Re: [RFC PATCH] fs: allow opening overlayfs/erofs layers through O_ALT

Miklos Szeredi <[email protected]> Mon, 20 Jul 2026 11:22:09 +0200

Newsgroups	org.kernel.vger.linux-unionfs,org.kernel.vger.linux-api,org.kernel.vger.linux-fsdevel,org.ozlabs.lists.linux-erofs
Message-ID	<CAJfpeguxCOw4cURjPVOMj5-A7JnLrmy8zx4qE9uwxg7x_Wr+zg@mail.gmail.com>

On Sun, 19 Jul 2026 at 16:13, Andy Lutomirski <[email protected]> wrote:

> This gives me the willies a bit.  It's very cool, but it has some
> potential security issues that I think we need to watch out for.  As
> sort-of prior art, NTFS supports alternate streams and Reiser4, ahem,
> supported files-as-directories.

I realized that we can keep the creepiness factor minimal by
introducing this first in procfs only.   That way zero changes are
needed in the path lookup code:

sprintf(buf, "/proc/self/fd-alt/%d/%s", base_fd, alt_path);
alt_fd = open(buf, O_PATH);

Thanks,
Miklos