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

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2026-03-09, Christian Brauner <brauner@kernel.org> wrote:
> > > On Sat, 2026-03-07 at 10:56 -0800, Andy Lutomirski wrote:
> > > > 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.

Indeed, I see OPENAT2_REGULAR as a way of optimising the tedious checks
that userspace does using O_PATH+/proc/self/fd/$n re-opening when
dealing with regular files.

For the problem of stuck NFS handles and so on, an idea I've had on my
backlog for a long time was RESOLVE_NO_REMOTE that would block those
kinds of things. IMHO it doesn't make sense to block those things with
an O_* flag because (especially in the NFS example) directory components
can also cause the syscall to block indefinitely and so RESOLVE_* flags
make more sense for this anyway. But in my mind this is a separate
problem to OPENAT2_REGULAR.

-- 
Aleksa Sarai
https://www.cyphar.com/

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