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

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Message-ID: <202310091034.4F58841@keescook>
Date:   Mon, 9 Oct 2023 10:37:10 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        kernel-dev@...lia.com, kernel@...ccoli.net, ebiederm@...ssion.com,
        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,
        sonicadvance1@...il.com, joshua@...ggi.es
Subject: Re: [RFC PATCH 0/2] Introduce a way to expose the interpreted file
 with binfmt_misc

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...

-- 
Kees Cook