Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20200401144109.GA29945@gardel-login> <CAJfpegs3uDzFTE4PCjZ7aZsEh8b=iy_LqO1DBJoQzkP+i4aBmw@mail.gmail.com>
 <2590640.1585757211@warthog.procyon.org.uk> <CAJfpegsXqxizOGwa045jfT6YdUpMxpXET-yJ4T8qudyQbCGkHQ@mail.gmail.com>
 <36e45eae8ad78f7b8889d9d03b8846e78d735d28.camel@themaw.net>
 <CAJfpegsCDWehsTRQ9UJYuQnghnE=M8L0_bJBTTPA+Upu87t90w@mail.gmail.com>
 <27994c53034c8f769ea063a54169317c3ee62c04.camel@themaw.net>
 <20200403111144.GB34663@gardel-login> <CAJfpeguQAw+Mgc8QBNd+h3KV8=Y-SOGT7TB_N_54wa8MCoOSzg@mail.gmail.com>
 <20200403151223.GB34800@gardel-login> <20200403203024.GB27105@fieldses.org>
In-Reply-To: <20200403203024.GB27105@fieldses.org>
From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Mon, 6 Apr 2020 10:35:55 +0200
Message-ID: <CAJfpegvxnp8N-o-iTXzj0UnYZbDPfms1zpwcHf1tdhRJ4au3Og@mail.gmail.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     Lennart Poettering <mzxreary@0pointer.de>,
        Ian Kent <raven@themaw.net>,
        David Howells <dhowells@redhat.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, dray@redhat.com,
        Karel Zak <kzak@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Jeff Layton <jlayton@redhat.com>, andres@anarazel.de,
        keyrings@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, Aleksa Sarai <cyphar@cyphar.com>
Content-Type: multipart/mixed; boundary="0000000000008acd9805a29b28ff"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

--0000000000008acd9805a29b28ff
Content-Type: text/plain; charset="UTF-8"

On Fri, Apr 3, 2020 at 10:30 PM J. Bruce Fields <bfields@fieldses.org> wrote:
>
> On Fri, Apr 03, 2020 at 05:12:23PM +0200, Lennart Poettering wrote:
> > BTW, while we are at it: one more thing I'd love to see exposed by
> > statx() is a simple flag whether the inode is a mount point. There's
> > plenty code that implements a test like this all over the place, and
> > it usually isn't very safe. There's one implementation in util-linux
> > for example (in the /usr/bin/mountpoint binary), and another one in
> > systemd. Would be awesome to just have a statx() return flag for that,
> > that would make things *so* much easier and more robust. because in
> > fact most code isn't very good that implements this, as much of it
> > just compares st_dev of the specified file and its parent. Better code
> > compares the mount ID, but as mentioned that's not as pretty as it
> > could be so far...
>
> nfs-utils/support/misc/mountpoint.c:check_is_mountpoint() stats the file
> and ".." and returns true if they have different st_dev or the same
> st_ino.  Comparing mount ids sounds better.
>
> So anyway, yes, everybody reinvents the wheel here, and this would be
> useful.  (And, yes, we want to know for the vfsmount, we don't care
> whether the same inode is used as a mountpoint someplace else.)

Attaching a patch.

There's some ambiguity about what is a "mountpoint" and what these
tools are interested in.  My guess is that they are not interested in
an object being a mount point (something where another object is
mounted) but being a mount root (this is the object mounted at the
mount point).  I.e

fd = open("/mnt", O_PATH);
mount("/bin", "/mnt", NULL, MS_BIND, NULL);
statx(AT_FDCWD, "/mnt", 0, 0, &stx1);
statx(fd, "", AT_EMPTY_PATH, 0, &stx2);
printf("mount_root(/mnt) = %c, mount_root(fd) = %c\n",
    stx1.stx_attributes & STATX_ATTR_MOUNT_ROOT ? 'y' : 'n',
    stx2.stx_attributes & STATX_ATTR_MOUNT_ROOT ? 'y' : 'n');

Would print:
mount_root(/mnt) = y, mount_root(fd) = n

Thanks,
Miklos

--0000000000008acd9805a29b28ff
Content-Type: text/x-patch; charset="US-ASCII"; name="statx-add-mount_root.patch"
Content-Disposition: attachment; filename="statx-add-mount_root.patch"
Content-Transfer-Encoding: base64
Content-ID: <f_k8o7ysgp0>
X-Attachment-Id: f_k8o7ysgp0

