Re: [Qemu-devel] [PATCH 1/2] block: make bdrv_find_backing_image compare canonical filenames

[Eric Blake <eblake@redhat.com>]

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On 10/09/2012 11:56 PM, Jeff Cody wrote:
> Currently, bdrv_find_backing_image compares bs->backing_file with
> what is passed in as a backing_file name.  Mismatches may occur,
> however, when bs->backing_file and backing_file are both not
> absolute or relative.
> 
> Use path_combine() to make sure any relative backing filenames are
> relative to the current image filename being searched, and then use
> realpath() to make all comparisons based on absolute filenames.
> 
> This also changes bdrv_find_backing_image to no longer be recursive,
> but iterative.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  block.c | 38 ++++++++++++++++++++++++++++++++------
>  1 file changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/block.c b/block.c
> index e95f613..641b8fa 100644
> --- a/block.c
> +++ b/block.c
> @@ -3123,18 +3123,44 @@ int bdrv_snapshot_load_tmp(BlockDriverState *bs,
>      return -ENOTSUP;
>  }
>  
> +/* backing_file can either be relative, or absolute.  If it is
> + * relative, it must be relative to the chain.  So, passing in
> + * bs->filename from a BDS as backing_file should not be done,
> + * as that may be relative to the CWD rather than the chain. */
>  BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>          const char *backing_file)
>  {
> -    if (!bs->drv) {
> +    char filename_full[PATH_MAX];
> +    char backing_file_full[PATH_MAX];
> +    char filename_tmp[PATH_MAX];

That's a LOT of stack space, which risks stack overflow, will mostly be
unused, and still doesn't work if you have super-deep hierarchies larger
than PATH_MAX.  Would you be better off using realpath(,NULL) for its
allocating semantics, and then free()ing the results?

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


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