Re: [PATCH v3] fs: introduce getfsxattrat and setfsxattrat syscalls

Jan Kara
2025-02-24T10:54:36Z

On Tue 11-02-25 18:22:47, Andrey Albershteyn wrote:
> From: Andrey Albershteyn <aalbersh@redhat.com>
>
> Introduce getfsxattrat and setfsxattrat syscalls to manipulate inode
> extended attributes/flags. The syscalls take parent directory fd and
> path to the child together with struct fsxattr.
>
> This is an alternative to FS_IOC_FSSETXATTR ioctl with a difference
> that file don't need to be open as we can reference it with a path
> instead of fd. By having this we can manipulated inode extended
> attributes not only on regular files but also on special ones. This
> is not possible with FS_IOC_FSSETXATTR ioctl as with special files
> we can not call ioctl() directly on the filesystem inode using fd.
>
> This patch adds two new syscalls which allows userspace to get/set
> extended inode attributes on special files by using parent directory
> and a path - *at() like syscall.
>
> Also, as vfs_fileattr_set() is now will be called on special files
> too, let's forbid any other attributes except projid and nextents
> (symlink can have an extent).
>
> CC: linux-api@vger.kernel.org
> CC: linux-fsdevel@vger.kernel.org
> CC: linux-xfs@vger.kernel.org
> Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>

Some comments below:

> +SYSCALL_DEFINE4(getfsxattrat, int, dfd, const char __user *, filename,
> + struct fsxattr __user *, fsx, unsigned int, at_flags)
> +{
> + CLASS(fd, dir)(dfd);
> + struct fileattr fa;
> + struct path filepath;
> + int error;
> + unsigned int lookup_flags = 0;
> +
> + if ((at_flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
> + return -EINVAL;
> +
> + if (at_flags & AT_SYMLINK_FOLLOW)
^^ This should be !(at_flags & AT_SYMLINK_NOFOLLOW)?

In the check above you verify for AT_SYMLINK_NOFOLLOW and that also matches
what setxattrat() does...

> + lookup_flags |= LOOKUP_FOLLOW;
> +
> + if (at_flags & AT_EMPTY_PATH)
> + lookup_flags |= LOOKUP_EMPTY;
> +
> + if (fd_empty(dir))
> + return -EBADF;

This check is wrong and in fact the whole dfd handling looks buggy.
openat(2) manpage describes the expected behavior:

The dirfd argument is used in conjunction with the pathname argument as
follows:

• If the pathname given in pathname is absolute, then dirfd is ig-
nored.
^^^^ This is what you break. If the pathname is absolute, you're
not expected to touch dirfd.

• If the pathname given in pathname is relative and dirfd is the spe-
cial value AT_FDCWD, then pathname is interpreted relative to the
current working directory of the calling process (like open()).
^^^ Also AT_FDCWD handling would be broken by the above check.

• If the pathname given in pathname is relative, then it is inter-
preted relative to the directory referred to by the file descriptor
dirfd (rather than relative to the current working directory of the
calling process, as is done by open() for a relative pathname). In
this case, dirfd must be a directory that was opened for reading
(O_RDONLY) or using the O_PATH flag.

If the pathname given in pathname is relative, and dirfd is not a valid
file descriptor, an error (EBADF) results. (Specifying an invalid file
descriptor number in dirfd can be used as a means to ensure that path-
name is absolute.)

> +
> + error = user_path_at(dfd, filename, lookup_flags, &filepath);
^^^ And user_path_at() isn't quite what you need either
because with AT_EMPTY_PATH we also want to allow for filename to be NULL
(not just empty string) and user_path_at() does not support that. That's
why I in my previous replies suggested you should follow what setxattrat()
does and that sadly it is more painful than it should be. You need
something like:

name = getname_maybe_null(filename, at_flags);
if (!name) {
CLASS(fd, f)(dfd);

if (fd_empty(f))
return -EBADF;
error = vfs_fileattr_get(file_dentry(fd_file(f)), &fa);
} else {
error = filename_lookup(dfd, filename, lookup_flags, &filepath,
NULL);
if (error)
goto out;
error = vfs_fileattr_get(filepath.dentry, &fa);
path_put(&filepath);
}
if (!error)
error = copy_fsxattr_to_user(&fa, fsx);
out:
putname(name);
return error;

Longer term, we need to provide user_path_maybe_null_at() for this but I
don't want to drag you into this cleanup :)

> + if (error)
> + return error;
> +
> + error = vfs_fileattr_get(filepath.dentry, &fa);
> + if (!error)
> + error = copy_fsxattr_to_user(&fa, fsx);
> +
> + path_put(&filepath);
> + return error;
> +}
> +
> +SYSCALL_DEFINE4(setfsxattrat, int, dfd, const char __user *, filename,
> + struct fsxattr __user *, fsx, unsigned int, at_flags)
> +{
> + CLASS(fd, dir)(dfd);
> + struct fileattr fa;
> + struct path filepath;
> + int error;
> + unsigned int lookup_flags = 0;
> +
> + if ((at_flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
> + return -EINVAL;
> +
> + if (at_flags & AT_SYMLINK_FOLLOW)
> + lookup_flags |= LOOKUP_FOLLOW;

I think using AT_SYMLINK_NOFOLLOW is actually more traditional and thus
less surprising to users so I'd prefer that. Definitely this needs to be
consistent with getfsxattrat().

> +
> + if (at_flags & AT_EMPTY_PATH)
> + lookup_flags |= LOOKUP_EMPTY;
> +
> + if (fd_empty(dir))
> + return -EBADF;

Same comment regarding dfd handling as above.

> +
> + if (copy_fsxattr_from_user(&fa, fsx))
> + return -EFAULT;
> +
> + error = user_path_at(dfd, filename, lookup_flags, &filepath);
> + if (error)
> + return error;
> +
> + error = mnt_want_write(filepath.mnt);
> + if (!error) {
> + error = vfs_fileattr_set(file_mnt_idmap(fd_file(dir)),
> + filepath.dentry, &fa);
> + mnt_drop_write(filepath.mnt);
> + }
> +
> + path_put(&filepath);
> + return error;
> +}

Otherwise the patch looks good to me.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR