Re: [PATCH v2 2/3] bpfilter: include bpfilter_umh in assembly instead of using objcopy

[Janne Karhunen]

Hi,

Hmm, does this approach work if the code is not in a kernel module? I
tried to use it as part of the kernel image and looks to me the
bounding symbols _start and _end are not correctly relocated?

--
Janne

On Fri, Jun 15, 2018 at 3:48 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Thu, Jun 14, 2018 at 11:39:31PM +0900, Masahiro Yamada wrote:
> > What we want here is to embed a user-space program into the kernel.
> > Instead of the complex ELF magic, let's simply wrap it in the assembly
> > with the '.incbin' directive.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > ---
> >
> > Changes in v2:
> >   - Rebase
> >
> >  net/bpfilter/Makefile            | 15 ++-------------
> >  net/bpfilter/bpfilter_kern.c     | 11 +++++------
> >  net/bpfilter/bpfilter_umh_blob.S |  7 +++++++
> >  3 files changed, 14 insertions(+), 19 deletions(-)
> >  create mode 100644 net/bpfilter/bpfilter_umh_blob.S
> >
> > diff --git a/net/bpfilter/Makefile b/net/bpfilter/Makefile
> > index e0bbe75..39c6980 100644
> > --- a/net/bpfilter/Makefile
> > +++ b/net/bpfilter/Makefile
> > @@ -15,18 +15,7 @@ ifeq ($(CONFIG_BPFILTER_UMH), y)
> >  HOSTLDFLAGS += -static
> >  endif
> >
> > -# a bit of elf magic to convert bpfilter_umh binary into a binary blob
> > -# inside bpfilter_umh.o elf file referenced by
> > -# _binary_net_bpfilter_bpfilter_umh_start symbol
> > -# which bpfilter_kern.c passes further into umh blob loader at run-time
> > -quiet_cmd_copy_umh = GEN $@
> > -      cmd_copy_umh = echo ':' > $(obj)/.bpfilter_umh.o.cmd; \
> > -      $(OBJCOPY) -I binary -O `$(OBJDUMP) -f $<|grep format|cut -d' ' -f8` \
> > -      -B `$(OBJDUMP) -f $<|grep architecture|cut -d, -f1|cut -d' ' -f2` \
> > -      --rename-section .data=.init.rodata $< $@
> > -
> > -$(obj)/bpfilter_umh.o: $(obj)/bpfilter_umh
> > -     $(call cmd,copy_umh)
> > +$(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
> >
> >  obj-$(CONFIG_BPFILTER_UMH) += bpfilter.o
> > -bpfilter-objs += bpfilter_kern.o bpfilter_umh.o
> > +bpfilter-objs += bpfilter_kern.o bpfilter_umh_blob.o
> > diff --git a/net/bpfilter/bpfilter_kern.c b/net/bpfilter/bpfilter_kern.c
> > index 0952257..6de3ae5 100644
> > --- a/net/bpfilter/bpfilter_kern.c
> > +++ b/net/bpfilter/bpfilter_kern.c
> > @@ -10,11 +10,8 @@
> >  #include <linux/file.h>
> >  #include "msgfmt.h"
> >
> > -#define UMH_start _binary_net_bpfilter_bpfilter_umh_start
> > -#define UMH_end _binary_net_bpfilter_bpfilter_umh_end
> > -
> > -extern char UMH_start;
> > -extern char UMH_end;
> > +extern char bpfilter_umh_start;
> > +extern char bpfilter_umh_end;
> >
> >  static struct umh_info info;
> >  /* since ip_getsockopt() can run in parallel, serialize access to umh */
> > @@ -93,7 +90,9 @@ static int __init load_umh(void)
> >       int err;
> >
> >       /* fork usermode process */
> > -     err = fork_usermode_blob(&UMH_start, &UMH_end - &UMH_start, &info);
> > +     err = fork_usermode_blob(&bpfilter_umh_end,
> > +                              &bpfilter_umh_end - &bpfilter_umh_start,
> > +                              &info);
> >       if (err)
> >               return err;
> >       pr_info("Loaded bpfilter_umh pid %d\n", info.pid);
> > diff --git a/net/bpfilter/bpfilter_umh_blob.S b/net/bpfilter/bpfilter_umh_blob.S
> > new file mode 100644
> > index 0000000..40311d1
> > --- /dev/null
> > +++ b/net/bpfilter/bpfilter_umh_blob.S
> > @@ -0,0 +1,7 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +     .section .init.rodata, "a"
> > +     .global bpfilter_umh_start
> > +bpfilter_umh_start:
> > +     .incbin "net/bpfilter/bpfilter_umh"
> > +     .global bpfilter_umh_end
> > +bpfilter_umh_end:
>
> for some reason it doesn't work.
> fork_usermode_blob() returns ENOEXEC
> You should be able to test it simply running 'iptables -L'.
> Without this patch you should see:
> [   12.696937] bpfilter: Loaded bpfilter_umh pid 225
> Started bpfilter
>
> where first line comes from kernel module and second from umh.
>
>