ovl: Provide a mount option metacopy=on/off for metadata copyup (d5791044) · Commits · e / devices / android_kernel_fairphone_FP5

Documentation/filesystems/overlayfs.txt

+29 −1

Original line number	Diff line number	Diff line
		@@ -262,6 +262,30 @@ rightmost one and going left. In the above example lower1 will be the
		top, lower2 the middle and lower3 the bottom layer.


		Metadata only copy up
		--------------------

		When metadata only copy up feature is enabled, overlayfs will only copy
		up metadata (as opposed to whole file), when a metadata specific operation
		like chown/chmod is performed. Full file will be copied up later when
		file is opened for WRITE operation.

		In other words, this is delayed data copy up operation and data is copied
		up when there is a need to actually modify data.

		There are multiple ways to enable/disable this feature. A config option
		CONFIG_OVERLAY_FS_METACOPY can be set/unset to enable/disable this feature
		by default. Or one can enable/disable it at module load time with module
		parameter metacopy=on/off. Lastly, there is also a per mount option
		metacopy=on/off to enable/disable this feature per mount.

		Do not use metacopy=on with untrusted upper/lower directories. Otherwise
		it is possible that an attacker can create a handcrafted file with
		appropriate REDIRECT and METACOPY xattrs, and gain access to file on lower
		pointed by REDIRECT. This should not be possible on local system as setting
		"trusted." xattrs will require CAP_SYS_ADMIN. But it should be possible
		for untrusted layers like from a pen drive.

		Sharing and copying layers
		--------------------------

		@@ -280,7 +304,7 @@ though it will not result in a crash or deadlock.
		Mounting an overlay using an upper layer path, where the upper layer path
		was previously used by another mounted overlay in combination with a
		different lower layer path, is allowed, unless the "inodes index" feature
		is enabled.
		or "metadata only copy up" feature is enabled.

		With the "inodes index" feature, on the first time mount, an NFS file
		handle of the lower layer root directory, along with the UUID of the lower
		@@ -293,6 +317,10 @@ lower root origin, mount will fail with ESTALE. An overlayfs mount with
		does not support NFS export, lower filesystem does not have a valid UUID or
		if the upper filesystem does not support extended attributes.

		For "metadata only copy up" feature there is no verification mechanism at
		mount time. So if same upper is mounted with different set of lower, mount
		probably will succeed but expect the unexpected later on. So don't do it.

		It is quite a common practice to copy overlay layers to a different
		directory tree on the same or different underlying filesystem, and even
		to a different machine. With the "inodes index" feature, trying to mount

fs/overlayfs/Kconfig

+19 −0

Original line number	Diff line number	Diff line
		@@ -64,6 +64,7 @@ config OVERLAY_FS_NFS_EXPORT
		bool "Overlayfs: turn on NFS export feature by default"
		depends on OVERLAY_FS
		depends on OVERLAY_FS_INDEX
		depends on !OVERLAY_FS_METACOPY
		help
		If this config option is enabled then overlay filesystems will use
		the index directory to decode overlay NFS file handles by default.
		@@ -103,3 +104,21 @@ config OVERLAY_FS_XINO_AUTO
		For more information, see Documentation/filesystems/overlayfs.txt

		If unsure, say N.

		config OVERLAY_FS_METACOPY
		bool "Overlayfs: turn on metadata only copy up feature by default"
		depends on OVERLAY_FS
		select OVERLAY_FS_REDIRECT_DIR
		help
		If this config option is enabled then overlay filesystems will
		copy up only metadata where appropriate and data copy up will
		happen when a file is opened for WRITE operation. It is still
		possible to turn off this feature globally with the "metacopy=off"
		module option or on a filesystem instance basis with the
		"metacopy=off" mount option.

		Note, that this feature is not backward compatible. That is,
		mounting an overlay which has metacopy only inodes on a kernel
		that doesn't support this feature will have unexpected results.

		If unsure, say N.

fs/overlayfs/ovl_entry.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -19,6 +19,7 @@ struct ovl_config {
		bool index;
		bool nfs_export;
		int xino;
		bool metacopy;
		};

		struct ovl_sb {

fs/overlayfs/super.c

+43 −3

Original line number	Diff line number	Diff line
		@@ -64,6 +64,11 @@ static void ovl_entry_stack_free(struct ovl_entry *oe)
		dput(oe->lowerstack[i].dentry);
		}

		static bool ovl_metacopy_def = IS_ENABLED(CONFIG_OVERLAY_FS_METACOPY);
		module_param_named(metacopy, ovl_metacopy_def, bool, 0644);
		MODULE_PARM_DESC(ovl_metacopy_def,
		"Default to on or off for the metadata only copy up feature");

		static void ovl_dentry_release(struct dentry *dentry)
		{
		struct ovl_entry *oe = dentry->d_fsdata;
		@@ -347,6 +352,9 @@ static int ovl_show_options(struct seq_file m, struct dentry dentry)
		"on" : "off");
		if (ofs->config.xino != ovl_xino_def())
		seq_printf(m, ",xino=%s", ovl_xino_str[ofs->config.xino]);
		if (ofs->config.metacopy != ovl_metacopy_def)
		seq_printf(m, ",metacopy=%s",
		ofs->config.metacopy ? "on" : "off");
		return 0;
		}

