Re: [RFC][PATCH] getname_maybe_null() - the third variant of pathname copy-in

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Oct 21, 2024 at 06:09:10PM +0100, Al Viro wrote:
> On Mon, Oct 21, 2024 at 02:39:58PM +0200, Christian Brauner wrote:
> 
> > > See #getname.fixup; on top of #base.getname and IMO worth folding into it.
> > 
> > Yes, please fold so I can rebase my series on top of it.
> 
> OK...  What I have is #base.getname-fixed, with two commits - trivial
> "teach filename_lookup() to accept NULL" and introducing getname_maybe_null(),
> with fix folded in.
> 
> #work.xattr2 and #work.statx2 are on top of that.

BTW, speaking of statx() - I would rather lift the call of cp_statx() out
of do_statx() and do_statx_fd() into the callers.  Yes, that needs making
it non-static, due to io_uring; not a problem, IMO - it fits into the
"how do we copy internal objects to userland ones" family of helpers.

Another fun issue: for by-pathname case vfs_fstatat() ends up hitting
the same vfs_statx_path() as statx(2); however, for by-descriptor case
they do vfs_getattr() and vfs_statx_path() resp.

The difference is, vfs_statx_path() has
        if (request_mask & STATX_MNT_ID_UNIQUE) {
                stat->mnt_id = real_mount(path->mnt)->mnt_id_unique;
                stat->result_mask |= STATX_MNT_ID_UNIQUE;
        } else {
                stat->mnt_id = real_mount(path->mnt)->mnt_id;
                stat->result_mask |= STATX_MNT_ID;
        }

        if (path_mounted(path))
                stat->attributes |= STATX_ATTR_MOUNT_ROOT;
        stat->attributes_mask |= STATX_ATTR_MOUNT_ROOT;

        /*
         * If this is a block device inode, override the filesystem
         * attributes with the block device specific parameters that need to be
         * obtained from the bdev backing inode.
         */
        if (S_ISBLK(stat->mode))
                bdev_statx(path, stat, request_mask);
done after vfs_getattr().  Questions:

1) why is STATX_MNT_ID set without checking if it's in the mask passed to
the damn thing?

2) why, in the name of everything unholy, does statx() on /dev/weird_shite
trigger modprobe on that thing?  Without even a permission check on it...