[PATCH 08/16] fs: add vfs_get_fscaps()

[Christian Brauner]

On Wed, Nov 29, 2023 at 03:50:26PM -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(), which skips security checks for
> later use from the capability code.
> 
> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee at kernel.org>
> ---
>  fs/xattr.c         | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h |  4 ++++
>  2 files changed, 70 insertions(+)
> 
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 09d927603433..3abaf9bef0a5 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -181,6 +181,72 @@ 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 = NULL;
> +	int ret;
> +
> +	ret = (int)vfs_getxattr_alloc(idmap, dentry, XATTR_NAME_CAPS,

I don't think you need that case here.

> +				      (char **)&nscaps, 0, GFP_NOFS);
> +
> +	if (ret >= 0)
> +		ret = vfs_caps_from_xattr(idmap, i_user_ns(inode), caps, nscaps, ret);
> +
> +	kfree(nscaps);
> +	return ret;
> +}
> +
> +/**
> + * __vfs_get_fscaps - 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(struct mnt_idmap *idmap, struct dentry *dentry,
> +		     struct vfs_caps *caps)

I would rename that to vfs_get_fscaps_nosec(). We do that for
vfs_getxattr_nosec() as well. It's not pretty but it's better than just
slapping underscores onto it imo.