ipc/sem.c: avoid overflow of semop undo (semadj) value

When trying to understand semop code, I found a small mistake in the check
for semadj (undo) value overflow.  The new undo value is not stored
immediately and next potential checks are done against the old value.

The failing scenario is not much practical.  One semop call has to do more
operations on the same semaphore.  Also semval and semadj must have
different values, so there has to be some operations without SEM_UNDO
flag.  For example:

	struct sembuf depositor_op[1];
	struct sembuf collector_op[2];

	depositor_op[0].sem_num = 0;
	depositor_op[0].sem_op = 20000;
	depositor_op[0].sem_flg = 0;

	collector_op[0].sem_num = 0;
	collector_op[0].sem_op = -10000;
	collector_op[0].sem_flg = SEM_UNDO;
	collector_op[1].sem_num = 0;
	collector_op[1].sem_op = -10000;
	collector_op[1].sem_flg = SEM_UNDO;

	if (semop(semid, depositor_op, 1) == -1)
		{ perror("Failed to do 1st deposit"); return 1; }

	if (semop(semid, collector_op, 2) == -1)
		{ perror("Failed to do 1st collect"); return 1; }

	if (semop(semid, depositor_op, 1) == -1)
		{ perror("Failed to do 2nd deposit"); return 1; }

	if (semop(semid, collector_op, 2) == -1)
		{ perror("Failed to do 2nd collect"); return 1; }

	return 0;

It passes without error now but the semadj value has overflown in the 2nd
collector operation.

Signed-off-by: Petr Mladek <pmladek@suse.cz>
Acked-by: Davidlohr Bueso <davidlohr@hp.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/ipc/sem.c b/ipc/sem.c
index db9d241af133d770cb0a95a22cacf257f79b1215..0d437576144974ebb667e8858884b0f00c02bc83 100644 (file)
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -599,7 +599,7 @@ SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg)
 static int perform_atomic_semop(struct sem_array *sma, struct sembuf *sops,
                             int nsops, struct sem_undo *un, int pid)
 {
-       int result, sem_op;
+       int result, undo, sem_op;
        struct sembuf *sop;
        struct sem * curr;
 
@@ -607,7 +607,7 @@ static int perform_atomic_semop(struct sem_array *sma, struct sembuf *sops,
                curr = sma->sem_base + sop->sem_num;
                sem_op = sop->sem_op;
                result = curr->semval;
-  
+
                if (!sem_op && result)
                        goto would_block;
 
@@ -616,25 +616,24 @@ static int perform_atomic_semop(struct sem_array *sma, struct sembuf *sops,
                        goto would_block;
                if (result > SEMVMX)
                        goto out_of_range;
+
                if (sop->sem_flg & SEM_UNDO) {
-                       int undo = un->semadj[sop->sem_num] - sem_op;
-                       /*
-                        *      Exceeding the undo range is an error.
-                        */
+                       undo = un->semadj[sop->sem_num] - sem_op;
+                       /* Exceeding the undo range is an error. */
                        if (undo < (-SEMAEM - 1) || undo > SEMAEM)
                                goto out_of_range;
+                       un->semadj[sop->sem_num] = undo;
                }
+
                curr->semval = result;
        }
 
        sop--;
        while (sop >= sops) {
                sma->sem_base[sop->sem_num].sempid = pid;
-               if (sop->sem_flg & SEM_UNDO)
-                       un->semadj[sop->sem_num] -= sop->sem_op;
                sop--;
        }
-       
+
        return 0;
 
 out_of_range:
@@ -650,7 +649,10 @@ would_block:
 undo:
        sop--;
        while (sop >= sops) {
-               sma->sem_base[sop->sem_num].semval -= sop->sem_op;
+               sem_op = sop->sem_op;
+               sma->sem_base[sop->sem_num].semval -= sem_op;
+               if (sop->sem_flg & SEM_UNDO)
+                       un->semadj[sop->sem_num] += sem_op;
                sop--;
        }