lift child restrictions after multi-threaded fork

as the outcome of Austin Group tracker issue #62, future editions of
POSIX have dropped the requirement that fork be AS-safe. this allows
but does not require implementations to synchronize fork with internal
locks and give forked children of multithreaded parents a partly or
fully unrestricted execution environment where they can continue to
use the standard library (per POSIX, they can only portably use
AS-safe functions).

up until recently, taking this allowance did not seem desirable.
however, commit 8ed2bd8 exposed the
extent to which applications and libraries are depending on the
ability to use malloc and other non-AS-safe interfaces in MT-forked
children, by converting latent very-low-probability catastrophic state
corruption into predictable deadlock. dealing with the fallout has
been a huge burden for users/distros.

while it looks like most of the non-portable usage in applications
could be fixed given sufficient effort, at least some of it seems to
occur in language runtimes which are exposing the ability to run
unrestricted code in the child as part of the contract with the
programmer. any attempt at fixing such contracts is not just a
technical problem but a social one, and is probably not tractable.

this patch extends the fork function to take locks for all libc
singletons in the parent, and release or reset those locks in the
child, so that when the underlying fork operation takes place, the
state protected by these locks is consistent and ready for the child
to use. locking is skipped in the case where the parent is
single-threaded so as not to interfere with legacy AS-safety property
of fork in single-threaded programs. lock order is mostly arbitrary,
but the malloc locks (including bump allocator in case it's used) must
be taken after the locks on any subsystems that might use malloc, and
non-AS-safe locks cannot be taken while the thread list lock is held,
imposing a requirement that it be taken last.

ldso/dynlink.c

@@ -20,6 +20,7 @@
 #include <semaphore.h>
 #include <sys/membarrier.h>
 #include "pthread_impl.h"
+#include "fork_impl.h"
 #include "libc.h"
 #include "dynlink.h"
 
@@ -1426,6 +1427,17 @@ void __libc_exit_fini()
 	}
 }
 
+void __ldso_atfork(int who)
+{
+	if (who<0) {
+		pthread_rwlock_wrlock(&lock);
+		pthread_mutex_lock(&init_fini_lock);
+	} else {
+		pthread_mutex_unlock(&init_fini_lock);
+		pthread_rwlock_unlock(&lock);
+	}
+}
+
 static struct dso **queue_ctors(struct dso *dso)
 {
 	size_t cnt, qpos, spos, i;
@@ -1484,6 +1496,13 @@ static struct dso **queue_ctors(struct dso *dso)
 	}
 	queue[qpos] = 0;
 	for (i=0; i<qpos; i++) queue[i]->mark = 0;
+	for (i=0; i<qpos; i++)
+		if (queue[i]->ctor_visitor && queue[i]->ctor_visitor->tid < 0) {
+			error("State of %s is inconsistent due to multithreaded fork\n",
+				queue[i]->name);
+			free(queue);
+			if (runtime) longjmp(*rtld_fail, 1);
+		}
 
 	return queue;
 }

src/exit/at_quick_exit.c

@@ -1,12 +1,14 @@
 #include <stdlib.h>
 #include "libc.h"
 #include "lock.h"
+#include "fork_impl.h"
 
 #define COUNT 32
 
 static void (*funcs[COUNT])(void);
 static int count;
 static volatile int lock[1];
+volatile int *const __at_quick_exit_lockptr = lock;
 
 void __funcs_on_quick_exit()
 {

src/exit/atexit.c

@@ -2,6 +2,7 @@
 #include <stdint.h>
 #include "libc.h"
 #include "lock.h"
+#include "fork_impl.h"
 
 #define malloc __libc_malloc
 #define calloc __libc_calloc
@@ -20,6 +21,7 @@ static struct fl
 
 static int slot;
 static volatile int lock[1];
+volatile int *const __atexit_lockptr = lock;
 
 void __funcs_on_exit()
 {

src/internal/fork_impl.h

@@ -0,0 +1,19 @@
+#include <features.h>
+
+extern hidden volatile int *const __at_quick_exit_lockptr;
+extern hidden volatile int *const __atexit_lockptr;
+extern hidden volatile int *const __dlerror_lockptr;
+extern hidden volatile int *const __gettext_lockptr;
+extern hidden volatile int *const __locale_lockptr;
+extern hidden volatile int *const __random_lockptr;
+extern hidden volatile int *const __sem_open_lockptr;
+extern hidden volatile int *const __stdio_ofl_lockptr;
+extern hidden volatile int *const __syslog_lockptr;
+extern hidden volatile int *const __timezone_lockptr;
+
+extern hidden volatile int *const __bump_lockptr;
+
+extern hidden volatile int *const __vmlock_lockptr;
+
+hidden void __malloc_atfork(int);
+hidden void __ldso_atfork(int);

src/ldso/dlerror.c

@@ -4,6 +4,7 @@
 #include "pthread_impl.h"
 #include "dynlink.h"
 #include "lock.h"
+#include "fork_impl.h"
 
