linux-kernel - Re: [PATCH] binder: fix descriptor lookup for context manager

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Message-ID: <ZpbDVXiiVVxyx84X@google.com>
Date: Tue, 16 Jul 2024 19:00:37 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: Todd Kjos <tkjos@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Christian Brauner <brauner@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Alice Ryhl <aliceryhl@...gle.com>, linux-kernel@...r.kernel.org,
	kernel-team@...roid.com, syzkaller-bugs@...glegroups.com,
	stable@...r.kernel.org,
	syzbot+3dae065ca76952a67257@...kaller.appspotmail.com
Subject: Re: [PATCH] binder: fix descriptor lookup for context manager

On Tue, Jul 16, 2024 at 06:48:54PM +0000, Carlos Llamas wrote:
> On Tue, Jul 16, 2024 at 10:40:20AM -0700, Todd Kjos wrote:
> > If context manager doesn't need to be bit 0 anymore, then why do we
> > bother to prefer bit 0? Does it matter?
> > 
> > It would simplify the code below if the offset is always 0 since you
> > wouldn't need an offset at all.
> 
> Yes, it would make things simplier if references to the context manager
> could get any descriptor id. However, there seems to be an expectation
> from libbinder that this descriptor would be zero. At least according to
> some folks more familiar with userspace binder than myself.
> 
> I think we can revisit this expectation though and also look closer at
> the scenario of a context manager "swap". The procs can still reach the
> new context manager using descriptor 0. However, this may cause some
> issues with operations with refs such as BC_INCREFS/BC_DECREFS.
> 
> AFAICT, the context manager doesn't even need a reference. But while we
> dig furhter into this I think the best option is to keep the behavior
> the same for now: reserve descriptor zero for the context manager node
> unless it's already taken. Changing this is non-trivial IMO.
> 
> --
> Carlos Llamas

Also, we need to consider that references to regular nodes (not context
manager) cannot get descriptor zero assigned to them for obvious
reasons. So descriptor zero is always reserved for the context manager,
but there might be certain scenarios in which references to the context
manager get a non-zero descriptor.