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Diffstat (limited to 'src/sighook.c')
| -rw-r--r-- | src/sighook.c | 566 |
1 files changed, 566 insertions, 0 deletions
diff --git a/src/sighook.c b/src/sighook.c new file mode 100644 index 0000000..4356fdb --- /dev/null +++ b/src/sighook.c @@ -0,0 +1,566 @@ +// 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 "sighook.h" + +#include "alloc.h" +#include "atomic.h" +#include "bfs.h" +#include "bfstd.h" +#include "diag.h" +#include "thread.h" + +#include <errno.h> +#include <pthread.h> +#include <signal.h> +#include <stdlib.h> +#include <unistd.h> + +// NetBSD opens a file descriptor for each sem_init() +#if defined(_POSIX_SEMAPHORES) && !__NetBSD__ +# define BFS_POSIX_SEMAPHORES _POSIX_SEMAPHORES +#else +# define BFS_POSIX_SEMAPHORES (-1) +#endif + +#if BFS_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 BFS_POSIX_SEMAPHORES >= 0 + /** A semaphore for arc_wait(). */ + sem_t sem; + /** sem_init() result. */ + int sem_status; +#endif +}; + +/** Initialize an arc. */ +static void arc_init(struct arc *arc) { + bfs_verify(atomic_is_lock_free(&arc->refs)); + + atomic_init(&arc->refs, 0); + arc->ptr = NULL; + +#if BFS_POSIX_SEMAPHORES >= 0 + if (sysoption(SEMAPHORES) > 0) { + arc->sem_status = sem_init(&arc->sem, false, 0); + } else { + arc->sem_status = -1; + } +#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 BFS_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 BFS_POSIX_SEMAPHORES >= 0 + if (arc->sem_status == 0) { + while (sem_wait(&arc->sem) != 0) { + bfs_everify(errno == EINTR, "sem_wait()"); + } + 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) == 0); + +#if BFS_POSIX_SEMAPHORES >= 0 + if (arc->sem_status == 0) { + bfs_everify(sem_destroy(&arc->sem) == 0, "sem_destroy()"); + } +#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; + +/** Map NULL -> RCU_NULL. */ +static void *rcu_encode(void *ptr) { + return ptr ? ptr : RCU_NULL; +} + +/** Map RCU_NULL -> NULL. */ +static void *rcu_decode(void *ptr) { + bfs_assert(ptr != NULL); + return ptr == RCU_NULL ? NULL : ptr; +} + +/** Initialize an RCU block. */ +static void rcu_init(struct rcu *rcu, void *ptr) { + bfs_verify(atomic_is_lock_free(&rcu->active)); + + atomic_init(&rcu->active, 0); + arc_init(&rcu->slots[0]); + arc_init(&rcu->slots[1]); + arc_set(&rcu->slots[0], rcu_encode(ptr)); +} + +/** Get the active slot. */ +static struct arc *rcu_active(struct rcu *rcu) { + size_t i = load(&rcu->active, relaxed); + return &rcu->slots[i]; +} + +/** Destroy an RCU block. */ +static void rcu_destroy(struct rcu *rcu) { + arc_wait(rcu_active(rcu)); + arc_destroy(&rcu->slots[1]); + arc_destroy(&rcu->slots[0]); +} + +/** 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) { + return rcu_decode(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); + return rcu_decode(arc->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, rcu_encode(ptr)); + store(&rcu->active, j, relaxed); + return rcu_decode(arc_wait(prev)); +} + +struct sighook { + /** The signal being hooked, or 0 for atsigexit(). */ + int sig; + /** Signal hook flags. */ + enum sigflags flags; + /** The function to call. */ + sighook_fn *fn; + /** An argument to pass to the function. */ + void *arg; + + /** The RCU pointer to this hook. */ + struct rcu *self; + /** The next hook in the list. */ + struct rcu next; +}; + +/** + * An RCU-protected linked list of signal hooks. + */ +struct siglist { + /** The first hook in the list. */ + struct rcu head; + /** &last->next */ + struct rcu *tail; +}; + +/** Initialize a siglist. */ +static void siglist_init(struct