Message-ID: <3f49c18e-fed6-b005-19ca-b11ad620f535@fb.com>
Date:   Tue, 15 Dec 2020 23:18:49 -0800
From:   Yonghong Song <yhs@...com>
To:     Brendan Jackman <jackmanb@...gle.com>
CC:     <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        <linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH bpf-next v4 10/11] bpf: Add tests for new BPF atomic
 operations



On 12/15/20 3:12 AM, Brendan Jackman wrote:
> On Tue, Dec 08, 2020 at 10:15:35AM -0800, Yonghong Song wrote:
>>
>>
>> On 12/8/20 8:59 AM, Brendan Jackman wrote:
>>> On Tue, Dec 08, 2020 at 08:38:04AM -0800, Yonghong Song wrote:
>>>>
>>>>
>>>> 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@...gle.com>
>>>>>>
>>>>>> Ack with minor comments below.
>>>>>>
>>>>>> Acked-by: Yonghong Song <yhs@...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.
>>>
>>> Ah nice, I didn't know about __sync_lock_test_and_set, let's switch to
>>> that I think.
>>>
>>>>> 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.
>>>
>>> Sorry I should have been more explicit: I meant that the fix would be to
>>> call atomic_fetch_add but discard the return value, purely for the
>>> implied barrier. The x86 JIT would stay the same. It would not break any
>>> existing code, only add ordering guarantees that the user probably
>>> already expected (since these builtins come from GCC originally and the
>>> GCC docs say "these builtins are considered a full barrier" [1])
>>>
>>> [1] https://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Atomic-Builtins.html
>>
>> This is indeed the issue. In the past, people already use gcc
>> __sync_fetch_and_add() for xadd instruction for which git generated
>> code does not implying barrier.
>>
>> The new atomics support has the following logic:
>>    . if return value is used, it is atomic_fetch_add
>>    . if return value is not used, it is xadd
>> The reason to do this is to preserve backward compabiility
>> and this way, we can get rid of -mcpu=v4.
>>
>> barrier issue is tricky and as we discussed earlier let us
>> delay this after basic atomics support landed. We may not
>> 100% conform to gcc __sync_fetch_and_add() or __atomic_*()
>> semantics. We do need to clearly document what is expected
>> in llvm and kernel.
> 
> OK, then I think we can probably justify not conforming to the
> __sync_fetch_and_add() semantics since that API is under-specified
> anyway.
> 
> However IMO it's unambiguously a bug for
> 
>    (void)__atomic_fetch_add(&x, y, __ATOMIC_SEQ_CST);
> 
> to compile down to a kernel atomic_add. I think for that specific API
> Clang really ought to always use BPF_FETCH | BPF_ADD when
> anything stronger than __ATOMIC_RELAXED is requested, or even just
> refuse to compile with when the return value is ignored and a
> none-relaxed memory ordering is specified.

Both the following codes:
    (void)__sync_fetch_and_add(p, a);
    (void)__atomic_fetch_add(p, a, __ATOMIC_SEQ_CST);

will generate the same IR:
    %0 = atomicrmw add i32* %p, i32 %a seq_cst

Basically that means for old compiler (<= llvm11),
   (void)__atomic_fetch_add(&x, y, __ATOMIC_SEQ_CST)
already generates xadd.

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