add arm64 cross compilation support to BPF samples

inline assembly has haunted building samples on arm64 for quite sometime. This patch uses the pre-processor to noop all occurences of inline asm when compiling the BPF sample for the BPF target.

This patch reintroduces inclusion of asm/sysregs.h which needs to be included to avoid compiler errors now, see [1]. Previously a hack prevented this inclusion [2] (to avoid the exact problem this patch fixes - skipping inline assembler) but the hack causes other errors now and no longer works.

Using the preprocessor to noop the inline asm occurences, we also avoid any future unstable hackery needed (such as those that skip asm headers) and provides information that asm headers may have which could have been used but which the inline asm issues prevented. This is the least messy of all hacks in my opinion.

[1] https://lkml.org/lkml/2017/8/5/143
[2] https://lists.linaro.org/pipermail/linaro-kernel/2015-November/024036.html

Signed-off-by Joel Fernandes <[email protected]>

---
 samples/bpf/Makefile           | 40 +++++++++++++++++++++++++++++++++-------
 samples/bpf/arm64_asmstubs.h   |  3 +++
 samples/bpf/bpf_helpers.h      | 12 ++++++------
 samples/bpf/generic_asmstubs.h |  4 ++++
 4 files changed, 46 insertions(+), 13 deletions(-)
 create mode 100644 samples/bpf/arm64_asmstubs.h
 create mode 100644 samples/bpf/generic_asmstubs.h

samples/bpf/Makefile +22 -7

index e5642c8c144d..7591cdd7fe69 100644
@@ -151,6 +151,8 @@ HOSTLOADLIBES_test_map_in_map += -lelf
 #  make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
 LLC ?= llc
 CLANG ?= clang
+PERL ?= perl
+RM ?= rm
 
 # Detect that we're cross compiling and use the right compilers and flags
 ifdef CROSS_COMPILE

@@ -186,14 +188,38 @@ verify_target_bpf: verify_cmds
 
 $(src)/*.c: verify_target_bpf
 
-# asm/sysreg.h - inline assembly used by it is incompatible with llvm.
-# But, there is no easy way to fix it, so just exclude it since it is
-# useless for BPF samples.
-$(obj)/%.o: $(src)/%.c
-       $(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
-               -D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
+curdir := $(shell dirname $(realpath $(lastword $(MAKEFILE_LIST))))
+ifeq ($(wildcard  $(curdir)/${ARCH}_asmstubs.h),)
+    ARCH_ASM_STUBS :=
+else
+    ARCH_ASM_STUBS := -include $(src)/${ARCH}_asmstubs.h
+endif
+
+ASM_STUBS := ${ARCH_ASM_STUBS} -include $(src)/generic_asmstubs.h
+
+CLANG_ARGS = $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
+               -D__KERNEL__ -Wno-unused-value -Wno-pointer-sign \
+               $(ASM_STUBS) \
                -Wno-compare-distinct-pointer-types \
                -Wno-gnu-variable-sized-type-not-at-end \
                -Wno-address-of-packed-member -Wno-tautological-compare \
                -Wno-unknown-warning-option \
-               -O2 -emit-llvm -c $< -o -| $(LLC) -march=bpf -filetype=obj -o $@
+               -O2 -emit-llvm
+
+$(obj)/%.o: $(src)/%.c
+       # Steps to compile BPF sample while getting rid of inline asm
+       # This has the advantage of not having to skip important asm headers
+       # Step 1. Use clang preprocessor to stub out asm() calls
+       # Step 2. Replace all "asm volatile" with single keyword "asmvolatile"
+       # Step 3. Use clang preprocessor to noop all asm volatile() calls

samples/bpf/Makefile +1 -0

+       $(RM) [email protected]

samples/bpf/arm64_asmstubs.h new file +3 -0

new file mode 100644
index 000000000000..23d47dbe61b1
@@ -0,0 +1,3 @@
+/* Special handing for current_stack_pointer */
+#define __ASM_STACK_POINTER_H
+#define current_stack_pointer 0

samples/bpf/bpf_helpers.h +6 -6

index 9a9c95f2c9fb..67c9c4438e4b 100644
@@ -64,12 +64,12 @@ static int (*bpf_xdp_adjust_head)(void *ctx, int offset) =
  * emit BPF_LD_ABS and BPF_LD_IND instructions
  */
 struct sk_buff;
-unsigned long long load_byte(void *skb,
-                            unsigned long long off) asm("llvm.bpf.load.byte");
-unsigned long long load_half(void *skb,
-                            unsigned long long off) asm("llvm.bpf.load.half");
-unsigned long long load_word(void *skb,
-                            unsigned long long off) asm("llvm.bpf.load.word");
+unsigned long long load_byte(void *skb, unsigned long long off)
+                            asm_bpf("llvm.bpf.load.byte");
+unsigned long long load_half(void *skb, unsigned long long off)
+                            asm_bpf("llvm.bpf.load.half");
+unsigned long long load_word(void *skb, unsigned long long off)
+                            asm_bpf("llvm.bpf.load.word");
 
 /* a helper structure used by eBPF C program
  * to describe map attributes to elf_bpf loader

samples/bpf/generic_asmstubs.h new file +4 -1

new file mode 100644
index 000000000000..1b9e9f5094d8
@@ -0,0 +1,4 @@
+#define bpf_noop_stub
+#define asm(...) bpf_noop_stub
+#define __asm__(...) bpf_noop_stub
+#define asmvolatile(...) bpf_noop_stub
--

2.14.0.rc1.383.gd1ce394fe2-goog

Please, no.

The amount of hellish hacks we are adding to deal with this is getting way out of control.

BPF programs MUST have their own set of asm headers, this is the only way to get around this issue in the long term.

I am also strongly against adding -static to the build.

Quoted 2 lines

The amount of hellish hacks we are adding to deal with this is getting way out of control.

I agree with you that hellish hacks are being added which is why it keeps breaking. I think one of the things my series does is to add back inclusion of asm headers that were previously removed (that is the worst hellish hack in my opinion that existing in mainline). So in that respect my patch is an improvement and makes it possible to build for arm64 platforms (which is currently broken in mainline).

From: Joel Fernandes <[email protected]>
Date: Mon, 7 Aug 2017 18:20:49 -0700

Quoted 10 lines · David Miller

Quoted 2 lines

The amount of hellish hacks we are adding to deal with this is getting way out of control.

I agree with you that hellish hacks are being added which is why it keeps breaking. I think one of the things my series does is to add back inclusion of asm headers that were previously removed (that is the worst hellish hack in my opinion that existing in mainline). So in that respect my patch is an improvement and makes it possible to build for arm64 platforms (which is currently broken in mainline).

Yeah that is a problem.

Perhaps another avenue of attack is to separate "type" header files from stuff that has functiond declarations and inline assembler code.