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Tsirkin" Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org References: <20200406124931.120768-1-mst@redhat.com> <045c84ed-151e-a850-9c72-5079bd2775e6@redhat.com> <20200406095424-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Mon, 6 Apr 2020 22:09:27 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20200406095424-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020/4/6 =E4=B8=8B=E5=8D=889:55, Michael S. Tsirkin wrote: > On Mon, Apr 06, 2020 at 09:34:00PM +0800, Jason Wang wrote: >> On 2020/4/6 =E4=B8=8B=E5=8D=888:50, Michael S. Tsirkin wrote: >>> The ring element addresses are passed between components with differe= nt >>> alignments assumptions. Thus, if guest/userspace selects a pointer an= d >>> host then gets and dereferences it, we might need to decrease the >>> compiler-selected alignment to prevent compiler on the host from >>> assuming pointer is aligned. >>> >>> This actually triggers on ARM with -mabi=3Dapcs-gnu - which is a >>> deprecated configuration, but it seems safer to handle this >>> generally. >>> >>> I verified that the produced binary is exactly identical on x86. >>> >>> Signed-off-by: Michael S. Tsirkin >>> --- >>> >>> This is my preferred way to handle the ARM incompatibility issues >>> (in preference to kconfig hacks). >>> I will push this into next now. >>> Comments? >> >> I'm not sure if it's too late to fix. It would still be still problema= tic >> for the userspace that is using old uapi headers? >> >> Thanks > It's not a problem in userspace. The problem is when > userspace/guest uses 2 byte alignment and passes it to kernel > assuming 8 byte alignment. The fix is for host not to > make these assumptions. Yes, but I meant when userspace is complied with apcs-gnu, then it still=20 assumes 8 byte alignment? Thanks > >>> drivers/vhost/vhost.h | 6 ++--- >>> include/uapi/linux/virtio_ring.h | 41 ++++++++++++++++++++++++----= ---- >>> 2 files changed, 34 insertions(+), 13 deletions(-) >>> >>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h >>> index cc82918158d2..a67bda9792ec 100644 >>> --- a/drivers/vhost/vhost.h >>> +++ b/drivers/vhost/vhost.h >>> @@ -74,9 +74,9 @@ struct vhost_virtqueue { >>> /* The actual ring of buffers. */ >>> struct mutex mutex; >>> unsigned int num; >>> - struct vring_desc __user *desc; >>> - struct vring_avail __user *avail; >>> - struct vring_used __user *used; >>> + vring_desc_t __user *desc; >>> + vring_avail_t __user *avail; >>> + vring_used_t __user *used; >>> const struct vhost_iotlb_map *meta_iotlb[VHOST_NUM_ADDRS]; >>> struct vhost_desc *descs; >>> diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/vi= rtio_ring.h >>> index 559f42e73315..cd6e0b2eaf2f 100644 >>> --- a/include/uapi/linux/virtio_ring.h >>> +++ b/include/uapi/linux/virtio_ring.h >>> @@ -118,16 +118,6 @@ struct vring_used { >>> struct vring_used_elem ring[]; >>> }; >>> -struct vring { >>> - unsigned int num; >>> - >>> - struct vring_desc *desc; >>> - >>> - struct vring_avail *avail; >>> - >>> - struct vring_used *used; >>> -}; >>> - >>> /* Alignment requirements for vring elements. >>> * When using pre-virtio 1.0 layout, these fall out naturally. >>> */ >>> @@ -164,6 +154,37 @@ struct vring { >>> #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num]) >>> #define vring_avail_event(vr) (*(__virtio16 *)&(vr)->used->ring[(v= r)->num]) >>> +/* >>> + * The ring element addresses are passed between components with dif= ferent >>> + * alignments assumptions. Thus, we might need to decrease the compi= ler-selected >>> + * alignment, and so must use a typedef to make sure the __aligned a= ttribute >>> + * actually takes hold: >>> + * >>> + * https://gcc.gnu.org/onlinedocs//gcc/Common-Type-Attributes.html#C= ommon-Type-Attributes >>> + * >>> + * When used on a struct, or struct member, the aligned attribute ca= n only >>> + * increase the alignment; in order to decrease it, the packed attri= bute must >>> + * be specified as well. When used as part of a typedef, the aligned= attribute >>> + * can both increase and decrease alignment, and specifying the pack= ed >>> + * attribute generates a warning. >>> + */ >>> +typedef struct vring_desc __attribute__((aligned(VRING_DESC_ALIGN_SI= ZE))) >>> + vring_desc_t; >>> +typedef struct vring_avail __attribute__((aligned(VRING_AVAIL_ALIGN_= SIZE))) >>> + vring_avail_t; >>> +typedef struct vring_used __attribute__((aligned(VRING_USED_ALIGN_SI= ZE))) >>> + vring_used_t; >>> + >>> +struct vring { >>> + unsigned int num; >>> + >>> + vring_desc_t *desc; >>> + >>> + vring_avail_t *avail; >>> + >>> + vring_used_t *used; >>> +}; >>> + >>> static inline void vring_init(struct vring *vr, unsigned int num, = void *p, >>> unsigned long align) >>> {