Re: [Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

[Kevin Wolf <kwolf@redhat.com>]

Am 11.08.2017 um 10:09 hat Fam Zheng geschrieben:
> People get surprised when, after "qemu-img create -f raw /dev/sdX", they
> still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
> header. While this is natural because raw doesn't need to write any
> magic bytes during creation, hdev_create is free to clear out the first
> sector to make sure the stale qcow2 header doesn't cause such confusion.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> 
> ---
> 
> v2: Use stack allocated buffer. [Eric]
>     Fix return value.
>     (Keep qemu_write_full instead of switching to qemu_pwritev because
>     the former handles short writes.)
>     Fix typo "qemu-img". [Changlong]
> ---
>  block/file-posix.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index f4de022ae0..a63bbf2b90 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2703,6 +2703,16 @@ static int hdev_create(const char *filename, QemuOpts *opts,
>          ret = -ENOSPC;
>      }

So the error paths above only set ret, but don't actually return or jump
to the end of the function.

> +    if (total_size) {
> +        uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
> +        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
> +        if (lseek(fd, 0, SEEK_SET) == -1) {
> +            ret = -errno;
> +        } else {
> +            ret = qemu_write_full(fd, buf, zero_size);
> +            ret = ret == zero_size ? 0 : -errno;

Which means that an error above (like a too small block device or using
a regular file) can be overwritten with a success value if clearing the
first sector works. That's probably not quite right.

> +        }
> +    }
>      qemu_close(fd);
>      return ret;
>  }

Kevin