Re: [Qemu-devel] [PATCH v3] snapshot: fixed bdrv_get_full_backing_filename can not get correct full_backing_filename

[Eric Blake <eblake@redhat.com>]

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On 05/10/2014 10:35 AM, Jun Li wrote:
> From: Jun Li <junmuzi@gmail.com>

I see three different "[PATCH v3]" mails with this subject.  To make
sure we are reviewing the right version, it might help to bump to version 4.

> 
> This patch fixed the following bug:
> https://bugzilla.redhat.com/show_bug.cgi?id=1084302 .
> 
> path_combine can not calculate the correct full path name for backing_file.
> Such as:
> create a snapshot chain:
> sn2->sn1->$BASE_IMG
> backing file is : /home/wookpecker/img.qcow2
> sn1 : /home/woodpecker/tmp/sn1
> sn2 : /home/woodpecker/tmp/sn2
> when create sn2, path_combine can not got a correct path for $BASE_IMG.

I'm having a hard time parsing that.  We usually represent backing
chains with the base image on the left:

base <- sn1

Can you show the output of 'qemu-img info --backing-chain
/home/woodpecker/tmp/sn2' to make it a bit more obvious what your setup
is?  What command are you issuing that triggers a path_combine that is
getting the wrong result?

> 
> In this patch, will check the backing_filename is a symlink or not firstly,
> then return the full(absolute) path via realpath.

This feels fishy to me, and liable to do the wrong thing.  I need more
context on how to reproduce the issue you are attempting to fix before I
can even decide if your fix is the right approach.

> 
> Signed-off-by: Jun Li <junmuzi@gmail.com>
> ---
>  block.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/block.c b/block.c
> index b749d31..6674719 100644
> --- a/block.c
> +++ b/block.c
> @@ -304,10 +304,26 @@ void path_combine(char *dest, int dest_size,
>  
>  void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz)
>  {
> +    struct stat sb;
> +    char *linkname;
> +
>      if (bs->backing_file[0] == '\0' || path_has_protocol(bs->backing_file)) {
>          pstrcpy(dest, sz, bs->backing_file);
>      } else {
> -        path_combine(dest, sz, bs->filename, bs->backing_file);
> +        if (lstat(bs->backing_file, &sb) == -1) {
> +            perror("lstat");
> +            exit(EXIT_FAILURE);
> +        }
> +
> +        /* Check linkname is a link or not */
> +        if (S_ISLNK(sb.st_mode)) {
> +            linkname = malloc(sb.st_size + 1);
> +            readlink(bs->backing_file, linkname, sb.st_size + 1);
> +            linkname[sb.st_size] = '\0';
> +            realpath(linkname, dest);
> +        } else {
> +            realpath(bs->backing_file, dest);
> +        }

Why are you tweaking just this caller, instead of path_combine() to
affect all other callers?

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


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