[Libguestfs] [PATCH nbdkit] xz: Do not pass can_write through to the plugin.

[Eric Blake]

On 1/30/19 8:36 AM, Richard W.M. Jones wrote:
> Using ‘nbdkit --filter=xz file disk.xz’ and loop mounting eventually
> results in this error:
> 
> nbdkit: file[1]: debug: xz: pwrite count=4096 offset=1048576 flags=0x0
> nbdkit: file[1]: debug: pwrite count=4096 offset=1048576 fua=0
> nbdkit: file[1]: error: pwrite: Bad file descriptor
> nbdkit: file[1]: debug: sending error reply: Bad file descriptor
> 
> Since the filter did not intercept can_write, it was passed through to
> the layer below (the file plugin).  The filter opens the file plugin
> passing readonly=1, but in the case where there are no filters, nbdkit
> core would never call can_write() (assuming that because we opened the
> server readonly, it would return false).  However with the filter,
> can_write is called and returns the default (true).  This caused
> writes to be generated which were also passed through to the plugin
> which generates "Bad file descriptor" because:
> 
>  EBADF  fd is not a valid file descriptor or is not open for writing.
> 
> This change also disables zero and trim because the server logic
> disables those when the top backend->can_write returns false.

ACK.

> ---
>  filters/xz/xz.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/filters/xz/xz.c b/filters/xz/xz.c
> index 28a6a81..5ff791e 100644
> --- a/filters/xz/xz.c
> +++ b/filters/xz/xz.c
> @@ -173,6 +173,13 @@ xz_get_size (struct nbdkit_next_ops *next_ops, void *nxdata, void *handle)
>    return xzfile_get_size (h->xz);
>  }
>  
> +static int
> +xz_can_write (struct nbdkit_next_ops *next_ops, void *nxdata,
> +              void *handle)

Perhaps worth a comment in the code, similar to cow.c's comment about
the override returning a constant because the filter enforces a
particular behavior that might be different from the backend's default
behavior.

> +{
> +  return 0;
> +}
> +
>  /* Read data from the file. */
>  static int
>  xz_pread (struct nbdkit_next_ops *next_ops, void *nxdata,
> @@ -225,6 +232,7 @@ static struct nbdkit_filter filter = {
>    .close             = xz_close,
>    .prepare           = xz_prepare,
>    .get_size          = xz_get_size,
> +  .can_write         = xz_can_write,
>    .pread             = xz_pread,
>  };
>  
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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