Libguestfs - Jul 2023 - [PATCH guestfs-tools] diff: Don't compare st_dev or st_ino fields

See comment for details.

Link: https://listman.redhat.com/archives/libguestfs/2023-July/032061.html
---
 diff/diff.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/diff/diff.c b/diff/diff.c
index fb66a2bbd3..b6344b4ec2 100644
--- a/diff/diff.c
+++ b/diff/diff.c
@@ -717,8 +717,20 @@ changed (guestfs_h *g1, struct file *file1,
     output_string ("changed:");
 #define COMPARE_STAT(n)						\
     if (file1->stat->n != file2->stat->n) output_string (#n)
-    COMPARE_STAT (st_dev);
-    COMPARE_STAT (st_ino);
+    /* Comparing st_dev and st_ino is disabled for now, see the longer
+     * discussion here:
+     * https://listman.redhat.com/archives/libguestfs/2023-July/032061.html
+     * Even if we fixed the libguestfs API to do translation of this
+     * field correctly, it seems unlikely that there would ever be a
+     * meaningful difference in the st_dev or st_ino fields.  We
+     * already know the fields refer to the same filename.  Is it
+     * interesting that the file might have moved to a different disk?
+     * Everything else is comparing the content or direct metadata of
+     * the file, but st_dev and st_ino represent the metadata of the
+     * filesystem which is (arguably) different.
+     */
+//    COMPARE_STAT (st_dev);
+//    COMPARE_STAT (st_ino);
     COMPARE_STAT (st_mode);
     COMPARE_STAT (st_nlink);
     COMPARE_STAT (st_uid);
-- 
2.41.0

On 7/20/23 16:15, Richard W.M. Jones wrote:
> See comment for details.
> 
> Link: https://listman.redhat.com/archives/libguestfs/2023-July/032061.html
> ---
>  diff/diff.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/diff/diff.c b/diff/diff.c
> index fb66a2bbd3..b6344b4ec2 100644
> --- a/diff/diff.c
> +++ b/diff/diff.c
> @@ -717,8 +717,20 @@ changed (guestfs_h *g1, struct file *file1,
>      output_string ("changed:");
>  #define COMPARE_STAT(n)						\
>      if (file1->stat->n != file2->stat->n) output_string (#n)
> -    COMPARE_STAT (st_dev);
> -    COMPARE_STAT (st_ino);
> +    /* Comparing st_dev and st_ino is disabled for now, see the longer
> +     * discussion here:
> +     *
https://listman.redhat.com/archives/libguestfs/2023-July/032061.html
> +     * Even if we fixed the libguestfs API to do translation of this
> +     * field correctly, it seems unlikely that there would ever be a
> +     * meaningful difference in the st_dev or st_ino fields.  We
> +     * already know the fields refer to the same filename.  Is it
> +     * interesting that the file might have moved to a different disk?
> +     * Everything else is comparing the content or direct metadata of
> +     * the file, but st_dev and st_ino represent the metadata of the
> +     * filesystem which is (arguably) different.
> +     */
> +//    COMPARE_STAT (st_dev);
> +//    COMPARE_STAT (st_ino);
>      COMPARE_STAT (st_mode);
>      COMPARE_STAT (st_nlink);
>      COMPARE_STAT (st_uid);
This seems to remain compatible with the existent virt-diff(1) manual,
which says:

https://libguestfs.org/virt-diff.1.html
> This tool will find differences in filenames, file sizes, checksums,
> extended attributes, file content and more from a virtual machine or
> disk image. However it does not look at the boot loader, unused space
> between partitions or within filesystems, "hidden" sectors and so
on.
> In other words, it is not a security or forensics tool.
Acked-by: Laszlo Ersek <lersek at redhat.com>

Laszlo