linux-kernel - Re: [PATCH 2/3] xfs: convert perag lookup to xarray

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Message-ID: <20240813083000.soja6ixtad3mq5k4@quentin>
Date: Tue, 13 Aug 2024 08:30:00 +0000
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Christoph Hellwig <hch@....de>,
	Chandan Babu R <chandan.babu@...cle.com>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, p.raghav@...sung.com
Subject: Re: [PATCH 2/3] xfs: convert perag lookup to xarray

On Mon, Aug 12, 2024 at 08:02:36PM +0100, Matthew Wilcox wrote:
> On Mon, Aug 12, 2024 at 04:39:56PM +0000, Pankaj Raghav (Samsung) wrote:
> > On Mon, Aug 12, 2024 at 08:31:01AM +0200, Christoph Hellwig wrote:
> > > diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
> > > index 7e80732cb54708..5efb1e8b4107a9 100644
> > > --- a/fs/xfs/libxfs/xfs_ag.c
> > > +++ b/fs/xfs/libxfs/xfs_ag.c
> > > @@ -46,7 +46,7 @@ xfs_perag_get(
> > >  	struct xfs_perag	*pag;
> > >  
> > >  	rcu_read_lock();
> > xa_load() already calls rcu_read_lock(). So we can get rid of this I
> > guess?
> 
> Almost certainly not; I assume pag is RCU-freed, so you'd be introducing
> a UAF if the RCU lock is dropped before getting a refcount on the pag.

Ah, yes. rcu lock is needed until we get a refcount on pag.