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

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On Thu, 2024-01-18 at 11:41 +0100, Miklos Szeredi wrote:
> Add a check on each 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.
> 
> Fixes: bc8df7a3dc03 ("ovl: Add an alternative type of whiteout")
> Cc: <stable@xxxxxxxxxxxxxxx> # v6.7
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> ---
> 
> Hi Alex,
> 
> Can you please test this in your environment?

Interesting, I was just finishing a patch for this, and it is mostly
identical.
It works in my testing.

Reviewed-by: Alexander Larsson <alexl@xxxxxxxxxx>

> 
> I xwhiteout test in xfstests needs this tweak:
> 
> --- a/tests/overlay/084
> +++ b/tests/overlay/084
> @@ -115,6 +115,7 @@ do_test_xwhiteout()
>  
>  	mkdir -p $basedir/lower $basedir/upper $basedir/work
>  	touch $basedir/lower/regular $basedir/lower/hidden 
> $basedir/upper/hidden
> +	setfattr -n $prefix.overlay.feature_xwhiteout -v "y"
> $basedir/upper
>  	setfattr -n $prefix.overlay.whiteouts -v "y" $basedir/upper
>  	setfattr -n $prefix.overlay.whiteout -v "y"
> $basedir/upper/hidden
>  

Also, I will need to update mkcomposefs to set this xattr and bump the
image version.

> 
> Thanks,
> Miklos
> 
> fs/overlayfs/namei.c     | 10 +++++++---
>  fs/overlayfs/overlayfs.h |  8 ++++++--
>  fs/overlayfs/ovl_entry.h |  2 ++
>  fs/overlayfs/readdir.c   | 11 ++++++++---
>  fs/overlayfs/super.c     | 13 ++++++++++++-
>  fs/overlayfs/util.c      |  9 ++++++++-
>  6 files changed, 43 insertions(+), 10 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..991eb5d5c66c 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -51,6 +51,7 @@ enum ovl_xattr {
>  	OVL_XATTR_PROTATTR,
>  	OVL_XATTR_XWHITEOUT,
>  	OVL_XATTR_XWHITEOUTS,
> +	OVL_XATTR_FEATURE_XWHITEOUT,
>  };
>  
>  enum ovl_inode_flag {
> @@ -492,7 +493,9 @@ 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 +677,8 @@ 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..51729e614f5a 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 feature_xwhiteout;
>  };
>  
>  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;
>  

I'm not sure exactly how impure dirs work, but I don't think this is
needed. When we hit the read_impure() codepath, we never then actually
look at the p->is_whiteout information.

>  	err = ovl_dir_read(&realpath, &rdd);
>  	if (err)
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index a0967bb25003..4e507ab780f3 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1291,7 +1291,7 @@ int ovl_fill_super(struct super_block *sb,
> struct fs_context *fc)
>  	struct ovl_entry *oe;
>  	struct ovl_layer *layers;
>  	struct cred *cred;
> -	int err;
> +	int err, i;
>  
>  	err = -EIO;
>  	if (WARN_ON(fc->user_ns != current_user_ns()))
> @@ -1414,6 +1414,17 @@ int ovl_fill_super(struct super_block *sb,
> struct fs_context *fc)
>  	if (err)
>  		goto out_free_oe;
>  
> +	for (i = 0; i < ofs->numlayer; i++) {
> +		struct path path = { .mnt = layers[i].mnt };
> +
> +		if (path.mnt) {
> +			path.dentry = path.mnt->mnt_root;
> +			err = ovl_path_getxattr(ofs, &path,
> OVL_XATTR_FEATURE_XWHITEOUT, NULL, 0);
> +			if (err >= 0)
> +				layers[i].feature_xwhiteout = true;
> +		}
> +	}
> +
>  	/* Show index=off in /proc/mounts for forced r/o mount */
>  	if (!ofs->indexdir) {
>  		ofs->config.index = false;
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index c3f020ca13a8..cae8219618e3 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->feature_xwhiteout)
> +		return false;
> +
>  	/* xattr.whiteouts must be a directory */
>  	if (!d_is_dir(dentry))
>  		return false;
> @@ -838,6 +843,7 @@ bool ovl_path_check_dir_xattr(struct ovl_fs *ofs,
> const struct path *path,
>  #define OVL_XATTR_PROTATTR_POSTFIX	"protattr"
>  #define OVL_XATTR_XWHITEOUT_POSTFIX	"whiteout"
>  #define OVL_XATTR_XWHITEOUTS_POSTFIX	"whiteouts"
> +#define OVL_XATTR_FEATURE_XWHITEOUT_POSTFIX	"feature_xwhiteout"
>  
>  #define OVL_XATTR_TAB_ENTRY(x) \
>  	[x] = { [false] = OVL_XATTR_TRUSTED_PREFIX x ## _POSTFIX, \
> @@ -855,6 +861,7 @@ const char *const ovl_xattr_table[][2] = {
>  	OVL_XATTR_TAB_ENTRY(OVL_XATTR_PROTATTR),
>  	OVL_XATTR_TAB_ENTRY(OVL_XATTR_XWHITEOUT),
>  	OVL_XATTR_TAB_ENTRY(OVL_XATTR_XWHITEOUTS),
> +	OVL_XATTR_TAB_ENTRY(OVL_XATTR_FEATURE_XWHITEOUT),
>  };
>  
>  int ovl_check_setxattr(struct ovl_fs *ofs, struct dentry
> *upperdentry,

-- 
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
=-=-=
 Alexander Larsson                                            Red Hat,
Inc 
       alexl@xxxxxxxxxx            alexander.larsson@xxxxxxxxx 
He's a gun-slinging hunchbacked boxer She's a psychotic 
antique-collecting politician who don't take no shit from nobody. They 
fight crime! 






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