[libvirt] [PATCH 2/4] qemu: report error if disk backing files doesn't exist

[Martin Kletzander]

On 07/18/2013 01:32 PM, Guannan Ren wrote:

s/doesn't/don't/ in $SUBJ

> Adding virFileAccessibleAs() to check if the backing file described in
> disk meta exist in real path. If not, report error. The uid and gid
> arguments don't take effect on F_OK mode for access, so use gid and gid
> of current process.
> 

This patch doesn't break anything new, but thanks to the
getuid()/getgid(), it leaves the previous problem in the code.  Even
though F_OK doesn't need uid/gid to check whether the file exists, root
may not have permissions for upper directories on NFS storage for
example, so ...

> ---
>  src/util/virstoragefile.c | 23 +++++++++++++++--------
>  tests/virstoragetest.c    | 16 ++++++++--------
>  2 files changed, 23 insertions(+), 16 deletions(-)
> 
> diff --git a/src/util/virstoragefile.c b/src/util/virstoragefile.c
> index cb6df91..b678eb8 100644
> --- a/src/util/virstoragefile.c
> +++ b/src/util/virstoragefile.c
> @@ -572,6 +572,13 @@ virFindBackingFile(const char *start, bool start_is_dir, const char *path,
>          goto cleanup;
>      }
>  
> +    if (virFileAccessibleAs(combined, F_OK, getuid(), getgid()) < 0) {
> +        virReportSystemError(errno,
> +                             _("Backing file '%s' does not exist"),
> +                             combined);
> +        goto cleanup;
> +    }
> +
>      if (!(*canonical = canonicalize_file_name(combined))) {
>          virReportSystemError(errno,
>                               _("Can't canonicalize path '%s'"), path);

... this hunk does make the code report better errors, but in the future
it should canonicalize the filename using root/qemu/domain users.

ACK to this hunk, with the error reworded (e.g. "Cannot access backing
file %s") and, of course, commit message changed appropriately, but ...

> @@ -857,14 +864,10 @@ virStorageFileGetMetadataInternal(const char *path,
>                                         !!directory, backing,
>                                         &meta->directory,
>                                         &meta->backingStore) < 0) {
> -                    /* the backing file is (currently) unavailable, treat this
> -                     * file as standalone:
> -                     * backingStoreRaw is kept to mark broken image chains */
> -                    meta->backingStoreIsFile = false;
> -                    backingFormat = VIR_STORAGE_FILE_NONE;
> -                    VIR_WARN("Backing file '%s' of image '%s' is missing.",
> -                             meta->backingStoreRaw, path);
> -
> +                    VIR_FREE(backing);
> +                    VIR_ERROR(_("Backing file '%s' of image '%s' is missing."),
> +                              meta->backingStoreRaw, path);
> +                    goto cleanup;

To fix a pre-existing error, we should (instead of this change) just add
virResetLastError() here as the error is used somewhere else in the code
and should be kept in virFindBackingFile().  Having it in the logs seems
OK to me.

>                  }
>              }
>              VIR_FREE(backing);

... NACK to the rest.  As written in the comment you are removing,
backingStoreRaw is kept to mark broken chains.  And that is the thing we
should use to check whether the backing chain is broken, not throw away
all the data we've got.

> @@ -1047,6 +1050,10 @@ virStorageFileGetMetadataRecurse(const char *path, const char *directory,
>                                                              uid, gid,
>                                                              allow_probe,
>                                                              cycle);
> +        if (!ret->backingMeta) {
> +            virStorageFileFreeMetadata(ret);
> +            ret = NULL;
> +        }
>      }
>  
>      return ret;

That means you can drop this hunk too, the tests as well, and the
detection of broken chains could be added in a separate function (just a
cycle checking for that) and called when we want to know whether the
chain is broken.

Martin