Messages in this thread | | | Date | Tue, 14 Nov 2023 17:14:59 +0100 | Subject | Re: [RFC PATCH 0/2] Introduce a way to expose the interpreted file with binfmt_misc | From | David Hildenbrand <> |
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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
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