linux-kernel - Re: [PATCH 02/20] STAGING/lustre: limit follow

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Date:	Mon, 20 Apr 2015 03:29:42 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	NeilBrown <neilb@...e.de>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Oleg Drokin <oleg.drokin@...el.com>,
	Andreas Dilger <andreas.dilger@...el.com>
Subject: Re: [PATCH 02/20] STAGING/lustre: limit follow_link recursion using
 stack space.

On Sun, Apr 19, 2015 at 10:33:48PM +0100, Al Viro wrote:
> On Sun, Apr 19, 2015 at 02:57:07PM -0600, Andreas Dilger wrote:
> 
> > I'd be happy if symlink recursion was removed completely, but so far the
> > added symlink recursion limit hasn't been a problem for Lustre users.
> 
> Well, it's gone in my tree; I've just pushed the current queue to
> vfs.git#link_path_walk.  Right now I'm looking at the unholy mess
> gcc does to stack footprint with inlining - the last commit in there
> is a result of exactly that.  Inlines in there really need tuning ;-/

FWIW, right now in my tree the maximal stack footprint of call chains through
fs/namei.c (amd64, my test config, including aushit) is 1408 bytes.
Goes via rename() -> renameat2() -> user_path_parent() -> filename_lookup() ->
path_lookupa() -> path_init() or follow_link() -> link_path_walk() ->
walk_component() -> lookup_fast() -> follow_managed().  And that does *not*
depend upon the depth of symlink nesting.  The maximal depth when calling
any methods present in lustre is 1328; similar path, except that its tail
goes like walk_component() -> __lookup_hash() -> lookup_dcache() ->
->d_revalidate().  Again, independent from the symlink nesting depth.
->lookup() calls are at 1296 maximum, similar call chain, for ->permission()
it's 1152, for ->follow_link() - 1088.

For mainline it's _much_ worse.  Maximal depth on the same config is
2986 bytes (with 8 levels of nesting) and each level costs 208 bytes.
->d_revalidate() is at 2880; for lustre it would be reduced a bit (again,
208 per level), but if you have any symlinks at all, you will end up
deeper than in non-recursive variant.

And frankly, the most scary thing in there isn't lustre-related - it's NFS4
(and AFS, etc.), where ->d_automount() might get called on _that_ depth.  With
quite a bit of stack footprint of its own - we are doing NFS referral handling.
With almost 3Kb of stack already eaten up.
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