Date:   Wed, 16 Dec 2020 12:00:35 -0800
From:   Yonghong Song <yhs@...com>
To:     Brendan Jackman <jackmanb@...gle.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        LKML <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 12/16/20 3:51 AM, Brendan Jackman wrote:
> On Wed, 16 Dec 2020 at 08:19, Yonghong Song <yhs@...com> wrote:
>>
>>
>>
>> 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.
> 
> Ah, I didn't realise that was already the case, that's unfortunate.
> 
> For users of newer Clang with alu32 enabled, unless I'm being naïve
> this could be fixed without breaking compatibility. Clang could just
> start generating a BPF_ADD|BPF_FETCH, and then handle the fact that
> the src_reg is clobbered, right?

This may cause regressions as old kernel won't support 
BPF_ADD|BPF_FETCH. Since we do not have llvm flags to control
this behavior, that means users with old kernel
cannot use llvm12 any more.

> 
> For users without alu32 enabled I would actually argue that the new
> Clang should start failing to build that code - as a user I'd much
> rather have my build suddenly fail than my explicitly-stated ordering
> assumptions violated. But I understand if that doesn't seem too
> palatable...
> 

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