		@@ -384,6 +392,8 @@ enum {
		OPT_XINO_ON,
		OPT_XINO_OFF,
		OPT_XINO_AUTO,
		OPT_METACOPY_ON,
		OPT_METACOPY_OFF,
		OPT_ERR,
		};

		@@ -400,6 +410,8 @@ static const match_table_t ovl_tokens = {
		{OPT_XINO_ON, "xino=on"},
		{OPT_XINO_OFF, "xino=off"},
		{OPT_XINO_AUTO, "xino=auto"},
		{OPT_METACOPY_ON, "metacopy=on"},
		{OPT_METACOPY_OFF, "metacopy=off"},
		{OPT_ERR, NULL}
		};

		@@ -452,6 +464,7 @@ static int ovl_parse_redirect_mode(struct ovl_config config, const char mode)
		static int ovl_parse_opt(char opt, struct ovl_config config)
		{
		char *p;
		int err;

		config->redirect_mode = kstrdup(ovl_redirect_mode_def(), GFP_KERNEL);
		if (!config->redirect_mode)
		@@ -526,6 +539,14 @@ static int ovl_parse_opt(char opt, struct ovl_config config)
		config->xino = OVL_XINO_AUTO;
		break;

		case OPT_METACOPY_ON:
		config->metacopy = true;
		break;

		case OPT_METACOPY_OFF:
		config->metacopy = false;
		break;

		default:
		pr_err("overlayfs: unrecognized mount option \"%s\" or missing value\n", p);
		return -EINVAL;
		@@ -540,7 +561,20 @@ static int ovl_parse_opt(char opt, struct ovl_config config)
		config->workdir = NULL;
		}

		return ovl_parse_redirect_mode(config, config->redirect_mode);
		err = ovl_parse_redirect_mode(config, config->redirect_mode);
		if (err)
		return err;

		/* metacopy feature with upper requires redirect_dir=on */
		if (config->upperdir && config->metacopy && !config->redirect_dir) {
		pr_warn("overlayfs: metadata only copy up requires \"redirect_dir=on\", falling back to metacopy=off.\n");
		config->metacopy = false;
		} else if (config->metacopy && !config->redirect_follow) {
		pr_warn("overlayfs: metadata only copy up requires \"redirect_dir=follow\" on non-upper mount, falling back to metacopy=off.\n");
		config->metacopy = false;
		}

		return 0;
		}

		#define OVL_WORKDIR_NAME "work"
		@@ -1013,7 +1047,8 @@ static int ovl_make_workdir(struct ovl_fs ofs, struct path workpath)
		if (err) {
		ofs->noxattr = true;
		ofs->config.index = false;
		pr_warn("overlayfs: upper fs does not support xattr, falling back to index=off.\n");
		ofs->config.metacopy = false;
		pr_warn("overlayfs: upper fs does not support xattr, falling back to index=off and metacopy=off.\n");
		err = 0;
		} else {
		vfs_removexattr(ofs->workdir, OVL_XATTR_OPAQUE);
		@@ -1035,7 +1070,6 @@ static int ovl_make_workdir(struct ovl_fs ofs, struct path workpath)
		pr_warn("overlayfs: NFS export requires \"index=on\", falling back to nfs_export=off.\n");
		ofs->config.nfs_export = false;
		}

		out:
		mnt_drop_write(mnt);
		return err;
		@@ -1346,6 +1380,7 @@ static int ovl_fill_super(struct super_block sb, void data, int silent)
		ofs->config.index = ovl_index_def;
		ofs->config.nfs_export = ovl_nfs_export_def;
		ofs->config.xino = ovl_xino_def();
		ofs->config.metacopy = ovl_metacopy_def;
		err = ovl_parse_opt((char *) data, &ofs->config);
		if (err)
		goto out_err;
		@@ -1416,6 +1451,11 @@ static int ovl_fill_super(struct super_block sb, void data, int silent)
		}
		}

		if (ofs->config.metacopy && ofs->config.nfs_export) {
		pr_warn("overlayfs: NFS export is not supported with metadata only copy up, falling back to nfs_export=off.\n");
		ofs->config.nfs_export = false;
		}

		if (ofs->config.nfs_export)
		sb->s_export_op = &ovl_export_operations;