 #define malloc __libc_malloc
 #define calloc __libc_calloc
@@ -24,6 +25,7 @@ char *dlerror()
 
 static volatile int freebuf_queue_lock[1];
 static void **freebuf_queue;
+volatile int *const __dlerror_lockptr = freebuf_queue_lock;
 
 void __dl_thread_cleanup(void)
 {

src/locale/dcngettext.c

@@ -10,6 +10,7 @@
 #include "atomic.h"
 #include "pleval.h"
 #include "lock.h"
+#include "fork_impl.h"
 
 #define malloc __libc_malloc
 #define calloc __libc_calloc
@@ -39,9 +40,11 @@ static char *gettextdir(const char *domainname, size_t *dirlen)
 	return 0;
 }
 
+static volatile int lock[1];
+volatile int *const __gettext_lockptr = lock;
+
 char *bindtextdomain(const char *domainname, const char *dirname)
 {
-	static volatile int lock[1];
 	struct binding *p, *q;
 
 	if (!domainname) return 0;

src/locale/locale_map.c

@@ -5,6 +5,7 @@
 #include "locale_impl.h"
 #include "libc.h"
 #include "lock.h"
+#include "fork_impl.h"
 
 #define malloc __libc_malloc
 #define calloc undef
@@ -27,9 +28,11 @@ static const char envvars[][12] = {
 	"LC_MESSAGES",
 };
 
+static volatile int lock[1];
+volatile int *const __locale_lockptr = lock;
+
 const struct __locale_map *__get_locale(int cat, const char *val)
 {
-	static volatile int lock[1];
 	static void *volatile loc_head;
 	const struct __locale_map *p;
 	struct __locale_map *new = 0;

src/malloc/lite_malloc.c

@@ -6,6 +6,7 @@
 #include "libc.h"
 #include "lock.h"
 #include "syscall.h"
+#include "fork_impl.h"
 
 #define ALIGN 16
 
@@ -31,10 +32,12 @@ static int traverses_stack_p(uintptr_t old, uintptr_t new)
 	return 0;
 }
 
+static volatile int lock[1];
+volatile int *const __bump_lockptr = lock;
+
 static void *__simple_malloc(size_t n)
 {
 	static uintptr_t brk, cur, end;
-	static volatile int lock[1];
 	static unsigned mmap_step;
 	size_t align=1;
 	void *p;

src/malloc/mallocng/glue.h

@@ -60,7 +60,8 @@ __attribute__((__visibility__("hidden")))
 extern int __malloc_lock[1];
 
 #define LOCK_OBJ_DEF \
-int __malloc_lock[1];
+int __malloc_lock[1]; \
+void __malloc_atfork(int who) { malloc_atfork(who); }
 
 static inline void rdlock()
 {
@@ -77,5 +78,16 @@ static inline void unlock()
 static inline void upgradelock()
 {
 }
+static inline void resetlock()
+{
+	__malloc_lock[0] = 0;
+}
+
+static inline void malloc_atfork(int who)
+{
+	if (who<0) rdlock();
+	else if (who>0) resetlock();
+	else unlock();
+}
 
 #endif

src/malloc/oldmalloc/malloc.c

@@ -9,6 +9,7 @@
 #include "atomic.h"
 #include "pthread_impl.h"
 #include "malloc_impl.h"
+#include "fork_impl.h"
 
 #define malloc __libc_malloc
 #define realloc __libc_realloc
@@ -531,3 +532,21 @@ void __malloc_donate(char *start, char *end)
 	c->csize = n->psize = C_INUSE | (end-start);
 	__bin_chunk(c);
 }
+
+void __malloc_atfork(int who)
+{
+	if (who<0) {
+		lock(mal.split_merge_lock);
+		for (int i=0; i<64; i++)
+			lock(mal.bins[i].lock);
+	} else if (!who) {
+		for (int i=0; i<64; i++)
+			unlock(mal.bins[i].lock);
+		unlock(mal.split_merge_lock);
+	} else {
+		for (int i=0; i<64; i++)
+			mal.bins[i].lock[0] = mal.bins[i].lock[1] = 0;
+		mal.split_merge_lock[1] = 0;
+		mal.split_merge_lock[0] = 0;
+	}
+}

src/misc/syslog.c

@@ -10,13 +10,15 @@
 #include <errno.h>
 #include <fcntl.h>
 #include "lock.h"
+#include "fork_impl.h"
 
 static volatile int lock[1];
 static char log_ident[32];
 static int log_opt;
 static int log_facility = LOG_USER;
 static int log_mask = 0xff;
 static int log_fd = -1;
+volatile int *const __syslog_lockptr = lock;
 
 int setlogmask(int maskpri)
 {

src/prng/random.c

@@ -1,6 +1,7 @@
 #include <stdlib.h>
 #include <stdint.h>
 #include "lock.h"
+#include "fork_impl.h"
 
