[PATCH bpf-next v2 1/2] samples/bpf: change Makefile to cope with latest llvm

[PATCH bpf-next v2 1/2] samples/bpf: change Makefile to cope with latest llvm

[Yonghong Song]

With latest llvm trunk, bpf programs under samples/bpf
directory, if using CORE, may experience the following
errors:

LLVM ERROR: Cannot select: intrinsic %llvm.preserve.struct.access.index
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
Stack dump:
0.      Program arguments: llc -march=bpf -filetype=obj -o samples/bpf/test_probe_write_user_kern.o
1.      Running pass 'Function Pass Manager' on module '<stdin>'.
2.      Running pass 'BPF DAG->DAG Pattern Instruction Selection' on function '@bpf_prog1'
 #0 0x000000000183c26c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int)
    (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x183c26c)
...
 #7 0x00000000017c375e (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x17c375e)
 #8 0x00000000016a75c5 llvm::SelectionDAGISel::CannotYetSelect(llvm::SDNode*)
    (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x16a75c5)
 #9 0x00000000016ab4f8 llvm::SelectionDAGISel::SelectCodeCommon(llvm::SDNode*, unsigned char const*,
    unsigned int) (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x16ab4f8)
...
Aborted (core dumped) | llc -march=bpf -filetype=obj -o samples/bpf/test_probe_write_user_kern.o

The reason is due to llvm change https://reviews.llvm.org/D87153
where the CORE relocation global generation is moved from the beginning
of target dependent optimization (llc) to the beginning
of target independent optimization (opt).

Since samples/bpf programs did not use vmlinux.h and its clang compilation
uses native architecture, we need to adjust arch triple at opt level
to do CORE relocation global generation properly. Otherwise, the above
error will appear.

This patch fixed the issue by introduce opt and llvm-dis to compilation chain,
which will do proper CORE relocation global generation as well as O2 level
optimization. Tested with llvm10, llvm11 and trunk/llvm12.

Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 samples/bpf/Makefile | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

Changelog:
  v1 -> v2:
    - add comments to explain the change (Andrii)

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 4f1ed0e3cf9f..e29b1e89dd3e 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -211,6 +211,8 @@ TPROGLDLIBS_xsk_fwd		+= -pthread
 #  make M=samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
 LLC ?= llc
 CLANG ?= clang
+OPT ?= opt
+LLVM_DIS ?= llvm-dis
 LLVM_OBJCOPY ?= llvm-objcopy
 BTF_PAHOLE ?= pahole
 
@@ -303,6 +305,11 @@ $(obj)/hbm_edt_kern.o: $(src)/hbm.h $(src)/hbm_kern.h
 # 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.
+# below we use long chain of commands, clang | opt | llvm-dis | llc,
+# to generate final object file. 'clang' compiles the source into IR
+# with native target, e.g., x64, arm64, etc. 'opt' does bpf CORE IR builtin
+# processing (llvm12) and IR optimizations. 'llvm-dis' converts
+# 'opt' output to IR, and finally 'llc' generates bpf byte code.
 $(obj)/%.o: $(src)/%.c
 	@echo "  CLANG-bpf " $@
 	$(Q)$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(BPF_EXTRA_CFLAGS) \
@@ -314,7 +321,9 @@ $(obj)/%.o: $(src)/%.c
 		-Wno-address-of-packed-member -Wno-tautological-compare \
 		-Wno-unknown-warning-option $(CLANG_ARCH_ARGS) \
 		-I$(srctree)/samples/bpf/ -include asm_goto_workaround.h \
-		-O2 -emit-llvm -c $< -o -| $(LLC) -march=bpf $(LLC_FLAGS) -filetype=obj -o $@
+		-O2 -emit-llvm -Xclang -disable-llvm-passes -c $< -o - | \
+		$(OPT) -O2 -mtriple=bpf-pc-linux | $(LLVM_DIS) | \
+		$(LLC) -march=bpf $(LLC_FLAGS) -filetype=obj -o $@
 ifeq ($(DWARF2BTF),y)
 	$(BTF_PAHOLE) -J $@
 endif
-- 
2.24.1

[PATCH bpf-next v2 2/2] samples/bpf: fix a compilation error with fallthrough marking

[Yonghong Song]

Compiling samples/bpf hits an error related to fallthrough marking.
    ...
    CC  samples/bpf/hbm.o
  samples/bpf/hbm.c: In function ‘main’:
  samples/bpf/hbm.c:486:4: error: ‘fallthrough’ undeclared (first use in this function)
      fallthrough;
      ^~~~~~~~~~~

The "fallthrough" is not defined under tools/include directory.
Rather, it is "__fallthrough" is defined in linux/compiler.h.
Including "linux/compiler.h" and using "__fallthrough" fixed the issue.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 samples/bpf/hbm.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Changelog:
  v1 -> v2: no change

diff --git a/samples/bpf/hbm.c b/samples/bpf/hbm.c
index 4b22ace52f80..ff4c533dfac2 100644
--- a/samples/bpf/hbm.c
+++ b/samples/bpf/hbm.c
@@ -40,6 +40,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <linux/unistd.h>
+#include <linux/compiler.h>
 
 #include <linux/bpf.h>
 #include <bpf/bpf.h>
@@ -483,7 +484,7 @@ int main(int argc, char **argv)
 					"Option -%c requires an argument.\n\n",
 					optopt);
 		case 'h':
-			fallthrough;
+			__fallthrough;
 		default:
 			Usage();
 			return 0;
-- 
2.24.1

Re: [PATCH bpf-next v2 1/2] samples/bpf: change Makefile to cope with latest llvm

[patchwork-bot+bpf]

Hello:

This series was applied to bpf/bpf-next.git (refs/heads/master):

On Mon, 5 Oct 2020 21:34:26 -0700 you wrote:
> With latest llvm trunk, bpf programs under samples/bpf
> directory, if using CORE, may experience the following
> errors:
> 
> LLVM ERROR: Cannot select: intrinsic %llvm.preserve.struct.access.index
> PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
> Stack dump:
> 0.      Program arguments: llc -march=bpf -filetype=obj -o samples/bpf/test_probe_write_user_kern.o
> 1.      Running pass 'Function Pass Manager' on module '<stdin>'.
> 2.      Running pass 'BPF DAG->DAG Pattern Instruction Selection' on function '@bpf_prog1'
>  #0 0x000000000183c26c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int)
>     (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x183c26c)
> ...
>  #7 0x00000000017c375e (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x17c375e)
>  #8 0x00000000016a75c5 llvm::SelectionDAGISel::CannotYetSelect(llvm::SDNode*)
>     (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x16a75c5)
>  #9 0x00000000016ab4f8 llvm::SelectionDAGISel::SelectCodeCommon(llvm::SDNode*, unsigned char const*,
>     unsigned int) (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x16ab4f8)
> ...
> Aborted (core dumped) | llc -march=bpf -filetype=obj -o samples/bpf/test_probe_write_user_kern.o
> 
> [...]

Here is the summary with links:
  - [bpf-next,v2,1/2] samples/bpf: change Makefile to cope with latest llvm
    https://git.kernel.org/bpf/bpf-next/c/9618bde489b2
  - [bpf-next,v2,2/2] samples/bpf: fix a compilation error with fallthrough marking
    https://git.kernel.org/bpf/bpf-next/c/544d6adf3c3d

You are awesome, thank you!
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