Add an exit hook, sem_forkhook(), which walks the list of POSIX semaphores

owned by a process when it forks, and creates a matching set of references
for the child process, as prescribed by POSIX.

In order to avoid races with other threads in the parent process during
fork(), it is necessary to allocate a temporary reference list while
holding the sem_lock, then transfer those references to the new process
once the sem_lock is released.  The implementation is inefficient but
appears functional; in order to improve the efficiency, it will be
necessary to modify the existing structures and logic, which generally
rely on O(n) operations over the global set of semaphores.

1 files changed, 113 insertions, 1 deletions

diff --git a/sys/kern/uipc_sem.c b/sys/kern/uipc_sem.c
index 429a6a1..1cf5e80 100644
--- a/sys/kern/uipc_sem.c
+++ b/sys/kern/uipc_sem.c
@@ -1,5 +1,6 @@
 /*-
  * Copyright (c) 2002 Alfred Perlstein <alfred@FreeBSD.org>
+ * Copyright (c) 2005 Robert N. M. Watson
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -53,6 +54,7 @@ __FBSDID("$FreeBSD$");
 #include <posix4/semaphore.h>
 #include <posix4/_semaphore.h>
 
+static int sem_count_proc(struct proc *p);
 static struct ksem *sem_lookup_byname(const char *name);
 static int sem_create(struct thread *td, const char *name,
     struct ksem **ksret, mode_t mode, unsigned int value);
@@ -61,6 +63,8 @@ static int sem_perm(struct thread *td, struct ksem *ks);
 static void sem_enter(struct proc *p, struct ksem *ks);
 static int sem_leave(struct proc *p, struct ksem *ks);
 static void sem_exithook(void *arg, struct proc *p);
+static void sem_forkhook(void *arg, struct proc *p1, struct proc *p2,
+    int flags);
 static int sem_hasopen(struct thread *td, struct ksem *ks);
 
 static int kern_sem_close(struct thread *td, semid_t id);
@@ -118,7 +122,7 @@ static int nsems = 0;
 SYSCTL_DECL(_p1003_1b);
 SYSCTL_INT(_p1003_1b, OID_AUTO, nsems, CTLFLAG_RD, &nsems, 0, "");
 
-static eventhandler_tag sem_exit_tag, sem_exec_tag;
+static eventhandler_tag sem_exit_tag, sem_exec_tag, sem_fork_tag;
 
 #ifdef SEM_DEBUG
 #define DP(x)	printf x
@@ -830,6 +834,112 @@ err:
 	return (error);
 }
 
+/*
+ * Count the number of kusers associated with a proc, so as to guess at how
+ * many to allocate when forking.
+ */
+static int
+sem_count_proc(p)
+	struct proc *p;
+{
+	struct ksem *ks;
+	struct kuser *ku;
+	int count;
+
+	mtx_assert(&sem_lock, MA_OWNED);
+
+	count = 0;
+	LIST_FOREACH(ks, &ksem_head, ks_entry) {
+		LIST_FOREACH(ku, &ks->ks_users, ku_next) {
+			if (ku->ku_pid == p->p_pid)
+				count++;
+		}
+	}
+	LIST_FOREACH(ks, &ksem_deadhead, ks_entry) {
+		LIST_FOREACH(ku, &ks->ks_users, ku_next) {
+			if (ku->ku_pid == p->p_pid)
+				count++;
+		}
+	}
+	return (count);
+}
+
+/*
+ * When a process forks, the child process must gain a reference to each open
+ * semaphore in the parent process, whether it is unlinked or not.  This
+ * requires allocating a kuser structure for each semaphore reference in the
+ * new process.  Because the set of semaphores in the parent can change while
+ * the fork is in progress, we have to handle races -- first we attempt to
+ * allocate enough storage to acquire references to each of the semaphores,
+ * then we enter the semaphores and release the temporary references.
+ */
+static void
+sem_forkhook(arg, p1, p2, flags)
+	void *arg;
+	struct proc *p1;
+	struct proc *p2;
+	int flags;
+{
+	struct ksem *ks, **sem_array;
+	int count, i, new_count;
+	struct kuser *ku;
+
+	mtx_lock(&sem_lock);
+	count = sem_count_proc(p1);
+race_lost:
+	mtx_assert(&sem_lock, MA_OWNED);
+	mtx_unlock(&sem_lock);
+	sem_array = malloc(sizeof(struct ksem *) * count, M_TEMP, M_WAITOK);
+	mtx_lock(&sem_lock);
+	new_count = sem_count_proc(p1);
+	if (count < new_count) {
+		/* Lost race, repeat and allocate more storage. */
+		free(sem_array, M_TEMP);
+		count = new_count;
+		goto race_lost;
+	}
+	/*
+	 * Given an array capable of storing an adequate number of semaphore
+	 * references, now walk the list of semaphores and acquire a new
+	 * reference for any semaphore opened by p1.
+	 */
+	count = new_count;
+	i = 0;
+	LIST_FOREACH(ks, &ksem_head, ks_entry) {
+		LIST_FOREACH(ku, &ks->ks_users, ku_next) {
+			if (ku->ku_pid == p1->p_pid) {
+				sem_ref(ks);
+				sem_array[i] = ks;
+				break;
+			}
+		}
+	}
+	LIST_FOREACH(ks, &ksem_deadhead, ks_entry) {
+		LIST_FOREACH(ku, &ks->ks_users, ku_next) {
+			if (ku->ku_pid == p1->p_pid) {
+				sem_ref(ks);
+				sem_array[i] = ks;
+				break;
+			}
+		}
+	}
+	mtx_unlock(&sem_lock);
+	KASSERT(i + 1 == count, ("sem_forkhook: i != count (%d, %d)", i,
+	    count));
+	/*
+	 * Now cause p2 to enter each of the referenced semaphores, then
+	 * release our temporary reference.  This is pretty inefficient.
+	 * Finally, free our temporary array.
+	 */
+	for (i = 0; i < count; i++) {
+		sem_enter(p2, sem_array[i]);
+		mtx_lock(&sem_lock);
+		sem_rel(sem_array[i]);
+		mtx_unlock(&sem_lock);
+	}
+	free(sem_array, M_TEMP);
+}
+
 static void
 sem_exithook(arg, p)
 	void *arg;
@@ -867,6 +977,7 @@ sem_modload(struct module *module, int cmd, void *arg)
 		    NULL, EVENTHANDLER_PRI_ANY);
 		sem_exec_tag = EVENTHANDLER_REGISTER(process_exec, sem_exithook,
 		    NULL, EVENTHANDLER_PRI_ANY);
+		sem_fork_tag = EVENTHANDLER_REGISTER(process_fork, sem_forkhook, NULL, EVENTHANDLER_PRI_ANY);
                 break;
         case MOD_UNLOAD:
 		if (nsems != 0) {
@@ -875,6 +986,7 @@ sem_modload(struct module *module, int cmd, void *arg)
 		}
 		EVENTHANDLER_DEREGISTER(process_exit, sem_exit_tag);
 		EVENTHANDLER_DEREGISTER(process_exec, sem_exec_tag);
+		EVENTHANDLER_DEREGISTER(process_fork, sem_fork_tag);
 		mtx_destroy(&sem_lock);
                 break;
         case MOD_SHUTDOWN: