Re: [PATCH bpf-next v4 10/11] bpf: Add tests for new BPF atomic operations

[Yonghong Song <yhs@fb.com>]

On 12/7/20 8:07 AM, Brendan Jackman wrote:
> The prog_test that's added depends on Clang/LLVM features added by
> Yonghong in commit 286daafd6512 (was https://reviews.llvm.org/D72184 ).
> 
> Note the use of a define called ENABLE_ATOMICS_TESTS: this is used
> to:
> 
>   - Avoid breaking the build for people on old versions of Clang
>   - Avoid needing separate lists of test objects for no_alu32, where
>     atomics are not supported even if Clang has the feature.
> 
> The atomics_test.o BPF object is built unconditionally both for
> test_progs and test_progs-no_alu32. For test_progs, if Clang supports
> atomics, ENABLE_ATOMICS_TESTS is defined, so it includes the proper
> test code. Otherwise, progs and global vars are defined anyway, as
> stubs; this means that the skeleton user code still builds.
> 
> The atomics_test.o userspace object is built once and used for both
> test_progs and test_progs-no_alu32. A variable called skip_tests is
> defined in the BPF object's data section, which tells the userspace
> object whether to skip the atomics test.
> 
> Signed-off-by: Brendan Jackman <jackmanb@google.com>

Ack with minor comments below.

Acked-by: Yonghong Song <yhs@fb.com>

> ---
>   tools/testing/selftests/bpf/Makefile          |  10 +
>   .../selftests/bpf/prog_tests/atomics.c        | 246 ++++++++++++++++++
>   tools/testing/selftests/bpf/progs/atomics.c   | 154 +++++++++++
>   .../selftests/bpf/verifier/atomic_and.c       |  77 ++++++
>   .../selftests/bpf/verifier/atomic_cmpxchg.c   |  96 +++++++
>   .../selftests/bpf/verifier/atomic_fetch_add.c | 106 ++++++++
>   .../selftests/bpf/verifier/atomic_or.c        |  77 ++++++
>   .../selftests/bpf/verifier/atomic_xchg.c      |  46 ++++
>   .../selftests/bpf/verifier/atomic_xor.c       |  77 ++++++
>   9 files changed, 889 insertions(+)
>   create mode 100644 tools/testing/selftests/bpf/prog_tests/atomics.c
>   create mode 100644 tools/testing/selftests/bpf/progs/atomics.c
>   create mode 100644 tools/testing/selftests/bpf/verifier/atomic_and.c
>   create mode 100644 tools/testing/selftests/bpf/verifier/atomic_cmpxchg.c
>   create mode 100644 tools/testing/selftests/bpf/verifier/atomic_fetch_add.c
>   create mode 100644 tools/testing/selftests/bpf/verifier/atomic_or.c
>   create mode 100644 tools/testing/selftests/bpf/verifier/atomic_xchg.c
>   create mode 100644 tools/testing/selftests/bpf/verifier/atomic_xor.c
> 
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index ac25ba5d0d6c..13bc1d736164 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -239,6 +239,12 @@ BPF_CFLAGS = -g -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN) 			\
>   	     -I$(INCLUDE_DIR) -I$(CURDIR) -I$(APIDIR)			\
>   	     -I$(abspath $(OUTPUT)/../usr/include)
>   
> +# BPF atomics support was added to Clang in llvm-project commit 286daafd6512
> +# (release 12.0.0).
> +BPF_ATOMICS_SUPPORTED = $(shell \
> +	echo "int x = 0; int foo(void) { return __sync_val_compare_and_swap(&x, 1, 2); }" \
> +	| $(CLANG) -x cpp-output -S -target bpf -mcpu=v3 - -o /dev/null && echo 1 || echo 0)

'-x c' here more intuitive?

> +
>   CLANG_CFLAGS = $(CLANG_SYS_INCLUDES) \
>   	       -Wno-compare-distinct-pointer-types
>   
> @@ -399,11 +405,15 @@ TRUNNER_EXTRA_FILES := $(OUTPUT)/urandom_read $(OUTPUT)/bpf_testmod.ko	\
>   		       $(wildcard progs/btf_dump_test_case_*.c)
>   TRUNNER_BPF_BUILD_RULE := CLANG_BPF_BUILD_RULE
>   TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS)
> +ifeq ($(BPF_ATOMICS_SUPPORTED),1)
> +  TRUNNER_BPF_CFLAGS += -DENABLE_ATOMICS_TESTS
> +endif
>   TRUNNER_BPF_LDFLAGS := -mattr=+alu32
>   $(eval $(call DEFINE_TEST_RUNNER,test_progs))
>   
>   # Define test_progs-no_alu32 test runner.
>   TRUNNER_BPF_BUILD_RULE := CLANG_NOALU32_BPF_BUILD_RULE
> +TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS)
>   TRUNNER_BPF_LDFLAGS :=
>   $(eval $(call DEFINE_TEST_RUNNER,test_progs,no_alu32))
>   
> diff --git a/tools/testing/selftests/bpf/prog_tests/atomics.c b/tools/testing/selftests/bpf/prog_tests/atomics.c
> new file mode 100644
> index 000000000000..c841a3abc2f7
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/atomics.c
> @@ -0,0 +1,246 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <test_progs.h>
> +
> +#include "atomics.skel.h"
> +
> +static void test_add(struct atomics *skel)
> +{
> +	int err, prog_fd;
> +	__u32 duration = 0, retval;
> +	struct bpf_link *link;
> +
> +	link = bpf_program__attach(skel->progs.add);
> +	if (CHECK(IS_ERR(link), "attach(add)", "err: %ld\n", PTR_ERR(link)))
> +		return;
> +
> +	prog_fd = bpf_program__fd(skel->progs.add);
> +	err = bpf_prog_test_run(prog_fd, 1, NULL, 0,
> +				NULL, NULL, &retval, &duration);
> +	if (CHECK(err || retval, "test_run add",
> +		  "err %d errno %d retval %d duration %d\n", err, errno, retval, duration))
> +		goto cleanup;
> +
> +	ASSERT_EQ(skel->data->add64_value, 3, "add64_value");
> +	ASSERT_EQ(skel->bss->add64_result, 1, "add64_result");
> +
> +	ASSERT_EQ(skel->data->add32_value, 3, "add32_value");
> +	ASSERT_EQ(skel->bss->add32_result, 1, "add32_result");
> +
> +	ASSERT_EQ(skel->bss->add_stack_value_copy, 3, "add_stack_value");
> +	ASSERT_EQ(skel->bss->add_stack_result, 1, "add_stack_result");
> +
> +	ASSERT_EQ(skel->data->add_noreturn_value, 3, "add_noreturn_value");
> +
> +cleanup:
> +	bpf_link__destroy(link);
> +}
> +
[...]
> +
> +__u64 xchg64_value = 1;
> +__u64 xchg64_result = 0;
> +__u32 xchg32_value = 1;
> +__u32 xchg32_result = 0;
> +
> +SEC("fentry/bpf_fentry_test1")
> +int BPF_PROG(xchg, int a)
> +{
> +#ifdef ENABLE_ATOMICS_TESTS
> +	__u64 val64 = 2;
> +	__u32 val32 = 2;
> +
> +	__atomic_exchange(&xchg64_value, &val64, &xchg64_result, __ATOMIC_RELAXED);
> +	__atomic_exchange(&xchg32_value, &val32, &xchg32_result, __ATOMIC_RELAXED);

Interesting to see this also works. I guess we probably won't advertise 
this, right? Currently for LLVM, the memory ordering parameter is ignored.

> +#endif
> +
> +	return 0;
> +}
[...]