Alexander Graf <ag...@suse.de> wrote on 2014/07/14 17:21:33:
> From: Alexander Graf <ag...@suse.de>
> To: Joakim Tjernlund <joakim.tjernl...@transmode.se>,
> Cc: qemu-devel@nongnu.org
> Date: 2014/07/14 17:21
> Subject: Re: [PATCH] linux-user: Add binfmt wrapper
>
>
> 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.tjernl...@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
>
> > ---
> > 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);
> >
>
