Re: [PATCH v2] ovl: require xwhiteout feature flag on layer roots

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On Fri, Jan 19, 2024 at 12:14 PM Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
>
> Add a check on each lower layer for the xwhiteout feature.  This prevents
> unnecessary checking the overlay.whiteouts xattr when reading a directory
> if this feature is not enabled, i.e. most of the time.
>
> Share the same xattr for the per-directory and the per-layer flag, which
> has the effect that if this is enabled for a layer, then the optimization
> to bypass checking of individual entries does not work on the root of the
> layer.  This was deemed better, than having a separate xattr for the layer
> and the directory.
>
> Fixes: bc8df7a3dc03 ("ovl: Add an alternative type of whiteout")
> Cc: <stable@xxxxxxxxxxxxxxx> # v6.7
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> ---
> v2:
>  - use overlay.whiteouts instead of overlay.feature_xwhiteout
>  - move initialization to ovl_get_layers()
>  - xwhiteouts can only be enabled on lower layer
>
>  fs/overlayfs/namei.c     | 10 +++++++---
>  fs/overlayfs/overlayfs.h |  7 +++++--
>  fs/overlayfs/ovl_entry.h |  2 ++
>  fs/overlayfs/readdir.c   | 11 ++++++++---
>  fs/overlayfs/super.c     | 13 +++++++++++++
>  fs/overlayfs/util.c      |  7 ++++++-
>  6 files changed, 41 insertions(+), 9 deletions(-)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index 03bc8d5dfa31..583cf56df66e 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -863,7 +863,8 @@ struct dentry *ovl_lookup_index(struct ovl_fs *ofs, struct dentry *upper,
>   * Returns next layer in stack starting from top.
>   * Returns -1 if this is the last layer.
>   */
> -int ovl_path_next(int idx, struct dentry *dentry, struct path *path)
> +int ovl_path_next(int idx, struct dentry *dentry, struct path *path,
> +                 const struct ovl_layer **layer)
>  {
>         struct ovl_entry *oe = OVL_E(dentry);
>         struct ovl_path *lowerstack = ovl_lowerstack(oe);
> @@ -871,13 +872,16 @@ int ovl_path_next(int idx, struct dentry *dentry, struct path *path)
>         BUG_ON(idx < 0);
>         if (idx == 0) {
>                 ovl_path_upper(dentry, path);
> -               if (path->dentry)
> +               if (path->dentry) {
> +                       *layer = &OVL_FS(dentry->d_sb)->layers[0];
>                         return ovl_numlower(oe) ? 1 : -1;
> +               }
>                 idx++;
>         }
>         BUG_ON(idx > ovl_numlower(oe));
>         path->dentry = lowerstack[idx - 1].dentry;
> -       path->mnt = lowerstack[idx - 1].layer->mnt;
> +       *layer = lowerstack[idx - 1].layer;
> +       path->mnt = (*layer)->mnt;
>
>         return (idx < ovl_numlower(oe)) ? idx + 1 : -1;
>  }
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index 05c3dd597fa8..6359cf5c66ff 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -492,7 +492,9 @@ 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_path_check_xwhiteout_xattr(struct ovl_fs *ofs, const struct path *path);
> -bool ovl_path_check_xwhiteouts_xattr(struct ovl_fs *ofs, const struct path *path);
> +bool ovl_path_check_xwhiteouts_xattr(struct ovl_fs *ofs,
> +                                    const struct ovl_layer *layer,
> +                                    const struct path *path);
>  bool ovl_init_uuid_xattr(struct super_block *sb, struct ovl_fs *ofs,
>                          const struct path *upperpath);
>
> @@ -674,7 +676,8 @@ int ovl_get_index_name(struct ovl_fs *ofs, struct dentry *origin,
>  struct dentry *ovl_get_index_fh(struct ovl_fs *ofs, struct ovl_fh *fh);
>  struct dentry *ovl_lookup_index(struct ovl_fs *ofs, struct dentry *upper,
>                                 struct dentry *origin, bool verify);
> -int ovl_path_next(int idx, struct dentry *dentry, struct path *path);
> +int ovl_path_next(int idx, struct dentry *dentry, struct path *path,
> +                 const struct ovl_layer **layer);
>  int ovl_verify_lowerdata(struct dentry *dentry);
>  struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
>                           unsigned int flags);
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index d82d2a043da2..33fcd3d3af30 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -40,6 +40,8 @@ struct ovl_layer {
>         int idx;
>         /* One fsid per unique underlying sb (upper fsid == 0) */
>         int fsid;
> +       /* xwhiteouts are enabled on this layer*/
> +       bool xwhiteouts;
>  };
>
>  struct ovl_path {
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index a490fc47c3e7..c2597075e3f8 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -305,8 +305,6 @@ static inline int ovl_dir_read(const struct path *realpath,
>         if (IS_ERR(realfile))
>                 return PTR_ERR(realfile);
>
> -       rdd->in_xwhiteouts_dir = rdd->dentry &&
> -               ovl_path_check_xwhiteouts_xattr(OVL_FS(rdd->dentry->d_sb), realpath);
>         rdd->first_maybe_whiteout = NULL;
>         rdd->ctx.pos = 0;
>         do {
> @@ -359,10 +357,14 @@ static int ovl_dir_read_merged(struct dentry *dentry, struct list_head *list,
>                 .is_lowest = false,
>         };
>         int idx, next;
> +       struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
> +       const struct ovl_layer *layer;
>
>         for (idx = 0; idx != -1; idx = next) {
> -               next = ovl_path_next(idx, dentry, &realpath);
> +               next = ovl_path_next(idx, dentry, &realpath, &layer);
>                 rdd.is_upper = ovl_dentry_upper(dentry) == realpath.dentry;
> +               if (ovl_path_check_xwhiteouts_xattr(ofs, layer, &realpath))
> +                       rdd.in_xwhiteouts_dir = true;
>
>                 if (next != -1) {
>                         err = ovl_dir_read(&realpath, &rdd);
> @@ -568,6 +570,7 @@ static int ovl_dir_read_impure(const struct path *path,  struct list_head *list,
>         int err;
>         struct path realpath;
>         struct ovl_cache_entry *p, *n;
> +       struct ovl_fs *ofs = OVL_FS(path->dentry->d_sb);
>         struct ovl_readdir_data rdd = {
>                 .ctx.actor = ovl_fill_plain,
>                 .list = list,
> @@ -577,6 +580,8 @@ static int ovl_dir_read_impure(const struct path *path,  struct list_head *list,
>         INIT_LIST_HEAD(list);
>         *root = RB_ROOT;
>         ovl_path_upper(path->dentry, &realpath);
> +       if (ovl_path_check_xwhiteouts_xattr(ofs, &ofs->layers[0], &realpath))
> +               rdd.in_xwhiteouts_dir = true;

