From 81af503610761a69476a3adbe8341fa8b6d078aa Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Thu, 10 Mar 2011 21:34:19 -0500
Subject: fix sem_open and sem_close to obey posix semantics

multiple opens of the same named semaphore must return the same
pointer, and only the last close can unmap it. thus the ugly global
state keeping track of mappings. the maximum number of distinct named
semaphores that can be opened is limited sufficiently small that the
linear searches take trivial time, especially compared to the syscall
overhead of these functions.
---
 src/thread/sem_open.c | 106 +++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 80 insertions(+), 26 deletions(-)

(limited to 'src/thread/sem_open.c')

diff --git a/src/thread/sem_open.c b/src/thread/sem_open.c
index 55d5ef6b..6fff71a8 100644
--- a/src/thread/sem_open.c
+++ b/src/thread/sem_open.c
@@ -9,32 +9,32 @@
 #include <time.h>
 #include <stdio.h>
 #include <sys/stat.h>
+#include <stdlib.h>
+#include <pthread.h>
 
-static void *find_map(int fd)
+static struct {
+	ino_t ino;
+	sem_t *sem;
+	int refcnt;
+} *semtab;
+
+static int semcnt;
+static pthread_spinlock_t lock;
+static pthread_once_t once;
+
+static void init()
 {
-	char c;
-	struct stat st;
-	FILE *f;
-	void *addr;
-	unsigned long long off, ino;
-	char buf[100];
+	semtab = calloc(sizeof *semtab, SEM_NSEMS_MAX);
+}
 
-	if (fstat(fd, &st) < 0) return 0;
-	if (!(f = fopen("/proc/self/maps", "rb"))) return 0;
-	
-	while (fgets(buf, sizeof buf, f)) {
-		sscanf(buf, "%lx-%*lx %*s %llx %*x:%*x %llu /dev/shm%c",
-			(long *)&addr, &off, &ino, &c);
-		while (!strchr(buf, '\n') && fgets(buf, sizeof buf, f));
-		if (c!='/') continue;
-		c = 0;
-		if (!off && st.st_ino == ino) {
-			fclose(f);
-			return addr;
-		}
-	}
-	fclose(f);
-	return 0;
+static sem_t *find_map(ino_t ino)
+{
+	int i;
+	for (i=0; i<SEM_NSEMS_MAX && semtab[i].ino != ino; i++);
+	if (i==SEM_NSEMS_MAX) return 0;
+	if (semtab[i].refcnt == INT_MAX) return (sem_t *)-1;
+	semtab[i].refcnt++;
+	return semtab[i].sem;
 }
 
 sem_t *sem_open(const char *name, int flags, ...)
@@ -47,6 +47,8 @@ sem_t *sem_open(const char *name, int flags, ...)
 	void *map;
 	char tmp[64];
 	struct timespec ts;
+	struct stat st;
+	int i;
 
 	while (*name=='/') name++;
 	if (strchr(name, '/')) {
@@ -54,6 +56,12 @@ sem_t *sem_open(const char *name, int flags, ...)
 		return SEM_FAILED;
 	}
 
+	pthread_once(&once, init);
+	if (!semtab) {
+		errno = ENOMEM;
+		return SEM_FAILED;
+	}
+
 	if (flags & O_CREAT) {
 		va_start(ap, flags);
 		mode = va_arg(ap, mode_t) & 0666;
@@ -81,6 +89,8 @@ sem_t *sem_open(const char *name, int flags, ...)
 	flags &= ~O_ACCMODE;
 	flags |= O_RDWR;
 
+	pthread_spin_lock(&lock);
+
 	for (;;) {
 		if (!(flags & O_EXCL)) {
 			fd = shm_open(name, flags&~O_CREAT, mode);
@@ -90,9 +100,21 @@ sem_t *sem_open(const char *name, int flags, ...)
 					close(tfd);
 					unlink(tmp);
 				}
-				if (fd < 0) return SEM_FAILED;
-				if ((map = find_map(fd)))
+				if (fstat(fd, &st) < 0) {
+					close(fd);
+					fd = -1;
+				}
+				if (fd < 0) {
+					pthread_spin_unlock(&lock);
+					return SEM_FAILED;
+				}
+				if ((map = find_map(st.st_ino))) {
+					pthread_spin_unlock(&lock);
+					close(fd);
+					if (map == (sem_t *)-1)
+						return SEM_FAILED;
 					return map;
+				}
 				break;
 			}
 		}
@@ -109,8 +131,40 @@ sem_t *sem_open(const char *name, int flags, ...)
 			return SEM_FAILED;
 		}
 	}
+	if (fstat(fd, &st) < 0) {
+		pthread_spin_unlock(&lock);
+		close(fd);
+		return SEM_FAILED;
+	}
+	if (semcnt == SEM_NSEMS_MAX) {
+		pthread_spin_unlock(&lock);
+		close(fd);
+		errno = EMFILE;
+		return SEM_FAILED;
+	}
+	for (i=0; i<SEM_NSEMS_MAX && semtab[i].sem; i++);
 	map = mmap(0, sizeof(sem_t), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
 	close(fd);
-	if (map == MAP_FAILED) return SEM_FAILED;
+	if (map == MAP_FAILED) {
+		pthread_spin_unlock(&lock);
+		return SEM_FAILED;
+	}
+	semtab[i].ino = st.st_ino;
+	semtab[i].sem = map;
+	semtab[i].refcnt = 1;
+	pthread_spin_unlock(&lock);
 	return map;
 }
+
+int sem_close(sem_t *sem)
+{
+	int i;
+	pthread_spin_lock(&lock);
+	for (i=0; i<SEM_NSEMS_MAX && semtab[i].sem != sem; i++);
+	if (!--semtab[i].refcnt) {
+		semtab[i].sem = 0;
+		semtab[i].ino = 0;
+	}
+	pthread_spin_unlock(&lock);
+	return munmap(sem, sizeof *sem);
+}
-- 
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