 + // + // After returning from a signal-catching function, the value of + // errno is unspecified if the signal-catching function or any + // function it called assigned a value to errno and the signal- + // catching function did not save and restore the original value of + // errno. + int error = errno; + + // Run the normal hooks + enum sigflags flags = run_hooks(&rcu_sighooks, sig, info); + + // Run the atsigexit() hooks, if we're exiting + if (!(flags & SH_CONTINUE) && is_fatal(sig)) { + run_hooks(&rcu_exithooks, sig, info); + reraise(sig); + } + + errno = error; +} + +/** Make sure our signal handler is installed for a given signal. */ +static int siginit(int sig) { + static struct sigaction action = { + .sa_sigaction = sigdispatch, + .sa_flags = SA_RESTART | SA_SIGINFO, + }; + + static sigset_t signals; + static bool initialized = false; + + if (!initialized) { + if (sigemptyset(&signals) != 0 + || sigemptyset(&action.sa_mask) != 0) { + return -1; + } + rcu_init(&rcu_sighooks); + rcu_init(&rcu_exithooks); + initialized = true; + } + + int installed = sigismember(&signals, sig); + if (installed < 0) { + return -1; + } else if (installed) { + return 0; + } + + if (sigaction(sig, &action, NULL) != 0) { + return -1; + } + + if (sigaddset(&signals, sig) != 0) { + return -1; + } + + return 0; +} + +/** Shared sighook()/atsigexit() implementation. */ +static struct sighook *sighook_impl(struct rcu *rcu, int sig, sighook_fn *fn, void *arg, enum sigflags flags) { + struct sighook *hook = ALLOC(struct sighook); + if (!hook) { + return NULL; + } + + hook->sig = sig; + hook->fn = fn; + hook->arg = arg; + hook->flags = flags; + + if (rcu_sigtable_add(rcu, hook) != 0) { + free(hook); + return NULL; + } + + return hook; +} + +struct sighook *sighook(int sig, sighook_fn *fn, void *arg, enum sigflags flags) { + mutex_lock(&sigmutex); + + struct sighook *ret = NULL; + if (siginit(sig) != 0) { + goto done; + } + + ret = sighook_impl(&rcu_sighooks, sig, fn, arg, flags); +done: + mutex_unlock(&sigmutex); + return ret; +} + +struct sighook *atsigexit(sighook_fn *fn, void *arg) { + mutex_lock(&sigmutex); + + struct sighook *ret = NULL; + + for (size_t i = 0; i < countof(FATAL_SIGNALS); ++i) { + if (siginit(FATAL_SIGNALS[i]) != 0) { + goto done; + } + } + +#ifdef SIGRTMIN + for (int i = SIGRTMIN; i <= SIGRTMAX; ++i) { + if (siginit(i) != 0) { + goto done; + } + } +#endif + + ret = sighook_impl(&rcu_exithooks, 0, fn, arg, 0); +done: + mutex_unlock(&sigmutex); + return ret; +} + +void sigunhook(struct sighook *hook) { + mutex_lock(&sigmutex); + + struct rcu *rcu = hook->sig ? &rcu_sighooks : &rcu_exithooks; + struct sigtable *table = rcu_peek(rcu); + bfs_verify(sigtable_del(table, hook) == 0); + + if (table->filled == 0) { + rcu_update(rcu, NULL); + sigtable_free(table); + } + + mutex_unlock(&sigmutex); + free(hook); +} diff --git a/src/sighook.h b/src/sighook.h new file mode 100644 index 0000000..74d18c0 --- /dev/null +++ b/src/sighook.h @@ -0,0 +1,73 @@ +// Copyright © Tavian Barnes <tavianator@tavianator.com> +// SPDX-License-Identifier: 0BSD + +/** + * Signal hooks. + */ + +#ifndef BFS_SIGHOOK_H +#define BFS_SIGHOOK_H + +#include <signal.h> + +/** + * A dynamic signal hook. + */ +struct sighook; + +/** + * Signal hook flags. + */ +enum sigflags { + /** Suppress the default action for this signal. */ + SH_CONTINUE = 1 << 0, +}; + +/** + * A signal hook callback. Hooks are executed from a signal handler, so must + * only call async-signal-safe functions. + * + * @param sig + * The signal number. + * @param info + * Additional information about the signal. + * @param arg + * An arbitrary pointer passed to the hook. + */ +typedef void sighook_fn(int sig, siginfo_t *info, void *arg); + +/** + * Install a hook for a signal. + * + * @param sig + * The signal to hook. + * @param fn + * The function to call. + * @param arg + * An argument passed to the function. + * @param flags + * Flags for the new hook. + * @return + * The installed hook, or NULL on failure. + */ +struct sighook *sighook(int sig, sighook_fn *fn, void *arg, enum sigflags flags); + +/** + * On a best-effort basis, invoke the given hook just before the program is + * abnormally terminated by a signal. + * + * @param fn + * The function to call. + * @param arg + * An argument passed to the function. + * @return + * The installed hook, or NULL on failure. + */ +struct sighook *atsigexit(sighook_fn *fn, void *arg); + +/** + * Remove a signal hook. + */ +void sigunhook(struct sighook *hook); + +#endif // BFS_SIGHOOK_H diff --git a/tests/main.c b/tests/main.c index 429772b..58def32 100644 --- a/tests/main.c +++ b/tests/main.c @@ -116,6 +116,7 @@ int main(int argc, char *argv[]) { run_test(&ctx, "bfstd", check_bfstd); run_test(&ctx, "bit", check_bit); run_test(&ctx, "ioq", check_ioq); + run_test(&ctx, "sighook", check_sighook); run_test(&ctx, "trie", check_trie); run_test(&ctx, "xspawn", check_xspawn); run_test(&ctx, "xtime", check_xtime); diff --git a/tests/sighook.c b/tests/sighook.c new file mode 100644 index 0000000..e5dd47f --- /dev/null +++ b/tests/sighook.c @@ -0,0 +1,97 @@ +// Copyright © Tavian Barnes <tavianator@tavianator.com> +// SPDX-License-Identifier: 0BSD + +#include "prelude.h" +#include "tests.h" +#include "sighook.h" +#include "atomic.h" +#include "thread.h" +#include <pthread.h> +#include <signal.h> +#include <sys/time.h> + +static atomic size_t count = 0; + +/** SIGALRM handler. */ +static void alrm_hook(int sig, siginfo_t *info, void *arg) { + fetch_add(&count, 1, relaxed); +} + +/** Swap out an old hook for a new hook. */ +static int swap_hooks(struct sighook **hook) { + struct sighook *next = sighook(SIGALRM, alrm_hook, NULL, SH_CONTINUE); + if (!bfs_pcheck(next, "sighook(SIGALRM)")) { + return -1; + } + + sigunhook(*hook); + *hook = next; + return 0; +} + +/** Background thread that rapidly (un)registers signal hooks. */ +static void *hook_thread(void *ptr) { + struct sighook *hook = sighook(SIGALRM, alrm_hook, NULL, SH_CONTINUE); + if (!bfs_pcheck(hook, "sighook(SIGALRM)")) { + return NULL; + } + + while (load(&count, relaxed) < 1000) { + if (swap_hooks(&hook) != 0) { + sigunhook(hook); + return NULL; + } + } + + sigunhook(hook); + return &count; +} + +bool check_sighook(void) { + bool ret = true; + + struct sighook *hook = sighook(SIGALRM, alrm_hook, NULL, SH_CONTINUE); + ret &= bfs_pcheck(hook, "sighook(SIGALRM)"); + if (!ret) { + goto done; + } + + struct itimerval ival = { + .it_value = { + .tv_usec = 100, + }, + .it_interval = { + .tv_usec = 100, + }, + }; + ret &= bfs_pcheck(setitimer(ITIMER_REAL, &ival, NULL) == 0); + if (!ret) { + goto unhook; + } + + pthread_t thread; + ret &= bfs_pcheck(thread_create(&thread, NULL, hook_thread, NULL) == 0); + if (!ret) { + goto untime; + } + + while (ret && load(&count, relaxed) < 1000) { + ret &= swap_hooks(&hook) == 0; + } + + void *ptr; + thread_join(thread, &ptr); + ret &= bfs_check(ptr); + +untime: + ival.it_value.tv_usec = 0; + ret &= bfs_pcheck(setitimer(ITIMER_REAL, &ival, NULL) == 0); + if (!ret) { + goto unhook; + } + +unhook: + sigunhook(hook); +done: + return ret; +} diff --git a/tests/tests.h b/tests/tests.h index 9078938..d26a5a0 100644 --- a/tests/tests.h +++ b/tests/tests.h @@ -26,6 +26,9 @@ bool check_bit(void); /** I/O queue tests. */ bool check_ioq(void); +/** Signal hook tests. */ +bool check_sighook(void); + /** Trie tests. */ bool check_trie(void); |