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Re: [Libguestfs] [PATCH] Correctly detect the size of a block device over SSH



On Wed, Aug 04, 2010 at 04:14:45PM +0100, Matthew Booth wrote:
> When fetching a storage volume over SSH, we were detecting its size with a
> simple 'stat -c %s'. While this works fine for files, it won't return the
> correct size of a block device.
> 
> This patch uses the output of 'blockdev --getsize64' for block devices, and stat
> as before for regular files.
> 
> Fixes RHBZ#620698
> ---
>  lib/Sys/VirtV2V/Transfer/SSH.pm |   16 +++++++++++++++-
>  1 files changed, 15 insertions(+), 1 deletions(-)
> 
> diff --git a/lib/Sys/VirtV2V/Transfer/SSH.pm b/lib/Sys/VirtV2V/Transfer/SSH.pm
> index 66ec294..5f54f0e 100644
> --- a/lib/Sys/VirtV2V/Transfer/SSH.pm
> +++ b/lib/Sys/VirtV2V/Transfer/SSH.pm
> @@ -138,7 +138,21 @@ sub _connect
>          push(@command, 'ssh');
>          push(@command, '-l', $username) if (defined($username));
>          push(@command, $host);
> -        push(@command, "stat -c %s $path; cat $path");
> +
> +        # Return the size of the remote path on the first line, followed by its
> +        # contents.
> +        # The bit arithmetic with the output of stat is a translation into shell
> +        # of the S_ISBLK macro. If the remote device is a block device, stat
> +        # will simply return the size of the block device inode. In this case,
> +        # we use the output of blockdev --getsize64 instead.
> +        push(@command,
> +            "dev=$path; ".
> +            'if [[ $(((0x$(stat -L -c %f $dev)&0170000)>>12)) == 6 ]]; then '.
> +              'blockdev --getsize64 $dev; '.
> +            'else '.
> +              'stat -L -c %s $dev; '.
> +            'fi; '.
> +            'cat $dev');
>  
>          # Close the ends of the pipes we don't need
>          close($stdin_write);

Yikes.

Not an easier way to ask if a filename is a block device?  Like
maybe: test -b $dev

ACK either way.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://et.redhat.com/~rjones/libguestfs/
See what it can do: http://et.redhat.com/~rjones/libguestfs/recipes.html


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