On Tue, Sep 12, 2023 at 12:56 PM Alexander Larsson <alexl@xxxxxxxxxx> wrote:
>
> There are cases where you want to use an overlayfs mount as a lowerdir
> for another overlayfs mount. For example, if the system rootfs is on
> overlayfs due to composefs, or to make it volatile (via tmps), then
> you cannot currently store a lowerdir on the rootfs. This means you
> can't e.g. store on the rootfs a prepared container image for use
> using overlayfs.
>
> To work around this, we introduce an escapment mechanism for overlayfs
> xattrs. Whenever the lower/upper dir has a xattr named
> "overlay.overlay.XYZ", we list it as "overlay.XYZ" in listxattrs, and
> when the user calls getxattr or setxattr on "overlay.XYZ", we apply to
> "overlay.overlay.XYZ" in the backing directories.
>
> This allows storing any kind of overlay xattrs in a overlayfs mount
> that can be used as a lowerdir in another mount. It is possible to
> stack this mechanism multiple times, such that
> "overlay.overlay.overlay.XYZ" will survive two levels of overlay mounts,
> however this is not all that useful in practice because of stack depth
> limitations of overlayfs mounts.
>
> Note: These escaped xattrs are copied to upper during copy-up.
>
> Signed-off-by: Alexander Larsson <alexl@xxxxxxxxxx>
> Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Now I really reviewed ;-)
I like this patch - it is much smaller.
I'd rename the helper s/ovl_is_prefixed_xattr/ovl_is_own_xattr
to match ovl_own_xattr_{get,set}, but don't worry about it,
I can rename on commit.
Thanks,
Amir.
> ---
> fs/overlayfs/overlayfs.h | 7 ++++
> fs/overlayfs/xattrs.c | 81 ++++++++++++++++++++++++++++++++++++++--
> 2 files changed, 85 insertions(+), 3 deletions(-)
>
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index dff7232b7bf5..736d7f952a8e 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -32,6 +32,13 @@ enum ovl_path_type {
> #define OVL_XATTR_USER_PREFIX XATTR_USER_PREFIX OVL_XATTR_NAMESPACE
> #define OVL_XATTR_USER_PREFIX_LEN (sizeof(OVL_XATTR_USER_PREFIX) - 1)
>
> +#define OVL_XATTR_ESCAPE_PREFIX OVL_XATTR_NAMESPACE
> +#define OVL_XATTR_ESCAPE_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_PREFIX) - 1)
> +#define OVL_XATTR_ESCAPE_TRUSTED_PREFIX OVL_XATTR_TRUSTED_PREFIX OVL_XATTR_ESCAPE_PREFIX
> +#define OVL_XATTR_ESCAPE_TRUSTED_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_TRUSTED_PREFIX) - 1)
> +#define OVL_XATTR_ESCAPE_USER_PREFIX OVL_XATTR_USER_PREFIX OVL_XATTR_ESCAPE_PREFIX
> +#define OVL_XATTR_ESCAPE_USER_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_USER_PREFIX) - 1)
> +
> enum ovl_xattr {
> OVL_XATTR_OPAQUE,
> OVL_XATTR_REDIRECT,
> diff --git a/fs/overlayfs/xattrs.c b/fs/overlayfs/xattrs.c
> index b8ea96606ea8..55ab56e982ea 100644
> --- a/fs/overlayfs/xattrs.c
> +++ b/fs/overlayfs/xattrs.c
> @@ -4,7 +4,19 @@
> #include <linux/xattr.h>
> #include "overlayfs.h"
>
> -bool ovl_is_private_xattr(struct super_block *sb, const char *name)
> +static bool ovl_is_escaped_xattr(struct super_block *sb, const char *name)
> +{
> + struct ovl_fs *ofs = sb->s_fs_info;
> +
> + if (ofs->config.userxattr)
> + return strncmp(name, OVL_XATTR_ESCAPE_USER_PREFIX,
> + OVL_XATTR_ESCAPE_USER_PREFIX_LEN) == 0;
> + else
> + return strncmp(name, OVL_XATTR_ESCAPE_TRUSTED_PREFIX,
> + OVL_XATTR_ESCAPE_TRUSTED_PREFIX_LEN - 1) == 0;
> +}
> +
> +static bool ovl_is_prefixed_xattr(struct super_block *sb, const char *name)
> {
> struct ovl_fs *ofs = OVL_FS(sb);
>
> @@ -16,6 +28,11 @@ bool ovl_is_private_xattr(struct super_block *sb, const char *name)
> OVL_XATTR_TRUSTED_PREFIX_LEN) == 0;
> }
>
> +bool ovl_is_private_xattr(struct super_block *sb, const char *name)
> +{
> + return ovl_is_prefixed_xattr(sb, name) && !ovl_is_escaped_xattr(sb, name);
> +}
> +
> static int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
> const void *value, size_t size, int flags)
> {
> @@ -97,10 +114,12 @@ static bool ovl_can_list(struct super_block *sb, const char *s)
> ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
> {
> struct dentry *realdentry = ovl_dentry_real(dentry);
> + struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
> ssize_t res;
> size_t len;
> char *s;
> const struct cred *old_cred;
> + size_t prefix_len, name_len;
>
> old_cred = ovl_override_creds(dentry->d_sb);
> res = vfs_listxattr(realdentry, list, size);
> @@ -108,6 +127,9 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
> if (res <= 0 || size == 0)
> return res;
>
> + prefix_len = ofs->config.userxattr ?
