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Date: Fri, 3 May 2024 11:37:25 +0200
From: Christian Brauner <brauner@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>, 
	linux-fsdevel@vger.kernel.org, Zack Rusin <zack.rusin@broadcom.com>, 
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	Rodrigo Vivi <rodrigo.vivi@intel.com>, Tvrtko Ursulin <tursulin@ursulin.net>, 
	Andi Shyti <andi.shyti@linux.intel.com>, Lucas De Marchi <lucas.demarchi@intel.com>, 
	Matt Atwood <matthew.s.atwood@intel.com>, Matthew Auld <matthew.auld@intel.com>, 
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Subject: Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long_t
Message-ID: <20240503-inventar-braut-c82e15e56a32@brauner>
References: <20240502222252.work.690-kees@kernel.org>
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 <202405021548.040579B1C@keescook>
 <20240502231228.GN2118490@ZenIV>
 <202405021620.C8115568@keescook>
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 <202405021708.267B02842@keescook>
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 <202405021736.574A688@keescook>
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In-Reply-To: <202405021736.574A688@keescook>

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.