netdev - Re: [net] bpfilter: include bpfilter_umh in assembly instead of using objcopy

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Message-ID: <20180627161855.GA12987@roeck-us.net>
Date:   Wed, 27 Jun 2018 09:18:55 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Alexei Starovoitov <ast@...nel.org>
Cc:     "David S . Miller" <davem@...emloft.net>, daniel@...earbox.net,
        torvalds@...ux-foundation.org, mcroce@...hat.com,
        yamada.masahiro@...ionext.com, netdev@...r.kernel.org,
        kernel-team@...com
Subject: Re: [net] bpfilter: include bpfilter_umh in assembly instead of
 using objcopy

On Tue, Jun 26, 2018 at 08:13:48PM -0700, Alexei Starovoitov wrote:
> From: Masahiro Yamada <yamada.masahiro@...ionext.com>
> 
> 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>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
> I think this patch should 'fix' bpfilter build issue on all archs.
> cflags for cross CC may still be incorrect and embedded blob
> may fail to execute via fork_usermode_blob()
> (like in case of 'make ARCH=i386 net/bpfilter/' CC will build and link 64-bit
> binary that will be included into bpfilter.o or vmlinux and that binary
> will fail to run on 32-bit kernel),
> but that is separate issue that will be addressed in net-next time frame.
> Long term we've discussed to switch to something like klibc and keep it
> as part of the kernel to avoid relying on glibc and cc-can-link.sh.
> 

At the very least it fixes the build issue when building i386 images
on x86_64 systems.

(Build-)Tested-by: Guenter Roeck <linux@...ck-us.net>

>  net/bpfilter/Makefile            | 17 ++---------------
>  net/bpfilter/bpfilter_kern.c     | 11 +++++------
>  net/bpfilter/bpfilter_umh_blob.S |  7 +++++++
>  3 files changed, 14 insertions(+), 21 deletions(-)
>  create mode 100644 net/bpfilter/bpfilter_umh_blob.S
> 
> diff --git a/net/bpfilter/Makefile b/net/bpfilter/Makefile
> index 051dc18b8ccb..39c6980b5d99 100644
> --- a/net/bpfilter/Makefile
> +++ b/net/bpfilter/Makefile
> @@ -15,20 +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 \
> -          `LC_ALL=C $(OBJDUMP) -f net/bpfilter/bpfilter_umh \
> -          |awk -F' |,' '/file format/{print "-O",$$NF} \
> -          /^architecture:/{print "-B",$$2}'` \
> -      --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 09522573f611..f0fc182d3db7 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_start,
> +				 &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 000000000000..40311d10d2f2
> --- /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: