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Date: Thu, 2 May 2024 15:52:21 -0700
From: Kees Cook <keescook@chromium.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
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Subject: Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long_t
Message-ID: <202405021548.040579B1C@keescook>
References: <20240502222252.work.690-kees@kernel.org>
 <20240502223341.1835070-5-keescook@chromium.org>
 <20240502224250.GM2118490@ZenIV>
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On Thu, May 02, 2024 at 11:42:50PM +0100, Al Viro wrote:
> On Thu, May 02, 2024 at 03:33:40PM -0700, Kees Cook wrote:
> > Underflow of f_count needs to be more carefully detected than it
> > currently is. The results of get_file() should be checked, but the
> > first step is detection. Redefine f_count from atomic_long_t to
> > refcount_long_t.
> 
> 	It is used on fairly hot paths.  What's more, it's not
> at all obvious what the hell would right semantics be.

I think we've put performance concerns between refcount_t and atomic_t
to rest long ago. If there is a real workload where it's a problem,
let's find it! :)

As for semantics, what do you mean? Detecting dec-below-zero means we
catch underflow, and detected inc-from-zero means we catch resurrection
attempts. In both cases we avoid double-free, but we have already lost
to a potential dangling reference to a freed struct file. But just
letting f_count go bad seems dangerous.

-- 
Kees Cook