Not needed since we do not support xwhiteouts on upper.

>
>         err = ovl_dir_read(&realpath, &rdd);
>         if (err)
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index a0967bb25003..04588721eb2a 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1027,6 +1027,7 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
>                 struct ovl_fs_context_layer *l = &ctx->lower[i];
>                 struct vfsmount *mnt;
>                 struct inode *trap;
> +               struct path root;
>                 int fsid;
>
>                 if (i < nr_merged_lower)
> @@ -1069,6 +1070,16 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
>                  */
>                 mnt->mnt_flags |= MNT_READONLY | MNT_NOATIME;
>
> +               /*
> +                * Check if xwhiteout (xattr whiteout) support is enabled on
> +                * this layer.
> +                */
> +               root.mnt = mnt;
> +               root.dentry = mnt->mnt_root;
> +               err = ovl_path_getxattr(ofs, &root, OVL_XATTR_XWHITEOUTS, NULL, 0);
> +               if (err >= 0)
> +                       layers[ofs->numlayer].xwhiteouts = true;
> +
>                 layers[ofs->numlayer].trap = trap;
>                 layers[ofs->numlayer].mnt = mnt;
>                 layers[ofs->numlayer].idx = ofs->numlayer;
> @@ -1079,6 +1090,8 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
>                 l->name = NULL;
>                 ofs->numlayer++;
>                 ofs->fs[fsid].is_lower = true;
> +
> +

extra spaces.

>         }
>
>         /*
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index c3f020ca13a8..6c6e6f5893ea 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -739,11 +739,16 @@ bool ovl_path_check_xwhiteout_xattr(struct ovl_fs *ofs, const struct path *path)
>         return res >= 0;
>  }
>
> -bool ovl_path_check_xwhiteouts_xattr(struct ovl_fs *ofs, const struct path *path)
> +bool ovl_path_check_xwhiteouts_xattr(struct ovl_fs *ofs,
> +                                    const struct ovl_layer *layer,
> +                                    const struct path *path)
>  {
>         struct dentry *dentry = path->dentry;
>         int res;
>
> +       if (!layer->xwhiteouts)
> +               return false;
> +
>         /* xattr.whiteouts must be a directory */
>         if (!d_is_dir(dentry))
>                 return false;
> --
> 2.43.0
>

Do you want me to fix/test and send this to Linus?

Alex, can we add your RVB to v2?

Thanks,
Amir.





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