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--- arm_kernel_panic [2018/11/28 12:55]
rpjday [Disassembling ''futex_wait_setup'']
+++ arm_kernel_panic [2018/11/28 13:02]
rpjday [The offending(?) kernbel code]
@@ Line 29: / Line 29: @@
   * register ''r5'' contains the value ''00000002''
+===== The offending(?) kernel code =====
+From ''kernel/futex.c'':
+<code>
+/* The key must be already stored in q->key. */
+static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
+        __acquires(&hb->lock)
+{
+        struct futex_hash_bucket *hb;
+        hb = hash_futex(&q->key);
+        /*
+         * Increment the counter before taking the lock so that
+         * a potential waker won't miss a to-be-slept task that is
+         * waiting for the spinlock. This is safe as all queue_lock()
+         * users end up calling queue_me(). Similarly, for housekeeping,
+         * decrement the counter at queue_unlock() when some error has
+         * occurred and we don't end up adding the task to the list.
+         */
+        hb_waiters_inc(hb);
+        q->lock_ptr = &hb->lock;
+        spin_lock(&hb->lock); /* implies MB (A) */
+        return hb;
+}
+</code>
+<code>
+/**
+ * futex_wait_setup() - Prepare to wait on a futex
+ * @uaddr:      the futex userspace address
+ * @val:        the expected value
+ * @flags:      futex flags (FLAGS_SHARED, etc.)
+ * @q:          the associated futex_q
+ * @hb:         storage for hash_bucket pointer to be returned to caller
+ *
+ * Setup the futex_q and locate the hash_bucket.  Get the futex value and
+ * compare it with the expected value.  Handle atomic faults internally.
+ * Return with the hb lock held and a q.key reference on success, and unlocked
+ * with no q.key reference on failure.
+ *
+ * Return:
+ *  0 - uaddr contains val and hb has been locked;
+ * <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
+ */
+static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
+                           struct futex_q *q, struct futex_hash_bucket **hb)
+{
+        u32 uval;
+        int ret;
+        /*
+         * Access the page AFTER the hash-bucket is locked.
+         * Order is important:
+         *
+         *   Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
+         *   Userspace waker:  if (cond(var)) { var = new; futex_wake(&var); }
+         *
+         * The basic logical guarantee of a futex is that it blocks ONLY
+         * if cond(var) is known to be true at the time of blocking, for
+         * any cond.  If we locked the hash-bucket after testing *uaddr, that
+         * would open a race condition where we could block indefinitely with
+         * cond(var) false, which would violate the guarantee.
+         *
+         * On the other hand, we insert q and release the hash-bucket only
+         * after testing *uaddr.  This guarantees that futex_wait() will NOT
+         * absorb a wakeup if *uaddr does not match the desired values
+         * while the syscall executes.
+         */
+retry:
+        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key, VERIFY_READ);
+        if (unlikely(ret != 0))
+                return ret;
+retry_private:
+        *hb = queue_lock(q);
+        ret = get_futex_value_locked(&uval, uaddr);
+        if (ret) {
+                queue_unlock(*hb);
+                ret = get_user(uval, uaddr);
+                if (ret)
+                        goto out;
+                if (!(flags & FLAGS_SHARED))
+                        goto retry_private;
+                put_futex_key(&q->key);
+                goto retry;
+        }
+        if (uval != val) {
+                queue_unlock(*hb);
+                ret = -EWOULDBLOCK;
+        }
+out:
+        if (ret)
+                put_futex_key(&q->key);
+        return ret;
+}
+</code>
 ===== Disassembling ''futex_wait_setup'' =====

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