Re: [PATCH v4 1/4] openat2: new OPENAT2_REGULAR flag support

On Sun, Mar 1, 2026 at 6:44 PM Jeff Layton <[email protected]> wrote:
>
> On Sat, 2026-02-21 at 20:45 +0600, Dorjoy Chowdhury 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].
> >
> > 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.
> >
> > When used in combination with O_CREAT, either the regular file is
> > created, or if the path already exists, it is opened if it's a regular
> > file. Otherwise, -EFTYPE is returned.
> >
>
> It would be good to mention that EFTYPE has precedent in BSD/Darwin.
> When an error code is already supported in another UNIX-y OS, then it
> bolsters the case for adding it here.
>

Good suggestion. Yes, I can include this information in the commit
message during the next posting.

> Your cover letter mentions that you only tested this on btrfs. At the
> very least, you should test NFS and SMB. It should be fairly easy to
> set up mounts over loopback for those cases.
>

I used virtme-ng (which I think reuses the host's filesystem) to run
the compiled bzImage and ran the openat2 kselftests there to verify
it's working. Is there a similar way I can test NFS/SMB by adding
kselftests? Or would I need to setup NFS/SMB inside a full VM distro
with a modified kernel to test this? I would appreciate any suggestion
on this.

> There are some places where it doesn't seem like -EFTYPE will be
> returned. It looks like it can send back -EISDIR and -ENOTDIR in some
> cases as well. With a new API like this, I think we ought to strive for
> consistency.
>

Good point. There was a comment in a previous posting of this patch
series "The most useful behavior would indicate what was found (e.g.,
a pipe)."
(ref: https://lore.kernel.org/linux-fsdevel/vhq3osjqs3nn764wrp2lxp66b4dxpb3n5x3dijhe2yr53qfgy3@tfswbjskc3y6/
)
So I thought maybe it would be useful to return -EISDIR where it was
already doing that. But it is a good point about consistency that we
won't be doing this for other different types so I guess it's better
to return -EFTYPE for all the cases anyway as you mention. Any
thoughts?


> Should this API return -EFTYPE for all cases where it's not S_IFREG? If
> not, then what other errors are allowed? Bear in mind that you'll need
> to document this in the manpages too.
>

Are the manpages in the kernel git repository or in a separate
repository? Do I make separate patch series for that? Sorry I don't
know about this in detail.

