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From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jann Horn <jannh@google.com>,
	Linux API <linux-api@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS
Date: Fri, 5 May 2017 04:00:58 +0100	[thread overview]
Message-ID: <20170505030058.GO29622@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CA+55aFyOKM7DW7+0sdDFKdZFXgptb5r1id9=Wvhd8AgSP7qjwQ@mail.gmail.com>

On Thu, May 04, 2017 at 06:27:10PM -0700, Linus Torvalds wrote:

> As mentioned last time, at least for the git usage, even relative
> symlinks are a no-no - not because they'd escape, but simply because
> git wants to see the *unique* name, and resolve relative symlinks to
> either the symlink, or to the actual file it points to.
> 
> So I think that we'd want an additional flag that says "no symlinks at all".

OK, that's easily done.

> And I think the "no mountpoint" traversal might be splittable too.
> 
> Yes, sometimes you'd probably want to say "stay exactly inside this
> filesystem" (like find -xdev). So no arguments against AT_XDEV that
> refuses any mount traversal (kind of like my "no symlink traversal"
> thing).
> 
> But at other points you might want to just guarantee that the walk
> stays below a certain starting point and doesn't escape.
> 
> That could still allow crossing mount-points, but only if they are
> non-bind mounts and cannot let us escape.
> 
> I'm not sure if that's testable, though.

This one isn't, unfortunately - there is no difference between bind and
no-bind; vfsmounts form a tree and both normal mount and bind add leaves
to it.  Moreover, mount -t ext2 /dev/sdc7 /mnt; mount -t ext2 /dev/sdc7 /tmp/a
yield the same state as mount -t ext2 /dev/sdc7; mount --bind /mnt /tmp/a.
There is no way to tell the difference, simply because there *is* no
difference.  Moreover, either can be followed by umount /mnt and you'll get
the same state as you would have after a solitary mount of the same fs on
/tmp/a.

Ho-hum...  So:

			AT_BENEATH	AT_XDEV		AT_NO_SYMLINKS
absolute pathname:	EXDEV
non-relative symlink:	EXDEV		?		ELOOP
relative symlink:					ELOOP
.. from starting point:	EXDEV
.. crossing mountpoint:			EXDEV
crossing into mountpoint:		EXDEV

1) What should AT_XDEV do about absolute symlinks?  Nothing special?  EXDEV?
EXDEV if we are not on root?
2) What should AT_BENEATH | AT_NO_SYMLINKS do on absolute symlinks?  My
preference would be "AT_NO_SYMLINKS wins, ELOOP for you", but that's based
mostly upon the convenience of implementation.
3) What effect should AT_NO_SYMLINKS have upon the final component?  Same
as AT_SYMLINK_NOFOLLOW?

  reply	other threads:[~2017-05-05  3:01 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-29 22:04 Al Viro
2017-04-29 23:17 ` Andy Lutomirski
2017-04-29 23:25   ` Al Viro
2017-04-30  1:13     ` Andy Lutomirski
2017-04-30  4:38     ` Matthew Wilcox
2017-04-30 16:10       ` Al Viro
2017-05-01  4:52         ` Andy Lutomirski
2017-05-01  5:15           ` Al Viro
2017-05-01 17:36 ` Jann Horn
2017-05-01 19:37   ` Andy Lutomirski
2017-05-05  0:30   ` Al Viro
2017-05-05  0:44     ` Andy Lutomirski
2017-05-05  1:06       ` Al Viro
2017-05-05  1:27     ` Linus Torvalds
2017-05-05  3:00       ` Al Viro [this message]
2017-05-05  4:01         ` Linus Torvalds
2017-05-05  4:31           ` Andy Lutomirski
2017-05-05  2:47     ` Jann Horn
2017-05-05  3:46       ` Linus Torvalds
2017-05-05  4:39         ` Al Viro
2017-05-05  4:44           ` Andy Lutomirski
2017-05-05 20:04             ` Eric W. Biederman
2017-05-05 20:28           ` Eric W. Biederman
2017-05-08 19:34             ` Mickaël Salaün
2017-05-18  8:50     ` David Drysdale
2017-09-10 20:26 Jürg Billeter

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