Merge patch series "OPENAT2_REGULAR flag support for openat2"

Dorjoy Chowdhury <dorjoychy111@gmail.com> says:

I came upon this "Ability to only open regular files" uapi feature
suggestion from
https://uapi-group.org/kernel-features/#ability-to-only-open-regular-files
and thought it would be something I could do as a first patch and get to
know the kernel code a bit better.

The following filesystems have been tested by building and booting the
kernel x86 bzImage in a Fedora 43 VM in QEMU. I have tested with
OPENAT2_REGULAR that regular files can be successfully opened and
non-regular files (directory, fifo etc) return -EFTYPE.

- btrfs
- NFS (loopback)
- SMB (loopback)

Christian Brauner (Amutable) <brauner@kernel.org>:

All atomic_open implementations were audited for OPENAT2_REGULAR handling.
Explicit checks were added to ceph, gfs2, nfs (v4), and cifs/smb — these
are the filesystems whose atomic_open can encounter an existing non-regular
file and would otherwise call finish_open() on it or return a misleading
error code. The checks allow these filesystems to return -EFTYPE directly
and avoid unnecessary open+close round-trips.

The remaining implementations (9p, fuse, vboxsf, nfs v2/v3) don't need
explicit checks. They only call finish_open() on freshly created files
(always S_IFREG) and use finish_no_open() for lookup hits, letting the
VFS catch non-regular files via the do_open() safety net. Notably, fuse
also validates the server response (S_ISREG check on the reply) before
reaching finish_open().

* patches from https://patch.msgid.link/20260328172314.45807-1-dorjoychy111@gmail.com:
  kselftest/openat2: test for OPENAT2_REGULAR flag
  openat2: new OPENAT2_REGULAR flag support

Link: https://patch.msgid.link/20260328172314.45807-1-dorjoychy111@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 49 insertions, 0 deletions

diff --git a/tools/testing/selftests/filesystems/openat2/openat2_test.c b/tools/testing/selftests/filesystems/openat2/openat2_test.c
index 5ea3eebb7b59..6f5afbe2d8d3 100644
--- a/tools/testing/selftests/filesystems/openat2/openat2_test.c
+++ b/tools/testing/selftests/filesystems/openat2/openat2_test.c
@@ -310,4 +310,53 @@ TEST_F(openat2, flag_validation)
 	}
 }
 
+#ifndef OPENAT2_REGULAR
+#define OPENAT2_REGULAR ((__u64)1 << 32)
+#endif
+
+#ifndef EFTYPE
+#define EFTYPE 134
+#endif
+
+/* Kernel-internal carrier for OPENAT2_REGULAR (see __O_REGULAR in fcntl.h). */
+#ifndef __O_REGULAR
+#define __O_REGULAR (1 << 30)
+#endif
+
+/* Verify that OPENAT2_REGULAR rejects non-regular files with EFTYPE. */
+TEST_F(openat2, regular_flag)
+{
+	struct open_how how = {
+		.flags = OPENAT2_REGULAR | O_RDONLY,
+	};
+	int fd;
+
+	fd = sys_openat2(AT_FDCWD, "/dev/null", &how);
+	if (fd == -ENOENT)
+		SKIP(return, "/dev/null does not exist");
+
+	EXPECT_EQ(-EFTYPE, fd) {
+		TH_LOG("openat2 with OPENAT2_REGULAR should fail with %d (%s), got %d (%s)",
+		       -EFTYPE, strerror(EFTYPE), fd, strerror(-fd));
+	}
+	if (fd >= 0)
+		close(fd);
+}
+
+/* open()/openat() must keep ignoring the internal __O_REGULAR bit. */
+TEST(legacy_openat_ignores_o_regular)
+{
+	int fd;
+
+	fd = openat(AT_FDCWD, "/dev/null", O_RDONLY | __O_REGULAR);
+	if (fd < 0 && errno == ENOENT)
+		SKIP(return, "/dev/null does not exist");
+
+	ASSERT_GE(fd, 0) {
+		TH_LOG("legacy openat() must ignore the __O_REGULAR carrier bit, got errno %d (%s)",
+		       errno, strerror(errno));
+	}
+	close(fd);
+}
+
 TEST_HARNESS_MAIN