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

[Yonghong Song <yhs@fb.com>]

On 12/8/20 4:41 AM, Brendan Jackman wrote:
> On Mon, Dec 07, 2020 at 07:18:57PM -0800, Yonghong Song wrote:
>>
>>
>> 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.
> 
> Well IIUC this specific case is fine: the ordering that you get with
> BPF_[CMP]XCHG (via kernel atomic_[cmpxchg]) is sequential consistency,
> and its' fine to provide a stronger ordering than the one requested. I
> should change it to say __ATOMIC_SEQ_CST to avoid confusing readers,
> though.
> 
> (I wrote it this way because I didn't see a __sync* function for
> unconditional atomic exchange, and I didn't see an __atomic* function
> where you don't need to specify the ordering).

For the above code,
    __atomic_exchange(&xchg64_value, &val64, &xchg64_result, 
__ATOMIC_RELAXED);
It tries to do an atomic exchange between &xchg64_value and
  &val64, and store the old &xchg64_value to &xchg64_result. So it is
equivalent to
     xchg64_result = __sync_lock_test_and_set(&xchg64_value, val64);

So I think this test case can be dropped.

> 
> However... this led me to double-check the semantics and realise that we
> do have a problem with ordering: The kernel's atomic_{add,and,or,xor} do
> not imply memory barriers and therefore neither do the corresponding BPF
> instructions. That means Clang can compile this:
> 
>   (void)__atomic_fetch_add(&val, 1, __ATOMIC_SEQ_CST)
> 
> to a {.code = (BPF_STX | BPF_DW | BPF_ATOMIC), .imm = BPF_ADD},
> which is implemented with atomic_add, which doesn't actually satisfy
> __ATOMIC_SEQ_CST.

This is the main reason in all my llvm selftests I did not use
__atomic_* intrinsics because we cannot handle *different* memory
ordering properly.

> 
> In fact... I think this is a pre-existing issue with BPF_XADD.
> 
> If all I've written here is correct, the fix is to use
> (void)atomic_fetch_add etc (these imply barriers) even when BPF_FETCH is
> not set. And that change ought to be backported to fix BPF_XADD.

We cannot change BPF_XADD behavior. If we change BPF_XADD to use
atomic_fetch_add, then suddenly old code compiled with llvm12 will
suddenly requires latest kernel, which will break userland very badly.