Re: [RFC PATCH 0/2] Introduce a way to expose the interpreted file with binfmt_misc

From: "Eric W. Biederman" <ebiederm@xmission.com>
To: "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Cc: Kees Cook <keescook@chromium.org>,
	David Hildenbrand <david@redhat.com>,
	sonicadvance1@gmail.com, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	kernel-dev@igalia.com, kernel@gpiccoli.net, oleg@redhat.com,
	yzaikin@google.com, mcgrof@kernel.org, akpm@linux-foundation.org,
	brauner@kernel.org, viro@zeniv.linux.org.uk, willy@infradead.org,
	dave@stgolabs.net, joshua@froggi.es
Subject: Re: [RFC PATCH 0/2] Introduce a way to expose the interpreted file with binfmt_misc
Date: Mon, 13 Nov 2023 12:29:30 -0600	[thread overview]
Message-ID: <871qctwlpx.fsf@email.froward.int.ebiederm.org> (raw)
In-Reply-To: <8dc5069f-5642-cc5b-60e0-0ed3789c780b@igalia.com> (Guilherme G. Piccoli's message of "Mon, 13 Nov 2023 14:33:13 -0300")

"Guilherme G. Piccoli" <gpiccoli@igalia.com> writes:

> On 09/10/2023 14:37, Kees Cook wrote:
>> On Fri, Oct 06, 2023 at 02:07:16PM +0200, David Hildenbrand wrote:
>>> On 07.09.23 22:24, Guilherme G. Piccoli wrote:
>>>> Currently the kernel provides a symlink to the executable binary, in the
>>>> form of procfs file exe_file (/proc/self/exe_file for example). But what
>>>> happens in interpreted scenarios (like binfmt_misc) is that such link
>>>> always points to the *interpreter*. For cases of Linux binary emulators,
>>>> like FEX [0] for example, it's then necessary to somehow mask that and
>>>> emulate the true binary path.
>>>
>>> I'm absolutely no expert on that, but I'm wondering if, instead of modifying
>>> exe_file and adding an interpreter file, you'd want to leave exe_file alone
>>> and instead provide an easier way to obtain the interpreted file.
>>>
>>> Can you maybe describe why modifying exe_file is desired (about which
>>> consumers are we worrying? ) and what exactly FEX does to handle that (how
>>> does it mask that?).
>>>
>>> So a bit more background on the challenges without this change would be
>>> appreciated.
>> 
>> Yeah, it sounds like you're dealing with a process that examines
>> /proc/self/exe_file for itself only to find the binfmt_misc interpreter
>> when it was run via binfmt_misc?
>> 
>> What actually breaks? Or rather, why does the process to examine
>> exe_file? I'm just trying to see if there are other solutions here that
>> would avoid creating an ambiguous interface...
>> 
>
> Thanks Kees and David! Did Ryan's thorough comment addressed your
> questions? Do you have any take on the TODOs?
>
> I can maybe rebase against 6.7-rc1 and resubmit , if that makes sense!
> But would be better having the TODOs addressed, I guess.

Currently there is a mechanism in the kernel for changing
/proc/self/exe.  Would that be reasonable to use in this case?

It came from the checkpoint/restart work, but given that it is already
implemented it seems like the path of least resistance to get your
binfmt_misc that wants to look like binfmt_elf to use that mechanism.

Eric