linux-kernel - Re: [RFC][PATCHSET v3] non-recursive pathname resolution & RCU symlinks

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150516013853.GN7232@ZenIV.linux.org.uk>
Date:	Sat, 16 May 2015 02:38:53 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	NeilBrown <neilb@...e.de>, Andreas Dilger <adilger@...ger.ca>,
	Dave Chinner <david@...morbit.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC][PATCHSET v3] non-recursive pathname resolution & RCU
 symlinks

On Fri, May 15, 2015 at 05:45:56PM -0700, Linus Torvalds wrote:

> Al, do you have any ideas? Personally, I've wanted to make I_mutex a
> rwsem for a long time, but right now pretty much everything uses it
> for exclusion. For example, filename lookup is clearly just reading
> the directory, so it should take a rwsem for reading, right? No. Not
> the way it is done now. Filename lookup wants the directory inode
> exclusively because that guarantees that we create just one dentry and
> call the filesystem ->lookup only once on that dentry.

rwsem by itself won't do us much good there.  Look: for multiple lookups on
the same existing entry we could try to teach d_splice_alias() to cope,
etc.  But what happens when a bunch of processes looks for the same
inexistent entry?  And no, "who cares about fuckloads of negatives with
the same name" isn't a good answer - suppose we do mkdir() after that.
OK, so we'll find a negative dentry in dcache.  And tell the filesystem
to create the sucker.  Done.  Made it positive.  Now, do we hunt down
all _other_ negative dentries for it?  Or never keep negative ones at
all.  Or slap some kind of ->d_revalidate() there to catch all negative
dentries creates before the last mkdir/creat/mknod/symlink/link in given
parent?

One possibility would be a new dentry state - "being looked up".  Hashed,
treated as "fall out of RCU mode" for lazy pathwalk purposes, and places
where we call ->lookup() would (while still holding ->i_mutex on parent
shared) wait for that state to end.  Places where we call ->d_revalidate()
(with or without ->i_mutex on parent) would also wait on those.

It would need a careful analysis of tree-walkers, though.  Doable, but there
might be dragons.  In case of e.g. ceph - swamp ones, with mirror in the line
of sight...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/