> > When OPENAT2_REGULAR is combined with O_DIRECTORY, -EINVAL is returned
> > as it doesn't make sense to open a path that is both a directory and a
> > regular file.
> >
> > [1]: https://uapi-group.org/kernel-features/#ability-to-only-open-regular-files
> >
> > Signed-off-by: Dorjoy Chowdhury <[email protected]>
> > ---
> >  arch/alpha/include/uapi/asm/errno.h        |  2 ++
> >  arch/alpha/include/uapi/asm/fcntl.h        |  1 +
> >  arch/mips/include/uapi/asm/errno.h         |  2 ++
> >  arch/parisc/include/uapi/asm/errno.h       |  2 ++
> >  arch/parisc/include/uapi/asm/fcntl.h       |  1 +
> >  arch/sparc/include/uapi/asm/errno.h        |  2 ++
> >  arch/sparc/include/uapi/asm/fcntl.h        |  1 +
> >  fs/ceph/file.c                             |  4 ++++
> >  fs/gfs2/inode.c                            |  2 ++
> >  fs/namei.c                                 |  4 ++++
> >  fs/nfs/dir.c                               |  4 +++-
> >  fs/open.c                                  |  4 +++-
> >  fs/smb/client/dir.c                        | 11 ++++++++++-
> >  include/linux/fcntl.h                      |  2 ++
> >  include/uapi/asm-generic/errno.h           |  2 ++
> >  include/uapi/asm-generic/fcntl.h           |  4 ++++
> >  tools/arch/alpha/include/uapi/asm/errno.h  |  2 ++
> >  tools/arch/mips/include/uapi/asm/errno.h   |  2 ++
> >  tools/arch/parisc/include/uapi/asm/errno.h |  2 ++
> >  tools/arch/sparc/include/uapi/asm/errno.h  |  2 ++
> >  tools/include/uapi/asm-generic/errno.h     |  2 ++
> >  21 files changed, 55 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/alpha/include/uapi/asm/errno.h b/arch/alpha/include/uapi/asm/errno.h
> > index 6791f6508632..1a99f38813c7 100644
> > --- a/arch/alpha/include/uapi/asm/errno.h
> > +++ b/arch/alpha/include/uapi/asm/errno.h
> > @@ -127,4 +127,6 @@
> >
> >  #define EHWPOISON    139     /* Memory page has hardware error */
> >
> > +#define EFTYPE               140     /* Wrong file type for the intended operation */
> > +
> >  #endif
> > diff --git a/arch/alpha/include/uapi/asm/fcntl.h b/arch/alpha/include/uapi/asm/fcntl.h
> > index 50bdc8e8a271..fe488bf7c18e 100644
> > --- a/arch/alpha/include/uapi/asm/fcntl.h
> > +++ b/arch/alpha/include/uapi/asm/fcntl.h
> > @@ -34,6 +34,7 @@
> >
> >  #define O_PATH               040000000
> >  #define __O_TMPFILE  0100000000
> > +#define OPENAT2_REGULAR      0200000000
> >
> >  #define F_GETLK              7
> >  #define F_SETLK              8
> > diff --git a/arch/mips/include/uapi/asm/errno.h b/arch/mips/include/uapi/asm/errno.h
> > index c01ed91b1ef4..1835a50b69ce 100644
> > --- a/arch/mips/include/uapi/asm/errno.h
> > +++ b/arch/mips/include/uapi/asm/errno.h
> > @@ -126,6 +126,8 @@
> >
> >  #define EHWPOISON    168     /* Memory page has hardware error */
> >
> > +#define EFTYPE               169     /* Wrong file type for the intended operation */
> > +
> >  #define EDQUOT               1133    /* Quota exceeded */
> >
> >
> > diff --git a/arch/parisc/include/uapi/asm/errno.h b/arch/parisc/include/uapi/asm/errno.h
> > index 8cbc07c1903e..93194fbb0a80 100644
> > --- a/arch/parisc/include/uapi/asm/errno.h
> > +++ b/arch/parisc/include/uapi/asm/errno.h
> > @@ -124,4 +124,6 @@
> >
> >  #define EHWPOISON    257     /* Memory page has hardware error */
> >
> > +#define EFTYPE               258     /* Wrong file type for the intended operation */
> > +
> >  #endif
> > diff --git a/arch/parisc/include/uapi/asm/fcntl.h b/arch/parisc/include/uapi/asm/fcntl.h
> > index 03dee816cb13..d46812f2f0f4 100644
> > --- a/arch/parisc/include/uapi/asm/fcntl.h
> > +++ b/arch/parisc/include/uapi/asm/fcntl.h
> > @@ -19,6 +19,7 @@
> >
> >  #define O_PATH               020000000
> >  #define __O_TMPFILE  040000000
> > +#define OPENAT2_REGULAR      0100000000
> >
> >  #define F_GETLK64    8
> >  #define F_SETLK64    9
> > diff --git a/arch/sparc/include/uapi/asm/errno.h b/arch/sparc/include/uapi/asm/errno.h
> > index 4a41e7835fd5..71940ec9130b 100644
> > --- a/arch/sparc/include/uapi/asm/errno.h
> > +++ b/arch/sparc/include/uapi/asm/errno.h
> > @@ -117,4 +117,6 @@
> >
> >  #define EHWPOISON    135     /* Memory page has hardware error */
> >
> > +#define EFTYPE               136     /* Wrong file type for the intended operation */
> > +
> >  #endif
> > diff --git a/arch/sparc/include/uapi/asm/fcntl.h b/arch/sparc/include/uapi/asm/fcntl.h
> > index 67dae75e5274..bb6e9fa94bc9 100644
> > --- a/arch/sparc/include/uapi/asm/fcntl.h
> > +++ b/arch/sparc/include/uapi/asm/fcntl.h
> > @@ -37,6 +37,7 @@
> >
> >  #define O_PATH               0x1000000
> >  #define __O_TMPFILE  0x2000000
> > +#define OPENAT2_REGULAR      0x4000000
> >
> >  #define F_GETOWN     5       /*  for sockets. */
> >  #define F_SETOWN     6       /*  for sockets. */
> > diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> > index 31b691b2aea2..0a4220f72ada 100644
> > --- a/fs/ceph/file.c
