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Date: Thu, 2 May 2024 16:21:13 -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: <202405021620.C8115568@keescook>
References: <20240502222252.work.690-kees@kernel.org>
 <20240502223341.1835070-5-keescook@chromium.org>
 <20240502224250.GM2118490@ZenIV>
 <202405021548.040579B1C@keescook>
 <20240502231228.GN2118490@ZenIV>
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On Fri, May 03, 2024 at 12:12:28AM +0100, Al Viro wrote:
> On Thu, May 02, 2024 at 03:52:21PM -0700, Kees Cook wrote:
> 
> > 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.
> 
> Detected inc-from-zero can also mean an RCU lookup detecting a descriptor
> in the middle of getting closed.  And it's more subtle than that, actually,
> thanks to SLAB_TYPESAFE_BY_RCU for struct file.

But isn't that already handled by __get_file_rcu()? i.e. shouldn't it be
impossible for a simple get_file() to ever see a 0 f_count under normal
conditions?

-- 
Kees Cook