Re: [PATCH 2/2] qemu-img convert: Unshare write permission for source

From: Eric Blake <eblake@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org
Cc: xuwei@redhat.com, qemu-devel@nongnu.org
Subject: Re: [PATCH 2/2] qemu-img convert: Unshare write permission for source
Date: Tue, 13 Apr 2021 08:43:35 -0500	[thread overview]
Message-ID: <5edcdaf9-2133-b426-82bb-8849264ff4fd@redhat.com> (raw)
In-Reply-To: <20210413132324.24043-3-kwolf@redhat.com>

On 4/13/21 8:23 AM, Kevin Wolf wrote:
> For a successful conversion of an image, we must make sure that its
> content doesn't change during the conversion.
> 
> A special case of this is using the same image file both as the source
> and as the destination. If both input and output format are raw, the
> operation would just be useless work, with other formats it is a sure
> way to destroy the image. This will now fail because the image file
> can't be opened a second time for the output when opening it for the
> input has already acquired file locks to unshare BLK_PERM_WRITE.
> 
> Nevertheless, if there is some reason in a special case why it is
> actually okay to allow writes to the image while it is being converted,
> -U can still be used to force sharing all permissions.
> 
> Note that for most image formats, BLK_PERM_WRITE would already be
> unshared by the format driver, so this only really makes a difference
> for raw source images (but any output format).
> 
> Reported-by: Xueqiang Wei <xuwei@redhat.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  qemu-img.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

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

> 
> diff --git a/qemu-img.c b/qemu-img.c
> index babb5573ab..a5993682aa 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -2146,7 +2146,7 @@ static void set_rate_limit(BlockBackend *blk, int64_t rate_limit)
>  
>  static int img_convert(int argc, char **argv)
>  {
> -    int c, bs_i, flags, src_flags = 0;
> +    int c, bs_i, flags, src_flags = BDRV_O_NO_SHARE;
>      const char *fmt = NULL, *out_fmt = NULL, *cache = "unsafe",
>                 *src_cache = BDRV_DEFAULT_CACHE, *out_baseimg = NULL,
>                 *out_filename, *out_baseimg_param, *snapshot_name = NULL;
> 

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