[PATCH v2 17/25] fs: add vfs_get_fscaps()

[Christian Brauner]

On Wed, Feb 21, 2024 at 03:24:48PM -0600, Seth Forshee (DigitalOcean) wrote:
> Provide a type-safe interface for retrieving filesystem capabilities and
> a generic implementation suitable for most filesystems. Also add an
> internal interface, vfs_get_fscaps_nosec(), which skips security checks
> for later use from the capability code.
> 
> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee at kernel.org>
> ---
>  fs/xattr.c         | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h |  4 ++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 06290e4ebc03..10d1b1f78fc2 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -181,6 +181,70 @@ xattr_supports_user_prefix(struct inode *inode)
>  }
>  EXPORT_SYMBOL(xattr_supports_user_prefix);
>  
> +static int generic_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +			      struct vfs_caps *caps)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct vfs_ns_cap_data nscaps;
> +	int ret;
> +
> +	ret = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, &nscaps, sizeof(nscaps));
> +
> +	if (ret >= 0)
> +		ret = vfs_caps_from_xattr(idmap, i_user_ns(inode), caps, &nscaps, ret);
> +
> +	return ret;
> +}
> +
> +/**
> + * vfs_get_fscaps_nosec - get filesystem capabilities without security checks
> + * @idmap: idmap of the mount the inode was found from
> + * @dentry: the dentry from which to get filesystem capabilities
> + * @caps: storage in which to return the filesystem capabilities
> + *
> + * This function gets the filesystem capabilities for the dentry and returns
> + * them in @caps. It does not perform security checks.
> + *
> + * Return: 0 on success, a negative errno on error.
> + */
> +int vfs_get_fscaps_nosec(struct mnt_idmap *idmap, struct dentry *dentry,
> +			 struct vfs_caps *caps)
> +{
> +	struct inode *inode = d_inode(dentry);
> +
> +	if (inode->i_op->get_fscaps)
> +		return inode->i_op->get_fscaps(idmap, dentry, caps);
> +	return generic_get_fscaps(idmap, dentry, caps);
> +}
> +
> +/**
> + * vfs_get_fscaps - get filesystem capabilities
> + * @idmap: idmap of the mount the inode was found from
> + * @dentry: the dentry from which to get filesystem capabilities
> + * @caps: storage in which to return the filesystem capabilities
> + *
> + * This function gets the filesystem capabilities for the dentry and returns
> + * them in @caps.
> + *
> + * Return: 0 on success, a negative errno on error.
> + */
> +int vfs_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +		   struct vfs_caps *caps)
> +{
> +	int error;
> +
> +	/*
> +	 * The VFS has no restrictions on reading security.* xattrs, so
> +	 * xattr_permission() isn't needed. Only LSMs get a say.
> +	 */
> +	error = security_inode_get_fscaps(idmap, dentry);
> +	if (error)
> +		return error;
> +
> +	return vfs_get_fscaps_nosec(idmap, dentry, caps);
> +}
> +EXPORT_SYMBOL(vfs_get_fscaps);
> +
>  int
>  __vfs_setxattr(struct mnt_idmap *idmap, struct dentry *dentry,
>  	       struct inode *inode, const char *name, const void *value,
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 89163e0f7aad..d7cd2467e1ea 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2116,6 +2116,10 @@ extern int vfs_dedupe_file_range(struct file *file,
>  extern loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
>  					struct file *dst_file, loff_t dst_pos,
>  					loff_t len, unsigned int remap_flags);
> +extern int vfs_get_fscaps_nosec(struct mnt_idmap *idmap, struct dentry *dentry,
> +				struct vfs_caps *caps);
> +extern int vfs_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +			  struct vfs_caps *caps);

Please drop the externs. Other than my usual complaing about this
falling back to the legacy vfs_*xattr() interfaces,
Reviewed-by: Christian Brauner <brauner at kernel.org>