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

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Message-ID: <9f83d97e-b7a1-4142-8316-088b3854c30d@redhat.com>
Date:   Mon, 13 Nov 2023 20:16:49 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     Kees Cook <keescook@...omium.org>, 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

On 13.11.23 19:29, Eric W. Biederman wrote:
> "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.

I had that in mind as well, but 
prctl_set_mm_exe_file()->replace_mm_exe_file() fails if the executable 
is still mmaped (due to denywrite handling); that should be the case for 
the emulator I strongly assume.

-- 
Cheers,

David / dhildenb