Re: [PATCH v5 1/4] openat2: new OPENAT2_REGULAR flag support

[Christian Brauner <brauner@kernel.org>]

On Sun, Mar 08, 2026 at 10:10:05AM -0700, Andy Lutomirski wrote:
> On Sun, Mar 8, 2026 at 4:40 AM Jeff Layton <jlayton@kernel.org> wrote:
> >
> > On Sat, 2026-03-07 at 10:56 -0800, Andy Lutomirski wrote:
> > > On Sat, Mar 7, 2026 at 6:09 AM Dorjoy Chowdhury <dorjoychy111@gmail.com> wrote:
> > > >
> > > > This flag indicates the path should be opened if it's a regular file.
> > > > This is useful to write secure programs that want to avoid being
> > > > tricked into opening device nodes with special semantics while thinking
> > > > they operate on regular files. This is a requested feature from the
> > > > uapi-group[1].
> > > >
> > >
> > > I think this needs a lot more clarification as to what "regular"
> > > means.  If it's literally
> > >
> > > > A corresponding error code EFTYPE has been introduced. For example, if
> > > > openat2 is called on path /dev/null with OPENAT2_REGULAR in the flag
> > > > param, it will return -EFTYPE. EFTYPE is already used in BSD systems
> > > > like FreeBSD, macOS.
> > >
> > > I think this needs more clarification as to what "regular" means,
> > > since S_IFREG may not be sufficient.  The UAPI group page says:
> > >
> > > Use-Case: this would be very useful to write secure programs that want
> > > to avoid being tricked into opening device nodes with special
> > > semantics while thinking they operate on regular files. This is
> > > particularly relevant as many device nodes (or even FIFOs) come with
> > > blocking I/O (or even blocking open()!) by default, which is not
> > > expected from regular files backed by “fast” disk I/O. Consider
> > > implementation of a naive web browser which is pointed to
> > > file://dev/zero, not expecting an endless amount of data to read.
> > >
> > > What about procfs?  What about sysfs?  What about /proc/self/fd/17
> > > where that fd is a memfd?  What about files backed by non-"fast" disk
> > > I/O like something on a flaky USB stick or a network mount or FUSE?
> > >
> > > Are we concerned about blocking open?  (open blocks as a matter of
> > > course.)  Are we concerned about open having strange side effects?
> > > Are we concerned about write having strange side effects?  Are we
> > > concerned about cases where opening the file as root results in
> > > elevated privilege beyond merely gaining the ability to write to that
> > > specific path on an ordinary filesystem?

I think this is opening up a barrage of question that I'm not sure are
all that useful. The ability to only open regular file isn't intended to
defend against hung FUSE or NFS servers or other random Linux
special-sauce murder-suicide file descriptor traps. For a lot of those
we have O_PATH which can easily function with the new extension. A lot
of the other special-sauce files (most anonymous inode fds) cannot even
be reopened via e.g., /proc.