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

On 2025-02-24 11:54:34, Jan Kara wrote:
> On Tue 11-02-25 18:22:47, Andrey Albershteyn wrote:
> > From: Andrey Albershteyn <[email protected]>
> > 
> > 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: [email protected]
> > CC: [email protected]
> > CC: [email protected]
> > Signed-off-by: Andrey Albershteyn <[email protected]>
> 
> 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...

Right, didn't notice that this is actually opposite to setxattrat(),
will change that.

> 
> 
> > +		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 :)

Oh, I missed that, thanks for pointing this out, I will change it as
suggested.

-- 
- Andrey