Re: [RFC][PATCH 3/3] ovl: use persistent s_uuid with index=on

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On Tue, Apr 25, 2023 at 4:22 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
> With index=on, overlayfs instances are non-migratable, meaning that
> the layers cannot be copied without breaking the index.
>
> So when indexdir exists, store a persistent uuid in xattr on the
> indexdir to give the overlayfs instance a persistent identifier.
>
> This also makes f_fsid persistent and more reliable for reporting
> fid info in fanotify events.
>
> With mount option uuid=nogen, a persistent uuid is not be initialized
> on indexdir, but if a persistent uuid already exists, it will be used.
>

This behavior (along with the grammatical mistakes) was changed in
https://github.com/amir73il/linux/commits/ovl_encode_fid

uuid=off or uuid=null both set ovl fsid to null regardless of persistent
uuid xattr.

Whether we need this backward compact option or not is to be determined.

> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> ---
>  fs/overlayfs/overlayfs.h |  3 +++
>  fs/overlayfs/super.c     |  7 +++++++
>  fs/overlayfs/util.c      | 41 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 51 insertions(+)
>
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index dcdb02d0ddf8..9927472a3aaa 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -36,6 +36,7 @@ enum ovl_xattr {
>         OVL_XATTR_IMPURE,
>         OVL_XATTR_NLINK,
>         OVL_XATTR_UPPER,
> +       OVL_XATTR_UUID,
>         OVL_XATTR_METACOPY,
>         OVL_XATTR_PROTATTR,
>  };
> @@ -431,6 +432,8 @@ bool ovl_already_copied_up(struct dentry *dentry, int flags);
>  bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
>                               enum ovl_xattr ox);
>  bool ovl_path_check_origin_xattr(struct ovl_fs *ofs, const struct path *path);
> +bool ovl_init_uuid_xattr(struct super_block *sb, struct ovl_fs *ofs,
> +                        struct dentry *upperdentry, bool set);
>
>  static inline bool ovl_check_origin_xattr(struct ovl_fs *ofs,
>                                           struct dentry *upperdentry)
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index ad2250f98b38..8364620e8722 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1535,6 +1535,9 @@ static int ovl_get_indexdir(struct super_block *sb, struct ovl_fs *ofs,
>                 if (err)
>                         pr_err("failed to verify index dir 'upper' xattr\n");
>
> +               /* Best effort get or set persistent uuid */
> +               ovl_init_uuid_xattr(sb, ofs, ofs->indexdir, true);
> +
>                 /* Cleanup bad/stale/orphan index entries */
>                 if (!err)
>                         err = ovl_indexdir_cleanup(ofs);
> @@ -2052,6 +2055,10 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>                         ovl_uuid_str[ofs->config.uuid]);
>         }
>
> +       /*
> +        * This uuid may be overridden by a persistent uuid stored in xattr on
> +        * index dir and it may be persisted in xattr on first index=on mount.
> +        */
>         if (ovl_want_uuid_gen(ofs))
>                 uuid_gen(&sb->s_uuid);
>
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 923d66d131c1..8902db4b2975 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -589,6 +589,45 @@ bool ovl_path_check_origin_xattr(struct ovl_fs *ofs, const struct path *path)
>         return false;
>  }
>
> +/*
> + * Load persistent uuid from xattr into s_uuid if found, possibly overriding
> + * the random generated value in s_uuid.
> + * Otherwise, if @set is true and s_uuid contains a valid value, store this
> + * value in xattr.
> + */
> +bool ovl_init_uuid_xattr(struct super_block *sb, struct ovl_fs *ofs,
> +                        struct dentry *upperdentry, bool set)
> +{
> +       struct path path = {
> +               .dentry = upperdentry,
> +               .mnt = ovl_upper_mnt(ofs),
> +       };
> +       uuid_t uuid;
> +       int res;
> +
> +       res = ovl_path_getxattr(ofs, &path, OVL_XATTR_UUID, uuid.b, UUID_SIZE);
> +       if (res == UUID_SIZE) {
> +               uuid_copy(&sb->s_uuid, &uuid);
> +               return true;
> +       }
> +
> +       if (res == -ENODATA) {
> +               if (!set || uuid_is_null(&sb->s_uuid))
> +                       return false;
> +
> +               res = ovl_setxattr(ofs, upperdentry, OVL_XATTR_UUID,
> +                                  sb->s_uuid.b, UUID_SIZE);
> +               if (res == 0)
> +                       return true;
> +       } else {
> +               set = false;
> +       }
> +
> +       pr_warn("failed to %s uuid (%pd2, err=%i)\n",
> +               set ? "set" : "get", upperdentry, res);
> +       return false;
> +}
> +
>  bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
>                                enum ovl_xattr ox)
>  {
> @@ -611,6 +650,7 @@ bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
>  #define OVL_XATTR_IMPURE_POSTFIX       "impure"
>  #define OVL_XATTR_NLINK_POSTFIX                "nlink"
>  #define OVL_XATTR_UPPER_POSTFIX                "upper"
> +#define OVL_XATTR_UUID_POSTFIX         "uuid"
>  #define OVL_XATTR_METACOPY_POSTFIX     "metacopy"
>  #define OVL_XATTR_PROTATTR_POSTFIX     "protattr"
>
> @@ -625,6 +665,7 @@ const char *const ovl_xattr_table[][2] = {
>         OVL_XATTR_TAB_ENTRY(OVL_XATTR_IMPURE),
>         OVL_XATTR_TAB_ENTRY(OVL_XATTR_NLINK),
>         OVL_XATTR_TAB_ENTRY(OVL_XATTR_UPPER),
> +       OVL_XATTR_TAB_ENTRY(OVL_XATTR_UUID),
>         OVL_XATTR_TAB_ENTRY(OVL_XATTR_METACOPY),
>         OVL_XATTR_TAB_ENTRY(OVL_XATTR_PROTATTR),
>  };
> --
> 2.34.1
>




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