siglist *list) { + rcu_init(&list->head, NULL); + list->tail = &list->head; +} + +/** Append a hook to a linked list. */ +static void sigpush(struct siglist *list, struct sighook *hook) { + hook->self = list->tail; + list->tail = &hook->next; + rcu_init(&hook->next, NULL); + rcu_update(hook->self, hook); +} + +/** Remove a hook from the linked list. */ +static void sigpop(struct siglist *list, struct sighook *hook) { + struct sighook *next = rcu_peek(&hook->next); + rcu_update(hook->self, next); + if (next) { + next->self = hook->self; + } +} + +/** The lists of signal hooks. */ +static struct siglist sighooks[64]; + +/** Get the hook list for a particular signal. */ +static struct siglist *siglist(int sig) { + return &sighooks[sig % countof(sighooks)]; +} + +/** 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/9799919799/functions/V2_chap02.html#tag_16_04_03_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/9799919799/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/9799919799/functions/V2_chap02.html#tag_16_04_03_01 + // + // 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. */ +_noreturn +static 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 siglist *list, int sig, siginfo_t *info) { + enum sigflags ret = 0; + + struct arc *slot = NULL; + struct sighook *hook = rcu_read(&list->head, &slot); + while (hook) { + if (hook->sig == sig || hook->sig == 0) { + hook->fn(sig, info, hook->arg); + ret |= hook->flags; + } + + struct arc *prev = slot; + hook = rcu_read(&hook->next, &slot); + arc_put(prev); + } + + 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/9799919799/functions/V2_chap02.html#tag_16_04_03_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/9799919799/functions/V2_chap02.html#tag_16_04_04 + // + // 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 + struct siglist *list = siglist(sig); + enum sigflags flags = run_hooks(list, sig, info); + + // Run the atsigexit() hooks, if we're exiting + if (!(flags & SH_CONTINUE) && is_fatal(sig)) { + list = siglist(0); + run_hooks(list, 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; + } + + for (size_t i = 0; i < countof(sighooks); ++i) { + siglist_init(&sighooks[i]); + } + + 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(int sig, sighook_fn *fn, void *arg, enum sigflags flags) { + struct sighook *hook = ALLOC(struct sighook); + if (!hook) { + return NULL; + } + + hook->sig = sig; + hook->flags = flags; + hook->fn = fn; + hook->arg = arg; + + struct siglist *list = siglist(sig); + sigpush(list, hook); + return hook; +} + +struct sighook *sighook(int sig, sighook_fn *fn, void *arg, enum sigflags flags) { + bfs_assert(sig > 0); + + mutex_lock(&sigmutex); + + struct sighook *ret = NULL; + if (siginit(sig) == 0) { + ret = sighook_impl(sig, fn, arg, flags); + } + + mutex_unlock(&sigmutex); + return ret; +} + +struct sighook *atsigexit(sighook_fn *fn, void *arg) { + mutex_lock(&sigmutex); + + for (size_t i = 0; i < countof(FATAL_SIGNALS); ++i) { + // Ignore errors; atsigexit() is best-effort anyway and things + // like sanitizer runtimes or valgrind may reserve signals for + // their own use + siginit(FATAL_SIGNALS[i]); + } + +#ifdef SIGRTMIN + for (int i = SIGRTMIN; i <= SIGRTMAX; ++i) { + siginit(i); + } +#endif + + struct sighook *ret = sighook_impl(0, fn, arg, 0); + mutex_unlock(&sigmutex); + return ret; +} + +void sigunhook(struct sighook *hook) { + if (!hook) { + return; + } + + mutex_lock(&sigmutex); + + struct siglist *list = siglist(hook->sig); + sigpop(list, hook); + + mutex_unlock(&sigmutex); + + rcu_destroy(&hook->next); + free(hook); +} |