RnJvbTogTWlrbG9zIFN6ZXJlZGkgPG1zemVyZWRpQHJlZGhhdC5jb20+ClN1YmplY3Q6IHN0YXR4
OiBhZGQgbW91bnRfcm9vdAoKRGV0ZXJtaW5pbmcgd2hldGhlciBhIHBhdGggb3IgZmlsZSBkZXNj
cmlwdG9yIHJlZmVycyB0byBhIG1vdW50cG9pbnQgKG9yCm1vcmUgcHJlY2lzZWx5IGEgbW91bnQg
cm9vdCkgaXMgbm90IHRyaXZpYWwgdXNpbmcgY3VycmVudCB0b29scy4KCkFkZCBhIGZsYWcgdG8g
c3RhdHggdGhhdCBpbmRpY2F0ZXMgd2hldGhlciB0aGUgcGF0aCBvciBmZCByZWZlcnMgdG8gdGhl
CnJvb3Qgb2YgYSBtb3VudCBvciBub3QuCgpSZXBvcnRlZC1ieTogTGVubmFydCBQb2V0dGVyaW5n
IDxtenhyZWFyeUAwcG9pbnRlci5kZT4KUmVwb3J0ZWQtYnk6IEouIEJydWNlIEZpZWxkcyA8YmZp
ZWxkc0BmaWVsZHNlcy5vcmc+ClNpZ25lZC1vZmYtYnk6IE1pa2xvcyBTemVyZWRpIDxtc3plcmVk
aUByZWRoYXQuY29tPgotLS0KIGZzL3N0YXQuYyAgICAgICAgICAgICAgICAgfCAgICAzICsrKwog
aW5jbHVkZS91YXBpL2xpbnV4L3N0YXQuaCB8ICAgIDEgKwogMiBmaWxlcyBjaGFuZ2VkLCA0IGlu
c2VydGlvbnMoKykKCi0tLSBhL2luY2x1ZGUvdWFwaS9saW51eC9zdGF0LmgKKysrIGIvaW5jbHVk
ZS91YXBpL2xpbnV4L3N0YXQuaApAQCAtMTcyLDYgKzE3Miw3IEBAIHN0cnVjdCBzdGF0eCB7CiAj
ZGVmaW5lIFNUQVRYX0FUVFJfTk9EVU1QCQkweDAwMDAwMDQwIC8qIFtJXSBGaWxlIGlzIG5vdCB0
byBiZSBkdW1wZWQgKi8KICNkZWZpbmUgU1RBVFhfQVRUUl9FTkNSWVBURUQJCTB4MDAwMDA4MDAg
LyogW0ldIEZpbGUgcmVxdWlyZXMga2V5IHRvIGRlY3J5cHQgaW4gZnMgKi8KICNkZWZpbmUgU1RB
VFhfQVRUUl9BVVRPTU9VTlQJCTB4MDAwMDEwMDAgLyogRGlyOiBBdXRvbW91bnQgdHJpZ2dlciAq
LworI2RlZmluZSBTVEFUWF9BVFRSX01PVU5UX1JPT1QJCTB4MDAwMDIwMDAgLyogUm9vdCBvZiBh
IG1vdW50ICovCiAjZGVmaW5lIFNUQVRYX0FUVFJfVkVSSVRZCQkweDAwMTAwMDAwIC8qIFtJXSBW
ZXJpdHkgcHJvdGVjdGVkIGZpbGUgKi8KIAogCi0tLSBhL2ZzL3N0YXQuYworKysgYi9mcy9zdGF0
LmMKQEAgLTIwMiw2ICsyMDIsOSBAQCBpbnQgdmZzX3N0YXR4KGludCBkZmQsIGNvbnN0IGNoYXIg
X191c2VyCiAJZXJyb3IgPSB2ZnNfZ2V0YXR0cigmcGF0aCwgc3RhdCwgcmVxdWVzdF9tYXNrLCBm
bGFncyk7CiAJc3RhdC0+bW50X2lkID0gcmVhbF9tb3VudChwYXRoLm1udCktPm1udF9pZDsKIAlz
dGF0LT5yZXN1bHRfbWFzayB8PSBTVEFUWF9NTlRfSUQ7CisJaWYgKHBhdGgubW50LT5tbnRfcm9v
dCA9PSBwYXRoLmRlbnRyeSkKKwkJc3RhdC0+YXR0cmlidXRlcyB8PSBTVEFUWF9BVFRSX01PVU5U
X1JPT1Q7CisJc3RhdC0+YXR0cmlidXRlc19tYXNrIHw9IFNUQVRYX0FUVFJfTU9VTlRfUk9PVDsK
IAlwYXRoX3B1dCgmcGF0aCk7CiAJaWYgKHJldHJ5X2VzdGFsZShlcnJvciwgbG9va3VwX2ZsYWdz
KSkgewogCQlsb29rdXBfZmxhZ3MgfD0gTE9PS1VQX1JFVkFMOwo=
--0000000000008acd9805a29b28ff--