Re: [PATCH v6] libxl: allow 'phy' backend to use empty files

From: Ian Jackson <Ian.Jackson@eu.citrix.com>
To: Roger Pau Monne <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org, Wei Liu <wei.liu2@citrix.com>,
	Ian Campbell <ian.campbell@citrix.com>,
	Alex Braunegg <alex.braunegg@gmail.com>
Subject: Re: [PATCH v6] libxl: allow 'phy' backend to use empty files
Date: Fri, 19 Feb 2016 17:30:53 +0000	[thread overview]
Message-ID: <22215.20813.59866.409199@mariner.uk.xensource.com> (raw)
In-Reply-To: <1455729638-41937-1-git-send-email-roger.pau@citrix.com>

Roger Pau Monne writes ("[PATCH v6] libxl: allow 'phy' backend to use empty files"):
> This was introduced by 97ee1f (~5 years ago), but was probably never
> surfaced because most people used regular files as CDROM images, so the PHY
> backend was actually never selected. A year ago this was changed, and now
> regular RAW files are also handled by the PHY backend, which has made this
> bug surface.
> 
> Fix it by allowing empty disks to use the PHY backend, skipping the stat
> tests.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> Reported-by: Alex Braunegg <alex.braunegg@gmail.com>

Thanks, and thanks to Alex for the testing, but:

> diff --git a/tools/libxl/libxl_device.c b/tools/libxl/libxl_device.c
> index 8bb5e93..2e08108 100644
> --- a/tools/libxl/libxl_device.c
> +++ b/tools/libxl/libxl_device.c
> @@ -196,6 +196,14 @@ static int disk_try_backend(disk_try_backend_args *a,
>              goto bad_format;
>          }
>  
> +        if (a->disk->format == LIBXL_DISK_FORMAT_EMPTY) {
> +            assert(a->disk->pdev_path == NULL ||
> +                   !strcmp(a->disk->pdev_path, ""));

I agree that these things ought to be true but I don't see what code
in libxl ensures that they definitely are.

I think this check ought to be moved to libxl__device_disk_set_backend
or perhaps even earlier, and should generate an ERROR_INVAL rather
than an assertion failure.

The rest of the logic LGTM.

Thanks,
Ian.