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

[David Hildenbrand <david@redhat.com>]

On 14.11.23 17:11, Eric W. Biederman wrote:
> David Hildenbrand <david@redhat.com> writes:
> 
>> On 13.11.23 19:29, Eric W. Biederman wrote:
>>> "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.
>>
>> 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.
> 
> Bah yes.  The sanity check that that the old executable is no longer
> mapped does make it so that we can't trivially change the /proc/self/exe
> using prctl(PR_SET_MM_EXE_FILE).

I was wondering if we should have a new file (yet have to come up witha 
fitting name) that defaults to /proc/self/exe as long as that new file 
doesn't explicitly get set via  a prctl.

So /proc/self/exe would indeed always show the emulator (executable), 
but the new file could be adjusted to something that is being executed 
by the emulator.

Just a thought ... I'd rather leave /proc/self/exe alone.

-- 
Cheers,

David / dhildenb