linux-kernel - Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution

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Date:   Wed, 4 Sep 2019 16:44:54 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     David Howells <dhowells@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>, Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
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Subject: Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape
 on ".." resolution

On Wed, Sep 4, 2019 at 4:29 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Wed, Sep 04, 2019 at 03:38:20PM -0700, Linus Torvalds wrote:
> > On Wed, Sep 4, 2019 at 3:31 PM David Howells <dhowells@...hat.com> wrote:
> > >
> > > It ought to be reasonably easy to make them per-sb at least, I think.  We
> > > don't allow cross-super rename, right?
> >
> > Right now the sequence count handling very much depends on it being a
> > global entity on the reader side, at least.
> >
> > And while the rename sequence count could (and probably should) be
> > per-sb, the same is very much not true of the mount one.
>
> Huh?  That will cost us having to have a per-superblock dentry
> hash table; recall that lockless lockup can give false negatives
> if something gets moved from chain to chain, and rename_lock is
> first and foremost used to catch those and retry.  If we split
> it on per-superblock basis, we can't have dentries from different
> superblocks in the same chain anymore...

That's exactly the "very much depends on it being a global entity on
the reader side" thing.

I'm not convinced that's the _only_ way to handle things. Maybe a
combination of (wild handwaving) per-hashqueue sequence count and some
clever scheme for pathname handling could work.

I've not personally seen a load where the global rename lock has been
a problem (very few things really do a lot of renames), but
system-wide locks do make me nervous.

We have other (and worse) ones. tasklist_lock comes to mind.

             Linus