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

Jeff Layton
2026-03-08T11:40:50Z

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?
>

Above the use-case, it also says:

"O_REGULAR (inspired by the existing O_DIRECTORY flag for open()),
which opens a file only if it is of type S_IFREG."

Since we allow programs to open a directory under /proc or /sys using
O_DIRECTORY, I don't think we should do anything different here. To the
VFS, all of the examples you gave above are S_IFREG "regular files",
even if they are backed by something quite irregular.
--
Jeff Layton <jlayton@kernel.org>