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    SubjectRe: [PATCH RFC] uaccess: user_access_begin_after_access_ok()
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    On 2020/6/3 下午2:30, Michael S. Tsirkin wrote:
    > On Wed, Jun 03, 2020 at 02:23:08PM +0800, Jason Wang wrote:
    >>> BTW now I re-read it I don't understand __vhost_get_user_slow:
    >>>
    >>>
    >>> static void __user *__vhost_get_user_slow(struct vhost_virtqueue *vq,
    >>> void __user *addr, unsigned int size,
    >>> int type)
    >>> {
    >>> int ret;
    >>>
    >>> ret = translate_desc(vq, (u64)(uintptr_t)addr, size, vq->iotlb_iov,
    >>> ARRAY_SIZE(vq->iotlb_iov),
    >>> VHOST_ACCESS_RO);
    >>>
    >>> ..
    >>> }
    >>>
    >>> how does this work? how can we cast a pointer to guest address without
    >>> adding any offsets?
    >>
    >> I'm not sure I get you here. What kind of offset did you mean?
    >>
    >> Thanks
    > OK so points:
    >
    > 1. type argument seems unused. Right?


    Yes, we can remove that.


    > 2. Second argument to translate_desc is a GPA, isn't it?


    No, it's IOVA, this function will be called only when IOTLB is enabled.

    Thanks


    > Here we cast a userspace address to this type. What if it
    > matches a valid GPA by mistake?
    >

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