linux-kernel - Re: [RFC PATCH 0/2] Introduce a way to expose the interpreted file with binfmt

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Date:   Mon, 13 Nov 2023 12:29:30 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     Kees Cook <keescook@...omium.org>,
        David Hildenbrand <david@...hat.com>, sonicadvance1@...il.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, kernel-dev@...lia.com, kernel@...ccoli.net,
        oleg@...hat.com, yzaikin@...gle.com, mcgrof@...nel.org,
        akpm@...ux-foundation.org, brauner@...nel.org,
        viro@...iv.linux.org.uk, willy@...radead.org, dave@...olabs.net,
        joshua@...ggi.es
Subject: Re: [RFC PATCH 0/2] Introduce a way to expose the interpreted file
 with binfmt_misc

"Guilherme G. Piccoli" <gpiccoli@...lia.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