Re: [Qemu-devel] [PATCH v6 2/3] block: Fix NULL deference for unaligned write if qiov is NULL

[Kevin Wolf <kwolf@redhat.com>]

Am 12.05.2015 um 08:09 hat Fam Zheng geschrieben:
> For zero write, callers pass in NULL qiov (qemu-io "write -z" or
> scsi-disk "write same").
> 
> Commit fc3959e466 fixed bdrv_co_write_zeroes which is the common case
> for this bug, but it still exists in bdrv_aio_write_zeroes. A simpler
> fix would be in bdrv_co_do_pwritev which is the NULL dereference point
> and covers both cases.
> 
> So don't access it in bdrv_co_do_pwritev in this case, use three aligned
> writes.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/io.c | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 95 insertions(+)
> 
> diff --git a/block/io.c b/block/io.c
> index 4e5a92e..d766220 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1174,6 +1174,97 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
>      return ret;
>  }
>  
> +static int coroutine_fn bdrv_co_do_zero_pwritev(BlockDriverState *bs,
> +                                                int64_t offset,
> +                                                unsigned int bytes,
> +                                                BdrvRequestFlags flags)
> +{
> +    BdrvTrackedRequest req;
> +    uint8_t *buf = NULL;
> +    QEMUIOVector local_qiov;
> +    struct iovec iov;
> +    uint64_t align = MAX(BDRV_SECTOR_SIZE, bs->request_alignment);
> +    unsigned int head_padding_bytes, tail_padding_bytes;
> +    int ret;
> +
> +    head_padding_bytes = offset & (align - 1);
> +    tail_padding_bytes = align - ((offset + bytes) & (align - 1));

Don't we have macros for these calculations?

> +    tracked_request_begin(&req, bs, offset, bytes, true);

Why duplicate this when it would already be the next line in
bdrv_co_do_pwritev()?

> +    mark_request_serialising(&req, align);
> +    wait_serialising_requests(&req);

So this patch serialises all zero writes, even if they are perfectly
aligned? Why?

Actually, even for misaligned requests, I think the part in the middle
doesn't require any serialisation, only the RMW parts do.

> +    assert(flags & BDRV_REQ_ZERO_WRITE);
> +    if (head_padding_bytes || tail_padding_bytes) {
> +        buf = qemu_blockalign(bs, align);
> +        iov = (struct iovec) {
> +            .iov_base   = buf,
> +            .iov_len    = align,
> +        };
> +        qemu_iovec_init_external(&local_qiov, &iov, 1);
> +    }
> +    if (head_padding_bytes) {
> +        uint64_t zero_bytes = MIN(bytes, align - head_padding_bytes);
> +
> +        /* RMW the unaligned part before head. */
> +        BLKDBG_EVENT(bs, BLKDBG_PWRITEV_RMW_HEAD);
> +        ret = bdrv_aligned_preadv(bs, &req, offset & ~(align - 1), align,
> +                                  align, &local_qiov, 0);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +        BLKDBG_EVENT(bs, BLKDBG_PWRITEV_RMW_AFTER_HEAD);
> +
> +        memset(buf + head_padding_bytes, 0, zero_bytes);
> +        ret = bdrv_aligned_pwritev(bs, &req, offset & ~(align - 1), align,
> +                                   &local_qiov,
> +                                   flags & ~BDRV_REQ_ZERO_WRITE);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +        offset += zero_bytes;
> +        bytes -= zero_bytes;
> +    }
> +
> +    assert((offset & (align - 1)) == 0);
> +    if (bytes >= align) {
> +        /* Write the aligned part in the middle. */
> +        uint64_t aligned_bytes = bytes & ~(align - 1);
> +        ret = bdrv_aligned_pwritev(bs, &req, offset, aligned_bytes,
> +                                   NULL, flags);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +        bytes -= aligned_bytes;
> +        offset += aligned_bytes;
> +    }
> +
> +    assert((offset & (align - 1)) == 0);
> +    if (bytes) {
> +        assert(align == tail_padding_bytes + bytes);
> +        /* RMW the unaligned part after tail. */
> +        BLKDBG_EVENT(bs, BLKDBG_PWRITEV_RMW_TAIL);
> +        ret = bdrv_aligned_preadv(bs, &req, offset, align,
> +                                  align, &local_qiov, 0);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +        BLKDBG_EVENT(bs, BLKDBG_PWRITEV_RMW_AFTER_TAIL);
> +
> +        memset(buf, 0, bytes);
> +        printf("tail part %ld %d\n", offset, bytes);
> +        ret = bdrv_aligned_pwritev(bs, &req, offset, align,
> +                                   &local_qiov, flags & ~BDRV_REQ_ZERO_WRITE);
> +    }
> +fail:
> +    tracked_request_end(&req);
> +    if (buf) {
> +        qemu_vfree(buf);
> +    }
> +    return ret;
> +
> +}
> +
>  /*
>   * Handle a write request in coroutine context
>   */
> @@ -1207,6 +1298,10 @@ static int coroutine_fn bdrv_co_do_pwritev(BlockDriverState *bs,
>          bdrv_io_limits_intercept(bs, bytes, true);
>      }
>  
> +    if (!qiov) {
> +        return bdrv_co_do_zero_pwritev(bs, offset, bytes, flags);
> +    }
> +
>      /*
>       * Align write if necessary by performing a read-modify-write cycle.
>       * Pad qiov with the read parts and be sure to have a tracked request not

Kevin