Re: [PATCH v4 2/3] fs: name_to_handle_at() support for "explicit connectable" file handles

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2024-10-11 at 11:00 +0200, Amir Goldstein wrote:
> nfsd encodes "connectable" file handles for the subtree_check feature,
> which can be resolved to an open file with a connected path.
> So far, userspace nfs server could not make use of this functionality.
> 
> Introduce a new flag AT_HANDLE_CONNECTABLE to name_to_handle_at(2).
> When used, the encoded file handle is "explicitly connectable".
> 
> The "explicitly connectable" file handle sets bits in the high 16bit of
> the handle_type field, so open_by_handle_at(2) will know that it needs
> to open a file with a connected path.
> 
> old kernels will now recognize the handle_type with high bits set,
> so "explicitly connectable" file handles cannot be decoded by
> open_by_handle_at(2) on old kernels.
> 
> The flag AT_HANDLE_CONNECTABLE is not allowed together with either
> AT_HANDLE_FID or AT_EMPTY_PATH.
> 
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
>  fs/fhandle.c               | 48 ++++++++++++++++++++++++++++++++++----
>  include/linux/exportfs.h   |  2 ++
>  include/uapi/linux/fcntl.h |  1 +
>  3 files changed, 46 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index 218511f38cbb..8339a1041025 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -31,6 +31,14 @@ static long do_sys_name_to_handle(const struct path *path,
>  	if (!exportfs_can_encode_fh(path->dentry->d_sb->s_export_op, fh_flags))
>  		return -EOPNOTSUPP;
>  
> +	/*
> +	 * A request to encode a connectable handle for a disconnected dentry
> +	 * is unexpected since AT_EMPTY_PATH is not allowed.
> +	 */
> +	if (fh_flags & EXPORT_FH_CONNECTABLE &&
> +	    WARN_ON(path->dentry->d_flags & DCACHE_DISCONNECTED))

Is this even possible? The dentry in this case will have been reached
by pathwalk. Oh, but I guess the dfd could point to a disconnected
dentry and then you pass in AT_EMPTY_PATH.

I'm not sure we want to warn in that case though, since this is a
situation that an unprivileged user could be able to arrange. Maybe we
should just return a more distinct error code in this case?

Since the scenario involves a dfd that is disconnected, how about:

    #define EBADFD          77      /* File descriptor in bad state */



> +		return -EINVAL;
> +
>  	if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle)))
>  		return -EFAULT;
>  
> @@ -45,7 +53,7 @@ static long do_sys_name_to_handle(const struct path *path,
>  	/* convert handle size to multiple of sizeof(u32) */
>  	handle_dwords = f_handle.handle_bytes >> 2;
>  
> -	/* we ask for a non connectable maybe decodeable file handle */
> +	/* Encode a possibly decodeable/connectable file handle */
>  	retval = exportfs_encode_fh(path->dentry,
>  				    (struct fid *)handle->f_handle,
>  				    &handle_dwords, fh_flags);
> @@ -67,8 +75,23 @@ static long do_sys_name_to_handle(const struct path *path,
>  		 * non variable part of the file_handle
>  		 */
>  		handle_bytes = 0;
> -	} else
> +	} else {
> +		/*
> +		 * When asked to encode a connectable file handle, encode this
> +		 * property in the file handle itself, so that we later know
> +		 * how to decode it.
> +		 * For sanity, also encode in the file handle if the encoded
> +		 * object is a directory and verify this during decode, because
> +		 * decoding directory file handles is quite different than
> +		 * decoding connectable non-directory file handles.
> +		 */
> +		if (fh_flags & EXPORT_FH_CONNECTABLE) {
> +			handle->handle_type |= FILEID_IS_CONNECTABLE;
> +			if (d_is_dir(path->dentry))
> +				fh_flags |= FILEID_IS_DIR;
> +		}
>  		retval = 0;
> +	}
>  	/* copy the mount id */
>  	if (unique_mntid) {
>  		if (put_user(real_mount(path->mnt)->mnt_id_unique,
> @@ -109,15 +132,30 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
>  {
>  	struct path path;
>  	int lookup_flags;
> -	int fh_flags;
> +	int fh_flags = 0;
>  	int err;
>  
>  	if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID |
> -		     AT_HANDLE_MNT_ID_UNIQUE))
> +		     AT_HANDLE_MNT_ID_UNIQUE | AT_HANDLE_CONNECTABLE))
> +		return -EINVAL;
> +
> +	/*
> +	 * AT_HANDLE_FID means there is no intention to decode file handle
> +	 * AT_HANDLE_CONNECTABLE means there is an intention to decode a
> +	 * connected fd (with known path), so these flags are conflicting.
> +	 * AT_EMPTY_PATH could be used along with a dfd that refers to a
> +	 * disconnected non-directory, which cannot be used to encode a
> +	 * connectable file handle, because its parent is unknown.
> +	 */
> +	if (flag & AT_HANDLE_CONNECTABLE &&

nit: might need parenthesis around the above & check.

> +	    flag & (AT_HANDLE_FID | AT_EMPTY_PATH))
>  		return -EINVAL;
> +	else if (flag & AT_HANDLE_FID)
> +		fh_flags |= EXPORT_FH_FID;
> +	else if (flag & AT_HANDLE_CONNECTABLE)
> +		fh_flags |= EXPORT_FH_CONNECTABLE;
>  
>  	lookup_flags = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
> -	fh_flags = (flag & AT_HANDLE_FID) ? EXPORT_FH_FID : 0;
>  	if (flag & AT_EMPTY_PATH)
>  		lookup_flags |= LOOKUP_EMPTY;
>  	err = user_path_at(dfd, name, lookup_flags, &path);
> diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> index 5e14d4500a75..4ee42b2cf4ab 100644
> --- a/include/linux/exportfs.h
> +++ b/include/linux/exportfs.h
> @@ -169,6 +169,8 @@ struct fid {
>  #define FILEID_USER_FLAGS(type) ((type) & FILEID_USER_FLAGS_MASK)
>  
>  /* Flags supported in encoded handle_type that is exported to user */
> +#define FILEID_IS_CONNECTABLE	0x10000
> +#define FILEID_IS_DIR		0x20000
>  #define FILEID_VALID_USER_FLAGS	(0)
>  
>  /**
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 87e2dec79fea..56ff2100e021 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -153,6 +153,7 @@
>  					   object identity and may not be
>  					   usable with open_by_handle_at(2). */
>  #define AT_HANDLE_MNT_ID_UNIQUE	0x001	/* Return the u64 unique mount ID. */
> +#define AT_HANDLE_CONNECTABLE	0x002	/* Request a connectable file handle */
>  
>  #if defined(__KERNEL__)
>  #define AT_GETATTR_NOSEC	0x80000000

-- 
Jeff Layton <jlayton@kernel.org>