 /*
 this code uses the same lagged fibonacci generator as the
@@ -23,6 +24,7 @@ static int i = 3;
 static int j = 0;
 static uint32_t *x = init+1;
 static volatile int lock[1];
+volatile int *const __random_lockptr = lock;
 
 static uint32_t lcg31(uint32_t x) {
 	return (1103515245*x + 12345) & 0x7fffffff;

src/process/fork.c

@@ -1,15 +1,85 @@
 #include <unistd.h>
 #include <errno.h>
 #include "libc.h"
+#include "lock.h"
+#include "pthread_impl.h"
+#include "fork_impl.h"
+
+static volatile int *const dummy_lockptr = 0;
+
+weak_alias(dummy_lockptr, __at_quick_exit_lockptr);
+weak_alias(dummy_lockptr, __atexit_lockptr);
+weak_alias(dummy_lockptr, __dlerror_lockptr);
+weak_alias(dummy_lockptr, __gettext_lockptr);
+weak_alias(dummy_lockptr, __locale_lockptr);
+weak_alias(dummy_lockptr, __random_lockptr);
+weak_alias(dummy_lockptr, __sem_open_lockptr);
+weak_alias(dummy_lockptr, __stdio_ofl_lockptr);
+weak_alias(dummy_lockptr, __syslog_lockptr);
+weak_alias(dummy_lockptr, __timezone_lockptr);
+weak_alias(dummy_lockptr, __bump_lockptr);
+
+weak_alias(dummy_lockptr, __vmlock_lockptr);
+
+static volatile int *const *const atfork_locks[] = {
+	&__at_quick_exit_lockptr,
+	&__atexit_lockptr,
+	&__dlerror_lockptr,
+	&__gettext_lockptr,
+	&__locale_lockptr,
+	&__random_lockptr,
+	&__sem_open_lockptr,
+	&__stdio_ofl_lockptr,
+	&__syslog_lockptr,
+	&__timezone_lockptr,
+	&__bump_lockptr,
+};
 
 static void dummy(int x) { }
 weak_alias(dummy, __fork_handler);
+weak_alias(dummy, __malloc_atfork);
+weak_alias(dummy, __ldso_atfork);
+
+static void dummy_0(void) { }
+weak_alias(dummy_0, __tl_lock);
+weak_alias(dummy_0, __tl_unlock);
 
 pid_t fork(void)
 {
+	sigset_t set;
 	__fork_handler(-1);
+	__block_app_sigs(&set);
+	int need_locks = libc.need_locks > 0;
+	if (need_locks) {
+		__ldso_atfork(-1);
+		__inhibit_ptc();
+		for (int i=0; i<sizeof atfork_locks/sizeof *atfork_locks; i++)
+			if (*atfork_locks[i]) LOCK(*atfork_locks[i]);
+		__malloc_atfork(-1);
+		__tl_lock();
+	}
+	pthread_t self=__pthread_self(), next=self->next;
 	pid_t ret = _Fork();
 	int errno_save = errno;
+	if (need_locks) {
+		if (!ret) {
+			for (pthread_t td=next; td!=self; td=td->next)
+				td->tid = -1;
+			if (__vmlock_lockptr) {
+				__vmlock_lockptr[0] = 0;
+				__vmlock_lockptr[1] = 0;
+			}
+		}
+		__tl_unlock();
+		__malloc_atfork(!ret);
+		for (int i=0; i<sizeof atfork_locks/sizeof *atfork_locks; i++)
+			if (*atfork_locks[i])
+				if (ret) UNLOCK(*atfork_locks[i]);
+				else **atfork_locks[i] = 0;
+		__release_ptc();
+		__ldso_atfork(!ret);
+	}
+	__restore_sigs(&set);
 	__fork_handler(!ret);
 	if (ret<0) errno = errno_save;
 	return ret;

src/stdio/ofl.c

@@ -1,8 +1,10 @@
 #include "stdio_impl.h"
 #include "lock.h"
+#include "fork_impl.h"
 
 static FILE *ofl_head;
 static volatile int ofl_lock[1];
+volatile int *const __stdio_ofl_lockptr = ofl_lock;
 
 FILE **__ofl_lock()
 {

src/thread/sem_open.c

@@ -12,6 +12,7 @@
 #include <stdlib.h>
 #include <pthread.h>
 #include "lock.h"
+#include "fork_impl.h"
 
 #define malloc __libc_malloc
 #define calloc __libc_calloc
@@ -24,6 +25,7 @@ static struct {
 	int refcnt;
 } *semtab;
 static volatile int lock[1];
+volatile int *const __sem_open_lockptr = lock;
 
 #define FLAGS (O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NONBLOCK)
 

src/thread/vmlock.c

@@ -1,6 +1,8 @@
 #include "pthread_impl.h"
+#include "fork_impl.h"
 
 static volatile int vmlock[2];
+volatile int *const __vmlock_lockptr = vmlock;
 
 void __vm_wait()
 {

src/time/__tz.c

@@ -6,6 +6,7 @@
 #include <sys/mman.h>
 #include "libc.h"
 #include "lock.h"
+#include "fork_impl.h"
 
 #define malloc __libc_malloc
 #define calloc undef
@@ -35,6 +36,7 @@ static char *old_tz = old_tz_buf;
 static size_t old_tz_size = sizeof old_tz_buf;
 
 static volatile int lock[1];
+volatile int *const __timezone_lockptr = lock;
 
 static int getint(const char **p)
 {