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Re: [Qemu-devel] [Bug 1835839] Re: qemu-user: $0 incorrectly always repo

From: Laurent Vivier
Subject: Re: [Qemu-devel] [Bug 1835839] Re: qemu-user: $0 incorrectly always reports absolute path
Date: Tue, 09 Jul 2019 13:47:08 -0000

Le 09/07/2019 à 14:16, Peter Maydell a écrit :
> Is the proposed patch backwards compatible (ie "old QEMU binary works
> with newer binfmt-misc registration" and "new QEMU binary works with
> older binfmt-misc registration") ? Because binfmt-misc stuff is whole-
> system but QEMU binaries are per-chroot, this kind of thing is awkward
> to change if we don't have back-compat (and typically the kernel
> semantics for these things often don't allow back-compat or any kind of
> migration-path to the new better setup :-( )

If you don't enable the preserve-arg[0] flag, old and new QEMU will

If you enable the flag, only new QEMU with -0/QEMU_ARGV0 will work.

The best solution would be to force preserve-arg[0] with open-binary
flag and rely on AT_FDEXEC to detect the binfmt-misc environment, but
this breaks compatibility with existing environment and old QEMU.

Regarding the "binfmt-misc stuff is whole-system" problem, I've proposed
months ago a kernel based solution [1] to have a configuration per
namespace (chroot), but no one seems really interested (I think
maintainer is afraid by potential security issues).

[1] ns: introduce binfmt_misc namespace

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  qemu-user: $0 incorrectly always reports absolute path

Status in QEMU:

Bug description:
  We just ran into an issue with the Perl package on Debian/m68k when
  being built with qemu-user [1].

  The problem can be boiled down to qemu-user always reporting absolute
  paths for the shell variable $0 no matter on how the command was

  A simple reproducer is this:

  On normal system (no emulation):

  root@nofan:~> sh -c 'echo $0'

  On qemu-user:

  (sid-m68k-sbuild)root@nofan:/# sh -c 'echo $0'

  > [1] https://lists.debian.org/debian-68k/2019/07/msg00007.html

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