Re: [Qemu-devel] [PATCH v3 1/3] raw-posix: Fix raw_co_get_block_status() after EOF

[Eric Blake <eblake@redhat.com>]

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On 10/22/2014 09:57 AM, Max Reitz wrote:
> As its comment states, raw_co_get_block_status() should unconditionally
> return 0 and set *pnum to 0 for after EOF.
> 
> An assertion after lseek(..., SEEK_HOLE) tried to catch this case by
> asserting that errno != -ENXIO (which would indicate a position after
> the EOF); but it should be errno != ENXIO instead. Regardless of that,
> there should be no such assertion at all. If bdrv_getlength() returned
> an outdated value and the image has been resized outside of qemu,
> lseek() will return with errno == ENXIO. Just return that value as an
> error then.
> 
> Setting *pnum to 0 and returning 0 should not be done here, as in that
> case we should update the device length as well. So, from qemu's
> perspective, the file has not been resized; it's just that there was an
> error querying sectors beyond a certain point (the actual file size).
> 
> Additionally, nb_sectors should be clamped against the image end. This
> was probably not an issue if FIEMAP or SEEK_HOLE/SEEK_DATA worked, but
> the fallback did not take this case into account.
> 
> Reported-by: Kevin Wolf <kwolf@redhat.com>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/raw-posix.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

> +    if (total_size < 0) {
> +        return total_size;
> +    } else if (start >= total_size) {
> +        *pnum = 0;
> +        return 0;
> +    } else if (start + nb_sectors * BDRV_SECTOR_SIZE > total_size) {
> +        nb_sectors = (total_size - start) / BDRV_SECTOR_SIZE;

Should this round up instead of truncate?  But it would only matter for
a file size that is not a multiple of sectors, where we probably have
other issues, and where reporting just the full sectors also seems
reasonable.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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