> + OVL_XATTR_USER_PREFIX_LEN : OVL_XATTR_TRUSTED_PREFIX_LEN;
> +
> /* filter out private xattrs */
> for (s = list, len = res; len;) {
> size_t slen = strnlen(s, len) + 1;
> @@ -120,6 +142,12 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
> if (!ovl_can_list(dentry->d_sb, s)) {
> res -= slen;
> memmove(s, s + slen, len);
> + } else if (ovl_is_escaped_xattr(dentry->d_sb, s)) {
> + res -= OVL_XATTR_ESCAPE_PREFIX_LEN;
> + name_len = slen - prefix_len - OVL_XATTR_ESCAPE_PREFIX_LEN;
> + s += prefix_len;
> + memmove(s, s + OVL_XATTR_ESCAPE_PREFIX_LEN, name_len + len);
> + s += name_len;
> } else {
> s += slen;
> }
> @@ -128,11 +156,47 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
> return res;
> }
>
> +static char *ovl_xattr_escape_name(const char *prefix, const char *name)
> +{
> + size_t prefix_len = strlen(prefix);
> + size_t name_len = strlen(name);
> + size_t escaped_len;
> + char *escaped, *s;
> +
> + escaped_len = prefix_len + OVL_XATTR_ESCAPE_PREFIX_LEN + name_len;
> + if (escaped_len > XATTR_NAME_MAX)
> + return ERR_PTR(-EOPNOTSUPP);
> +
> + escaped = kmalloc(escaped_len + 1, GFP_KERNEL);
> + if (escaped == NULL)
> + return ERR_PTR(-ENOMEM);
> +
> + s = escaped;
> + memcpy(s, prefix, prefix_len);
> + s += prefix_len;
> + memcpy(s, OVL_XATTR_ESCAPE_PREFIX, OVL_XATTR_ESCAPE_PREFIX_LEN);
> + s += OVL_XATTR_ESCAPE_PREFIX_LEN;
> + memcpy(s, name, name_len + 1);
> +
> + return escaped;
> +}
> +
> static int ovl_own_xattr_get(const struct xattr_handler *handler,
> struct dentry *dentry, struct inode *inode,
> const char *name, void *buffer, size_t size)
> {
> - return -EOPNOTSUPP;
> + char *escaped;
> + int r;
> +
> + escaped = ovl_xattr_escape_name(handler->prefix, name);
> + if (IS_ERR(escaped))
> + return PTR_ERR(escaped);
> +
> + r = ovl_xattr_get(dentry, inode, escaped, buffer, size);
> +
> + kfree(escaped);
> +
> + return r;
> }
>
> static int ovl_own_xattr_set(const struct xattr_handler *handler,
> @@ -141,7 +205,18 @@ static int ovl_own_xattr_set(const struct xattr_handler *handler,
> const char *name, const void *value,
> size_t size, int flags)
> {
> - return -EOPNOTSUPP;
> + char *escaped;
> + int r;
> +
> + escaped = ovl_xattr_escape_name(handler->prefix, name);
> + if (IS_ERR(escaped))
> + return PTR_ERR(escaped);
> +
> + r = ovl_xattr_set(dentry, inode, escaped, value, size, flags);
> +
> + kfree(escaped);
> +
> + return r;
> }
>
> static int ovl_other_xattr_get(const struct xattr_handler *handler,
> --
> 2.41.0
>
