Re: [PATCH net-next v6 4/5] selftests/net: Add a shared BPF skeleton rule

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- [Medium] Under LLVM=1, the host bpftool bootstrap rule in
  tools/testing/selftests/net/bpf.mk probes one compiler and builds with…
- [Low] The patch makes two changes outside its subject.

--

selftests/net: Add a shared BPF skeleton rule

This commit adds a shared %.skel.h rule to selftests/net/bpf.mk. When
BPFTOOL is not supplied, it also builds a host bootstrap bpftool. The
BPF compiler now takes KHDR_INCLUDES instead of the source-tree
usr/include, and the libbpf sub-make gets an absolute srctree.

> diff --git a/tools/testing/selftests/net/bpf.mk b/tools/testing/selftests/net/bpf.mk
> index a4f6755dd8941..016e32960280a 100644
> --- a/tools/testing/selftests/net/bpf.mk
> +++ b/tools/testing/selftests/net/bpf.mk
> @@ -6,8 +6,8 @@ BUILD_DIR := $(SCRATCH_DIR)/build
>  BPFDIR := $(top_srcdir)/tools/lib/bpf
>  APIDIR := $(top_srcdir)/tools/include/uapi
>  
> +CCINCLUDE += $(KHDR_INCLUDES)
>  CCINCLUDE += -I$(selfdir)/bpf
> -CCINCLUDE += -I$(top_srcdir)/usr/include/
>  CCINCLUDE += -I$(SCRATCH_DIR)/include
>  
>  BPFOBJ := $(BUILD_DIR)/libbpf/libbpf.a
> @@ -46,8 +46,37 @@ $(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
>  	   $(APIDIR)/linux/bpf.h					       \
>  	   | $(BUILD_DIR)/libbpf
>  	$(call msg,MAKE,,$@)
> -	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) OUTPUT=$(BUILD_DIR)/libbpf/ \
> +	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) srctree=$(abspath $(top_srcdir)) \
> +		    OUTPUT=$(BUILD_DIR)/libbpf/ \
>  		    EXTRA_CFLAGS='-g -O0'				       \
>  		    DESTDIR=$(SCRATCH_DIR) prefix= all install_headers

[Severity: Low]
This isn't a bug, but the patch also changes two existing parts of
bpf.mk. CCINCLUDE now uses $(KHDR_INCLUDES) in place of
-I$(top_srcdir)/usr/include/, and the libbpf sub-make now gets an
absolute srctree.

Both changes fix behaviour that dates back to commit 3f584c211d8c
("selftests: net: extract BPF building logic from the Makefile").

The commit message does describe both changes. Would it be worth moving
them into their own patch, or adding a Fixes: tag, so they can be picked
up separately from the new skeleton rule?

>  
>  EXTRA_CLEAN += $(SCRATCH_DIR)
> +
> +# Build a host bpftool only when a test needs a generated skeleton.
> +HOSTCC ?= gcc
> +HOSTLD ?= ld
> +BPFTOOL_BUILD := $(BUILD_DIR)/bpftool

[ ... ]

> +$(BPFTOOL_BUILD)/bootstrap/bpftool: \
> +	$(wildcard $(top_srcdir)/tools/bpf/bpftool/*.[ch]) \
> +	$(top_srcdir)/tools/bpf/bpftool/Makefile \
> +	$(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)
> +	$(call msg,MAKE,,$@)
> +	$(Q)mkdir -p $(BPFTOOL_BUILD)
> +	$(Q)$(MAKE) $(submake_extras) -C $(top_srcdir)/tools/bpf/bpftool \
> +		OUTPUT=$(BPFTOOL_BUILD)/ srctree=$(abspath $(top_srcdir)) \
> +		ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD="$(HOSTLD)" bootstrap

[Severity: Medium]
Can this bootstrap build fail with LLVM=1?

bpf.mk sets HOSTCC to gcc and HOSTLD to ld whatever LLVM=, LLVM_PREFIX
and LLVM_SUFFIX are, and it only hands them to the bpftool sub-make as
CC and LD. HOSTCC itself is never passed or exported, so inside the
sub-make tools/scripts/Makefile.include sets it from LLVM:

  $(call allow-override,HOSTCC,$(LLVM_PREFIX)clang$(LLVM_SUFFIX))
  $(call allow-override,HOSTAR,$(LLVM_PREFIX)llvm-ar$(LLVM_SUFFIX))
  $(call allow-override,HOSTLD,$(LLVM_PREFIX)ld.lld$(LLVM_SUFFIX))

bpftool then compiles the bootstrap objects and the bootstrap libbpf with
that HOSTCC, which is clang:

tools/bpf/bpftool/Makefile:
	$(QUIET_CC)$(HOSTCC) $(HOST_CFLAGS) -c -MMD $< -o $@
	...
		ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)" \
		CFLAGS="$(LIBBPF_BOOTSTRAP_CFLAGS)" EXTRA_CFLAGS= $@ install_headers

The CC_NO_CLANG probe in Makefile.include, however, runs against $(CC).
That is the gcc forced on the command line, so the probe adds the
GCC-only warning:

  ifeq ($(CC_NO_CLANG), 1)
  EXTRA_WARNINGS += -Wstrict-aliasing=3

bpftool copies EXTRA_WARNINGS into CFLAGS, HOST_CFLAGS and
LIBBPF_BOOTSTRAP_CFLAGS, and tools/lib/bpf/Makefile adds -Werror. Clang
would then stop with:

  error: unknown warning option '-Wstrict-aliasing=3' [-Werror,-Wunknown-warning-option]

Commit 093b75ef5995 ("tools: Suppress request for warning options not
existent in clang") added the CC_NO_CLANG guard to avoid exactly this
error.

On a clang-only host the result is the same, because a missing gcc
leaves CC_NO_CLANG at 1. The libelf-zstd feature probe would also run
with gcc, not with the clang that does the linking.

The following command should hit this once "selftests/net: Move the XDP
feature test to drivers/net/hw" makes xdp_features depend on
xdp_features.skel.h:

  make LLVM=1 -C tools/testing/selftests TARGETS=drivers/net/hw

selftests/bpf and selftests/hid include tools/scripts/Makefile.include,
so their HOSTCC follows LLVM=. Could bpf.mk set its HOSTCC/HOSTLD
defaults the same way, or pass HOSTCC, HOSTLD and HOSTAR to the bpftool
sub-make as well as CC and LD?

[ ... ]

-- 
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