bpfilter: include bpfilter_umh in assembly instead of using objcopy

From: Masahiro Yamada <[email protected]>

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 <[email protected]>

Signed-off-by Alexei Starovoitov <[email protected]>

---
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.

 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

net/bpfilter/Makefile +2 -15

index 051dc18b8ccb..39c6980b5d99 100644
@@ -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

net/bpfilter/bpfilter_kern.c +5 -6

index 09522573f611..f0fc182d3db7 100644
@@ -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);

net/bpfilter/bpfilter_umh_blob.S new file +7 -0

new file mode 100644
index 000000000000..40311d10d2f2
@@ -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:

From: Masahiro Yamada <[email protected]>

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 <[email protected]>

Signed-off-by Alexei Starovoitov <[email protected]>