SYSV semaphore adjustment is not done correctly for arrays with more than one semathore.


#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/sem.h>


#define BUG1
#define BUG2


#define SEMKEY 1


union semun {
    int val;                    /* value for SETVAL */
    struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */
    unsigned short  *array;     /* array for GETALL, SETALL */
};


static void doCriticalWork(void)
{
    return;
}


int main(void)
{
    struct sembuf lock[2];
    int n, semid;

    if ((semid = semget(SEMKEY, 2, IPC_CREAT | 0666)) < 0) {
        perror("semget(IPC_CREATE)");
        return 1;
    }

#ifdef BUG1
    lock[0].sem_num = 0;
    lock[0].sem_op  = 0;
    lock[0].sem_flg = IPC_NOWAIT;
    lock[1].sem_num = 0;
    lock[1].sem_op  = 1;

#ifdef BUG2
    lock[1].sem_flg = SEM_UNDO;
#else
    lock[1].sem_flg = 0;
#endif

    if (semop(semid, lock, 2) != 0) {
        perror("semop(LOCK[0])");
        return 1;
    }
#endif

    for (n = 0; ; ++n) {
        static const union semun arg = { 0 };
        int error;

        printf("Iteration %d\n", n);

        lock[0].sem_num = 1;
        lock[0].sem_op  = 0; /* precondition:  [1] == 0 */
        lock[0].sem_flg = 0;
        lock[1].sem_num = 1;
        lock[1].sem_op  = 1; /* postcondition: [1] == 1 */
        lock[1].sem_flg = SEM_UNDO;

        if (semop(semid, lock, 2) < 0) {
            error = errno;
            fprintf(stderr, "semop(LOCK[1]): %d, %s\n",
                    error, strerror(error));
            break;
        }

        doCriticalWork();

#if 1
        if (semctl(semid, 1, SETVAL, arg) < 0) {
            error = errno;
            fprintf(stderr, "semctl(UNLOCK[1]): %d, %s\n",
                    error, strerror(error));
            break;
        }
#else
        lock[0].sem_num =  1;
        lock[0].sem_op  = -1;
        lock[0].sem_flg =  SEM_UNDO | IPC_NOWAIT;

        if (semop(semid, lock, 1) < 0) {
            error = errno;
            fprintf(stderr, "semop(UNLOCK[1]): %d, %s\n",
                    error, strerror(error));
            break;
        }
#endif

    }

    return 1;
}

Fix: 

The probable cause is the implementation semundo_clear() found in
sys/kern/sysv_sem.c .  FYI, originally the bug was found on CYGWIN, which
adopts the similar FreeBSD code to implement the semaphore arrays.
How-To-Repeat: Compile and run the code shown in the problem description.  It will stop
at iteration 16384 suggesting the UNDO adjustment value overflow with
ERANGE (which should have not happened because SETVAL should have cleared
all the UNDO adjustments on each loop iteration).