Re: [PATCH v1] linux-user: Add option to run `execve`d programs through QEMU

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Wed, 2024-10-02 at 16:08 +0200, Laurent Vivier wrote:
> Le 02/10/2024 à 10:08, Ilya Leoshkevich a écrit :
> > On Fri, 2024-08-30 at 15:36 -0700, Noah Goldstein wrote:
> > > The new option '-qemu-children' makes it so that on `execve` the
> > > child
> > > process will be launch by the same `qemu` executable that is
> > > currently
> > > running along with its current commandline arguments.
> > > 
> > > The motivation for the change is to make it so that plugins
> > > running
> > > through `qemu` can continue to run on children.  Why not just
> > > `binfmt`?: Plugins can be desirable regardless of
> > > system/architecture
> > > emulation, and can sometimes be useful for elf files that can run
> > > natively. Enabling `binfmt` for all natively runnable elf files
> > > may
> > > not be desirable.
> > 
> > Another reason to have this is that one may not have root
> > permissions
> > to configure binfmt-misc.
> 
> A little note on that: binfmt_misc is now part of the user namespace
> (since linux v6.7), so you can 
> configure binfmt_misc as a non root user in a given namepace.
> 
> There is helper to use it with unshare from util-linux, you can do
> things like that:
> 
>    With 'F' flag, load the interpreter from the initial namespace:
> 
>      $ /bin/qemu-m68k-static --version
>      qemu-m68k version 8.2.2 (qemu-8.2.2-1.fc40)
>      Copyright (c) 2003-2023 Fabrice Bellard and the QEMU Project
> developers
>      $ unshare --map-root-user --fork --pid 
> --load-interp=":qemu-
> m68k:M::\\x7fELF\\x01\\x02\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x
> 00\\x00\\x00\\x02\\x00\\x04:\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\x00\
> \xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff:/bin/qemu
> -m68k-static:OCF" 
> --root=chroot/m68k/sid
>      # QEMU_VERSION= ls
>      qemu-m68k version 8.2.2 (qemu-8.2.2-1.fc40)
>      Copyright (c) 2003-2023 Fabrice Bellard and the QEMU Project
> developers
>      # /qemu-m68k  --version
>      qemu-m68k version 8.0.50 (v8.0.0-340-gb1cff5e2da95)
>      Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project
> developers
> 
>    Without 'F' flag, from inside the namespace:
> 
>      $ unshare --map-root-user --fork --pid 
> --load-interp=":qemu-
> m68k:M::\\x7fELF\\x01\\x02\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x
> 00\\x00\\x00\\x02\\x00\\x04:\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\x00\
> \xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff:/qemu-
> m68k:OC" 
> --root=chroot/m68k/sid
>      # QEMU_VERSION= ls
>      qemu-m68k version 8.0.50 (v8.0.0-340-gb1cff5e2da95)
>      Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project
> developers
>      # /qemu-m68k  --version
>      qemu-m68k version 8.0.50 (v8.0.0-340-gb1cff5e2da95)
>      Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project
> developers
> 
> Thanks,
> Laurent
> 

Thanks for posting this, I wasn't aware of this feature and it looks
really useful. 

IIUC it also resolves the main problem this patch is dealing with:

  Enabling `binfmt` for all natively runnable elf files may
  not be desirable.