Re: [Qemu-devel] [PATCH] linux-user: Add binfmt wrapper

[Riku Voipio <riku.voipio@iki.fi>]

On Mon, Jul 14, 2014 at 05:38:49PM +0200, Joakim Tjernlund wrote:
> Alexander Graf <agraf@suse.de> wrote on 2014/07/14 17:21:33:
> > On 14.07.14 16:38, Joakim Tjernlund wrote:
> > > The popular binfmt-wrapper patch adds an additional
> > > executable which mangle argv suitable for binfmt flag P.
> > > In a chroot you need the both (statically linked) qemu-$arch
> > > and qemu-$arch-binfmt-wrapper. This is sub optimal and a
> > > better approach is to recognize the -binfmt-wrapper extension
> > > within linux-user(qemu-$arch) and mangle argv there.
> > > This just produces on executable which can be either copied to
> > > the chroot or bind mounted with the appropriate -binfmt-wrapper
> > > suffix.
> > >
> > > Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
> > 
> > Please make sure to CC Riku on patches like this - he is the linux-user 
> > maintainer.
> 
> Doesn't he read the devel list? Anyhow CC:ed

I do - but CC gets directly to my inbox while qemu-devel goes to an
folder.

I take from this discussion, that this patch has been superceded by the
Patch at: http://patchwork.ozlabs.org/patch/369770/ ?

Riku

> > 
> > > ---
> > >   linux-user/main.c | 13 +++++++++++++
> > >   1 file changed, 13 insertions(+)
> > >
> > > diff --git a/linux-user/main.c b/linux-user/main.c
> > > index 71a33c7..212067a 100644
> > > --- a/linux-user/main.c
> > > +++ b/linux-user/main.c
> > > @@ -3828,6 +3828,19 @@ int main(int argc, char **argv, char **envp)
> > >       int i;
> > >       int ret;
> > >       int execfd;
> > > +    char *binfmt;
> > > +
> > > +    i = strlen( argv[0] ) - strlen ( "-binfmt-wrapper" );
> > 
> > The spaces are odd. Did this patch pass checkpatch.pl? Same comment goes 
> 
> > for almost all function invocations.
> 
> ehh, didn't run it through checkpatch.pl. Easy to fix next time.
> 
> > 
> > > +    binfmt = argv[0] + i;
> > > +    if (i > 0 && strcmp ( binfmt, "-binfmt-wrapper" ) == 0) {
> > 
> > This magic needs to be documented somewhere. In fact, I find it pretty 
> > hard to use in real world scenarios. Imagine a distribution - should it 
> > package every target binary twice? Should it create hardlinks all over?
> 
> How does dists. handle your original binfmt-wrapper? This is not much
> different I think. Here you got a choice to create a hardlink or a copy.
> Any chroot will only have to bind mount binfmt-wrapper into the chroot or 
> lxc container.
> 
> > 
> > I think we should try and find better magic :). Looking at the 
> > binfmt_misc loading code, I think we can cheat a bit. If we pass the 'O' 
> 
> > flag (open target binary for handler), binfmt_misc will tell us the 
> > binary fd in AT_EXECFD:
> > 
> >                  NEW_AUX_ENT(AT_EXECFD, bprm->interp_data);
> > 
> > We could then use this as a hint that we were spawned by binfmt_misc 
> > rather than directly and interpret the first argv as target_argv[0].
> > 
> > Then we can also add the P and O flags to scripts/qemu-binfmt-conf.sh 
> > and have a solution that works well for everyone.
> 
> What to do with P only then? Seems like most dists uses only P 
> 
> > 
> > > +   if (argc < 3 ) {
> > > +       fprintf ( stderr, "%s: Please use me through binfmt with P 
> flag\n", argv[0] );
> > > +       exit(1);
> > > +   }
> > > +   handle_arg_argv0(argv[2]); /* binfmt wrapper */
> > > +   memmove(&argv[2], &argv[3], (argc-2)*sizeof(argv));
> > 
> > I can't say I'm a big fan of this memmove, but everything else I can 
> > think of is going to be even uglier.
> 
> Me too :)
> 
> > > +   argc--;
> > > +    }
> > > 
> > >       module_call_init(MODULE_INIT_QOM);
> > > 
> > 
>