Re: [Qemu-devel] [PATCH v2 3/4] raw-posix: Fix try_seek_hole()'s handling of SEEK_DATA failure

[Kevin Wolf <kwolf@redhat.com>]

Am 13.11.2014 um 11:17 hat Markus Armbruster geschrieben:
> When SEEK_HOLE tells us we're in a hole, we try SEEK_DATA to find its
> end.  When that fails, we pretend the hole extends to the end of file.
> Wrong.

Wrong only in some cases, see below.

> Except when SEEK_END fails, we screw up and claim it extends
> to offset -1.  More wrong.
> 
> Fortunately, these seeks are very unlikely to fail.  Fix it anyway, by
> returning failure.  The caller will then pretend there are no holes.
> Inaccurate, but safe.
> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  block/raw-posix.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index fd80d84..2a12a50 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -1494,8 +1494,9 @@ static int try_seek_hole(BlockDriverState *bs, off_t start, off_t *data,
>      } else {
>          /* On a hole.  We need another syscall to find its end.  */
>          *data = lseek(s->fd, start, SEEK_DATA);
> -        if (*data == -1) {
> -            *data = lseek(s->fd, 0, SEEK_END);
> +        if (*data < 0) {
> +            /* no idea where the hole ends, give up (unlikely to happen) */

Not quite unlikely. If the file ends with a sparse area, we'll get
-1/ENXIO here.

lseek() with SEEK_DATA starting in a hole when there is no data until
EOF is actually the part that isn't documented in the man page, but
ENXIO is what I'm seeing here on RHEL 7.

> +            return -errno;
>          }
>      }

Kevin