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

[David Hildenbrand]

On 14.11.23 17:11, Eric W. Biederman wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
>
> > On 13.11.23 19:29, Eric W. Biederman wrote:
> > > "Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx> 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