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Date:   Mon, 9 Oct 2023 10:37:10 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Cc:     David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        kernel-dev@igalia.com, kernel@gpiccoli.net, ebiederm@xmission.com,
        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,
        sonicadvance1@gmail.com, joshua@froggi.es
Subject: Re: [RFC PATCH 0/2] Introduce a way to expose the interpreted file
 with binfmt_misc
Message-ID: <202310091034.4F58841@keescook>
References: <20230907204256.3700336-1-gpiccoli@igalia.com>
 <e673d8d6-bfa8-be30-d1c1-fe09b5f811e3@redhat.com>
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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