linux-kernel - Re: [PATCH v2 18/25] fs: add vfs_set

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Date: Fri, 23 Feb 2024 09:38:59 +0100
From: Christian Brauner <brauner@...nel.org>
To: "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
Cc: Serge Hallyn <serge@...lyn.com>, Paul Moore <paul@...l-moore.com>, 
	Eric Paris <eparis@...hat.com>, James Morris <jmorris@...ei.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	Stephen Smalley <stephen.smalley.work@...il.com>, Ondrej Mosnacek <omosnace@...hat.com>, 
	Casey Schaufler <casey@...aufler-ca.com>, Mimi Zohar <zohar@...ux.ibm.com>, 
	Roberto Sassu <roberto.sassu@...wei.com>, Dmitry Kasatkin <dmitry.kasatkin@...il.com>, 
	Eric Snowberg <eric.snowberg@...cle.com>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Jonathan Corbet <corbet@....net>, Miklos Szeredi <miklos@...redi.hu>, 
	Amir Goldstein <amir73il@...il.com>, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-security-module@...r.kernel.org, audit@...r.kernel.org, selinux@...r.kernel.org, 
	linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org, linux-unionfs@...r.kernel.org
Subject: Re: [PATCH v2 18/25] fs: add vfs_set_fscaps()

On Wed, Feb 21, 2024 at 03:24:49PM -0600, Seth Forshee (DigitalOcean) wrote:
> Provide a type-safe interface for setting filesystem capabilities and a
> generic implementation suitable for most filesystems.
> 
> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@...nel.org>
> ---
>  fs/xattr.c         | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h |  2 ++
>  2 files changed, 81 insertions(+)
> 
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 10d1b1f78fc2..96de43928a51 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -245,6 +245,85 @@ int vfs_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
>  }
>  EXPORT_SYMBOL(vfs_get_fscaps);
>  
> +static int generic_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +			      const struct vfs_caps *caps, int setxattr_flags)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct vfs_ns_cap_data nscaps;
> +	int size;

ssize_t, I believe.

> +
> +	size = vfs_caps_to_xattr(idmap, i_user_ns(inode), caps,
> +				 &nscaps, sizeof(nscaps));
> +	if (size < 0)
> +		return size;
> +
> +	return __vfs_setxattr_noperm(idmap, dentry, XATTR_NAME_CAPS,
> +				     &nscaps, size, setxattr_flags);
> +}
> +
> +/**
> + * vfs_set_fscaps - set filesystem capabilities
> + * @idmap: idmap of the mount the inode was found from
> + * @dentry: the dentry on which to set filesystem capabilities
> + * @caps: the filesystem capabilities to be written
> + * @setxattr_flags: setxattr flags to use when writing the capabilities xattr
> + *
> + * This function writes the supplied filesystem capabilities to the dentry.
> + *
> + * Return: 0 on success, a negative errno on error.
> + */
> +int vfs_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +		   const struct vfs_caps *caps, int setxattr_flags)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct inode *delegated_inode = NULL;
> +	int error;
> +
> +retry_deleg:
> +	inode_lock(inode);
> +
> +	error = xattr_permission(idmap, inode, XATTR_NAME_CAPS, MAY_WRITE);
> +	if (error)
> +		goto out_inode_unlock;

I think this should be

        /*
         * We only care about restrictions the inode struct itself places upon
         * us otherwise fscaps aren't subject to any VFS restrictions.
         */
        error = may_write_xattr(idmap, inode);
        if (error)
                goto out_inode_unlock;

which is a 1:1 copy of what POSIX ACLs do?

> +	error = security_inode_set_fscaps(idmap, dentry, caps, setxattr_flags);
> +	if (error)
> +		goto out_inode_unlock;
> +
> +	error = try_break_deleg(inode, &delegated_inode);
> +	if (error)
> +		goto out_inode_unlock;
> +
> +	if (inode->i_opflags & IOP_XATTR) {

Fwiw, I think that if we move fscaps off of xattr handlers completely
this can go away and we can simply rely on ->{g,s}et_fscaps() being
implemented. But again, that can be in a follow-up series.

> +		if (inode->i_op->set_fscaps)
> +			error = inode->i_op->set_fscaps(idmap, dentry, caps,
> +							setxattr_flags);
> +		else
> +			error = generic_set_fscaps(idmap, dentry, caps,
> +						   setxattr_flags);
> +		if (!error) {
> +			fsnotify_xattr(dentry);
> +			security_inode_post_set_fscaps(idmap, dentry, caps,
> +						       setxattr_flags);
> +		}
> +	} else if (unlikely(is_bad_inode(inode))) {
> +		error = -EIO;
> +	} else {
> +		error = -EOPNOTSUPP;
> +	}
> +
> +out_inode_unlock:
> +	inode_unlock(inode);
> +
> +	if (delegated_inode) {
> +		error = break_deleg_wait(&delegated_inode);
> +		if (!error)
> +			goto retry_deleg;
> +	}
> +
> +	return error;
> +}
> +EXPORT_SYMBOL(vfs_set_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 d7cd2467e1ea..4f5d7ed44644 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2120,6 +2120,8 @@ 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);
> +extern int vfs_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +			  const struct vfs_caps *caps, int setxattr_flags);

Please drop the extern.