archive mirror
 help / color / mirror / Atom feed
From: Al Viro <>
To: Linus Torvalds <>
Cc: Jann Horn <>,
	Linux API <>,
	Linux Kernel Mailing List <>,
	linux-fsdevel <>
Subject: Re: new flag: AT_NO_JUMPS
Date: Fri, 5 May 2017 04:00:58 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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

Ho-hum...  So:

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

  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 new flag: AT_NO_JUMPS 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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).