> > +++ b/fs/ceph/file.c
> > @@ -977,6 +977,10 @@ int ceph_atomic_open(struct inode *dir, struct dentry *dentry,
> >                       ceph_init_inode_acls(newino, &as_ctx);
> >                       file->f_mode |= FMODE_CREATED;
> >               }
> > +             if ((flags & OPENAT2_REGULAR) && !d_is_reg(dentry)) {
> > +                     err = -EFTYPE;
> > +                     goto out_req;
> > +             }
> >               err = finish_open(file, dentry, ceph_open);
> >       }
> >  out_req:
> > diff --git a/fs/gfs2/inode.c b/fs/gfs2/inode.c
> > index 8344040ecaf7..0dc3e4240d9e 100644
> > --- a/fs/gfs2/inode.c
> > +++ b/fs/gfs2/inode.c
> > @@ -749,6 +749,8 @@ static int gfs2_create_inode(struct inode *dir, struct dentry *dentry,
> >               if (file) {
> >                       if (S_ISREG(inode->i_mode))
> >                               error = finish_open(file, dentry, gfs2_open_common);
> > +                     else if (file->f_flags & OPENAT2_REGULAR)
> > +                             error = -EFTYPE;
> >                       else
> >                               error = finish_no_open(file, NULL);
> >               }
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 5fe6cac48df8..aa5fb2672881 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -4651,6 +4651,10 @@ static int do_open(struct nameidata *nd,
> >               if (unlikely(error))
> >                       return error;
> >       }
> > +
> > +     if ((open_flag & OPENAT2_REGULAR) && !d_is_reg(nd->path.dentry))
> > +             return -EFTYPE;
> > +
> >       if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
> >               return -ENOTDIR;
> >
> > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > index b3f5c9461204..ef61db67d06e 100644
> > --- a/fs/nfs/dir.c
> > +++ b/fs/nfs/dir.c
> > @@ -2195,7 +2195,9 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry,
> >                       break;
> >               case -EISDIR:
> >               case -ENOTDIR:
> > -                     goto no_open;
> > +                     if (!(open_flags & OPENAT2_REGULAR))
> > +                             goto no_open;
> > +                     break;
>
> Shouldn't this also set the error to -EFTYPE?
>
> >               case -ELOOP:
> >                       if (!(open_flags & O_NOFOLLOW))
> >                               goto no_open;
> > diff --git a/fs/open.c b/fs/open.c
> > index 91f1139591ab..1524f52a1773 100644
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -1198,7 +1198,7 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
> >        * values before calling build_open_flags(), but openat2(2) checks all
> >        * of its arguments.
> >        */
> > -     if (flags & ~VALID_OPEN_FLAGS)
> > +     if (flags & ~VALID_OPENAT2_FLAGS)
> >               return -EINVAL;
> >       if (how->resolve & ~VALID_RESOLVE_FLAGS)
> >               return -EINVAL;
> > @@ -1237,6 +1237,8 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
> >                       return -EINVAL;
> >               if (!(acc_mode & MAY_WRITE))
> >                       return -EINVAL;
> > +     } else if ((flags & O_DIRECTORY) && (flags & OPENAT2_REGULAR)) {
> > +             return -EINVAL;
> >       }
> >       if (flags & O_PATH) {
> >               /* O_PATH only permits certain other flags to be set. */
> > diff --git a/fs/smb/client/dir.c b/fs/smb/client/dir.c
> > index cb10088197d2..d12ed0c87599 100644
> > --- a/fs/smb/client/dir.c
> > +++ b/fs/smb/client/dir.c
> > @@ -236,6 +236,11 @@ static int cifs_do_create(struct inode *inode, struct dentry *direntry, unsigned
> >                                * lookup.
> >                                */
> >                               CIFSSMBClose(xid, tcon, fid->netfid);
> > +                             if (oflags & OPENAT2_REGULAR) {
> > +                                     iput(newinode);
> > +                                     rc = -EFTYPE;
> > +                                     goto out;
> > +                             }
> >                               goto cifs_create_get_file_info;
> >                       }
> >                       /* success, no need to query */
> > @@ -433,11 +438,15 @@ static int cifs_do_create(struct inode *inode, struct dentry *direntry, unsigned
> >               goto out_err;
> >       }
> >
> > -     if (newinode)
> > +     if (newinode) {
> >               if (S_ISDIR(newinode->i_mode)) {
> >                       rc = -EISDIR;
> >                       goto out_err;
>
> This logic doesn't look quite right. If you do a create and race with a
> directory create, then it looks like you'll send back -EISDIR here
> instead of -EFTYPE?
>

Yes, I thought it would be useful for the caller to know what type of
file it actually is so I had kept this the same. Let me know if it is
preferable to return -EFTYPE always.

Thanks for reviewing!

Regards,
Dorjoy