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

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Date:	Thu, 24 Apr 2008 17:37:39 +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

> > Since all the vfs_* functions will become static with path_* being the
> > only caller, the compiler will be happy to get rid of that stack frame
> > too.
> > 
> > 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
> > the r/o checks.
> > 
> > I think it's definitely worth it.
> 
> It would be, if we had only single vfs_...() as critical sections.  We
> do not - see previous mail or read the fucking tree, already.

Why are those so important?  Yes, if we have multiple vfs_() calls,
surround them with an extra want/drop pair.  We do already do multiple
overlapping want/drop pairs with O_TRUNC and O_CREAT (AFAIR).

> We don't even have many callers of each, and with a few we do it's not
> obvious that we want to go through vfsmounts (and vfsmount-based checks)
> in all of them.  So no, I don't buy your argument.  Sorry.
> 
> I'm not even convinced that they are useful as helpers, at least until
> we'd decided what to do with checks in nfsd.  Until then we do, as
> far as I'm concerned, one place where they would definitely DTRT - fs/namei.c.
> And I want more than one caller before merging those,

unix_bind() -> vfs_mknod()
sys_mq_unlink() -> vfs_unlink()
open.c (several) -> notify_change()
*setxattr() -> vfs_setxattr()
*removexattr() -> vfs_removexattr()

> 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.

Miklos

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