linux-kernel - Re: [patch 00/13] vfs: add helpers to check r/o bind mounts

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Message-Id: <E1Jp47x-0004MY-1W@pomaz-ex.szeredi.hu>
Date:	Thu, 24 Apr 2008 18:16:53 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	viro@...IV.linux.org.uk
CC:	miklos@...redi.hu, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, dave@...ux.vnet.ibm.com,
	ezk@...sunysb.edu, mhalcrow@...ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 00/13] vfs: add helpers to check r/o bind mounts

> > > > What is left is the guarantee, that the race-free r/o remounts will
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > > always work and some obscure caller didn't forget to surround it with
> ^^^^^^^^^^^^^^^^^
> 
> > Why are those so important?  Yes, if we have multiple vfs_() calls,
> > surround them with an extra want/drop pair.
> 
> Which leaves you with the same need to audit all these suckers anyway.

Not really.  Missing such calls would just make the *caller* buggy
(i.e. racy with remount r/o), but it would not make the *filesystem*
buggy.  Big difference.

> I'm in principle fine with having such helper functions, *IF* they are
> not sold as providing all protection one needs,

I'm not selling them as that.

> *IF* you are not expecting
> to be able to fold all areas down into them and *IF* original ones are
> left intact.

Left intact for whom, specifically?  Another question you've managed
to avoid answering.

> Modulo the like path_rename(), BTW - that one is just plain ugly API.

I'm all open to improvements.

> > > let alone removing the interface that doesn't require checks to be
> > > vfsmount-based for all users.
> > 
> > What users?  There are paractically _no_ other users.  The ones that
> > there are (like reiserfs) should not be using them, and there are
> > already some patches cleaning that mess up.
> 
> OK, explain me, in small words, WTF should something that wants to do
> operations on filesystem tree have a vfsmount.  Slowly.  And "r/o
> bind loses value if it can be bypassed" is a hogwash - fs methods are
> still there, so it *can* be bypassed just fine, thank you very much.

And we know what to do with such users.

> It's really up to caller.  "But they won't be able to do open()" also
> doesn't fly - again, it's up to whoever writes particular piece of code.

I understand your theory.  But it has zero practical significance.

IOW it doesn't matter that someone _may_ want to access the filesystem
without a vfsmount, if that someone doesn't exist.

Miklos
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