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Date: Fri, 3 May 2024 12:36:14 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <keescook@chromium.org>, Al Viro <viro@zeniv.linux.org.uk>,
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Subject: Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long_t
Message-ID: <20240503103614.GF30852@noisy.programming.kicks-ass.net>
References: <20240502223341.1835070-5-keescook@chromium.org>
 <20240502224250.GM2118490@ZenIV>
 <202405021548.040579B1C@keescook>
 <20240502231228.GN2118490@ZenIV>
 <202405021620.C8115568@keescook>
 <20240502234152.GP2118490@ZenIV>
 <202405021708.267B02842@keescook>
 <20240503001445.GR2118490@ZenIV>
 <202405021736.574A688@keescook>
 <20240503-inventar-braut-c82e15e56a32@brauner>
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In-Reply-To: <20240503-inventar-braut-c82e15e56a32@brauner>

On Fri, May 03, 2024 at 11:37:25AM +0200, Christian Brauner wrote:
> On Thu, May 02, 2024 at 05:41:23PM -0700, Kees Cook wrote:
> > On Fri, May 03, 2024 at 01:14:45AM +0100, Al Viro wrote:
> > > On Thu, May 02, 2024 at 05:10:18PM -0700, Kees Cook wrote:
> > > 
> > > > But anyway, there needs to be a general "oops I hit 0"-aware form of
> > > > get_file(), and it seems like it should just be get_file() itself...
> > > 
> > > ... which brings back the question of what's the sane damage mitigation
> > > for that.  Adding arseloads of never-exercised failure exits is generally
> > > a bad idea - it's asking for bitrot and making the thing harder to review
> > > in future.
> > 
> > Linus seems to prefer best-effort error recovery to sprinkling BUG()s
> > around.  But if that's really the solution, then how about get_file()
> > switching to to use inc_not_zero and BUG on 0?
> 
> Making get_file() return an error is not an option. For all current
> callers that's pointless churn for a condition that's not supposed to
> happen at all.
> 
> Additionally, iirc *_inc_not_zero() variants are implemented with
> try_cmpxchg() which scales poorly under contention for a condition
> that's not supposed to happen.

	unsigned long old = atomic_long_fetch_inc_relaxed(&f->f_count);
	WARN_ON(!old);

Or somesuch might be an option?