linux-kernel - Re: [RFC PATCH 19/24] erofs: introduce namei alternative to C

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Message-ID: <20240917074429.GE3107530@ZenIV>
Date: Tue, 17 Sep 2024 08:44:29 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: Yiyang Wu <toolmanp@...p.cc>, linux-erofs@...ts.ozlabs.org,
	rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 19/24] erofs: introduce namei alternative to C

On Tue, Sep 17, 2024 at 08:31:49AM +0100, Al Viro wrote:

> > After d_splice_alias() and d_add(), rename() could change d_name.  So
> > either we take d_lock or with rcu_read_lock() to take a snapshot of
> > d_name in the RCU walk path.  That is my overall understanding.
> 
> No, it's more complicated than that, sadly.  ->d_name and ->d_parent are
> the trickiest parts of dentry field stability.
> 
> > But for EROFS, since we don't have rename, so it doesn't matter.
> 
> See above.  IF we could guarantee that all filesystem images are valid
> and will remain so, life would be much simpler.

In any case, currently it is safe - d_splice_alias() is the last thing
done by erofs_lookup().  Just don't assume that names can't change in
there - and the fewer places in filesystem touch ->d_name, the better.

In practice, for ->lookup() you are safe until after d_splice_alias()
and for directory-modifying operations you are safe unless you start
playing insane games with unlocking and relocking the parent directories
(apparmorfs does; the locking is really obnoxious there).  That covers
the majority of ->d_name and ->d_parent accesses in filesystem code.

->d_hash() and ->d_compare() are separate story; I've posted a text on
that last year (or this winter - not sure, will check once I get some
sleep).

d_path() et.al. are taking care to do the right thing; those (and %pd
format) can be used safely.

Anyway, I'm half-asleep at the moment and I'd rather leave writing these
rules up until tomorrow.  Sorry...