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Date: Tue, 10 Mar 2026 20:24:27 +0100
From: Florian Westphal <fw@strlen.de>
To: Leon Romanovsky <leon@kernel.org>
Cc: Sabrina Dubroca <sd@queasysnail.net>, netdev@vger.kernel.org,
	Steffen Klassert <steffen.klassert@secunet.com>,
	Herbert Xu <herbert@gondor.apana.org.au>
Subject: Re: [PATCH ipsec-next 01/10] xfrm: state: fix sparse warnings on
 xfrm_state_hold_rcu
Message-ID: <abBv6zR4Qq_lrzuC@strlen.de>
References: <cover.1773051558.git.sd@queasysnail.net>
 <7388df7238672a92be0e4048f0225e6db294e736.1773051558.git.sd@queasysnail.net>
 <20260310103135.GB12611@unreal>
 <abABg6aO9twGIk7F@krikkit>
 <20260310182012.GF12611@unreal>
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In-Reply-To: <20260310182012.GF12611@unreal>

Leon Romanovsky <leon@kernel.org> wrote:
> > > This change makes me wonder why we need both xfrm_state_hold_rcu() and
> > > xfrm_state_hold().
> > 
> > Commit 02efdff7e209 ("xfrm: state: use atomic_inc_not_zero to
> > increment refcount") and the series around it [0] introduced the
> > possibility of that refcount increment failing.
> > 
> > I can't tell you why a 10-years-old commit made some choice, but
> > keeping both variants has the benefit of documenting that one
> > increment is expected to never fail (because we already hold a ref on
> > the object on that path) and we can skip the error handling. We don't
> > want to add error handling that will never get reached, it always goes
> > wrong (because it's untested) and it adds uneeded complexity to the
> > code.
> > 
> > So I wouldn't get rid of xfrm_state_hold.
> 
> So let's add some comment when this function should be used.

Before rcu-ification, everything was serialized via state lock.
Entries still in the state table thus always had > 0 refcounts.

You can only use xfrm_state_hold() if the refcount is already > 0.
xfrm_state_hold uses refcount_inc(), so you get a UaF warn splat
if this assuption doesn't hold true.