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

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Message-ID: <20180615004704.u5gofft7k6ehmhwi@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 14 Jun 2018 17:47:06 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-kernel@...r.kernel.org, YueHaibing <yuehaibing@...wei.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v2 2/3] bpfilter: include bpfilter_umh in assembly
 instead of using objcopy

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@...ionext.com>
> ---
> 
> 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.