Re: [PATCH bpf v4 13/14] bpf/tests: Fix error in tail call limit tests

From: Johan Almbladh <johan.almbladh@anyfinetworks.com>
To: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Ilya Leoshkevich <iii@linux.ibm.com>,
	Paul Chaignon <paul@cilium.io>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH bpf v4 13/14] bpf/tests: Fix error in tail call limit tests
Date: Tue, 14 Sep 2021 15:09:58 +0200	[thread overview]
Message-ID: <CAM1=_QRzTXZ=WRrN4WFezPSLpm1yh3V0gX8HHP0f8yBYEYs2_A@mail.gmail.com> (raw)
In-Reply-To: <8e2404d6-f226-3749-2e35-5519b2c90754@loongson.cn>

On Tue, Sep 14, 2021 at 2:55 PM Tiezhu Yang <yangtiezhu@loongson.cn> wrote:
>
> On 09/14/2021 08:41 PM, Tiezhu Yang wrote:
> > On 09/14/2021 05:18 PM, Johan Almbladh wrote:
> >> This patch fixes an error in the tail call limit test that caused the
> >> test to fail on for x86-64 JIT. Previously, the register R0 was used to
> >> report the total number of tail calls made. However, after a tail call
> >> fall-through, the value of the R0 register is undefined. Now, all tail
> >> call error path tests instead use context state to store the count.
> >>
> >> Fixes: 874be05f525e ("bpf, tests: Add tail call test suite")
> >> Reported-by: Paul Chaignon <paul@cilium.io>
> >> Reported-by: Tiezhu Yang <yangtiezhu@loongson.cn>
> >> Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
> >> ---
> >>   lib/test_bpf.c | 37 +++++++++++++++++++++++++++----------
> >>   1 file changed, 27 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/lib/test_bpf.c b/lib/test_bpf.c
> >> index 7475abfd2186..ddb9a8089d2e 100644
> >> --- a/lib/test_bpf.c
> >> +++ b/lib/test_bpf.c
> >> @@ -12179,10 +12179,15 @@ static __init int test_bpf(void)
> >>   struct tail_call_test {
> >>       const char *descr;
> >>       struct bpf_insn insns[MAX_INSNS];
> >> +    int flags;
> >>       int result;
> >>       int stack_depth;
> >>   };
> >>   +/* Flags that can be passed to tail call test cases */
> >> +#define FLAG_NEED_STATE        BIT(0)
> >> +#define FLAG_RESULT_IN_STATE    BIT(1)
> >> +
> >>   /*
> >>    * Magic marker used in test snippets for tail calls below.
> >>    * BPF_LD/MOV to R2 and R2 with this immediate value is replaced
> >> @@ -12252,32 +12257,38 @@ static struct tail_call_test
> >> tail_call_tests[] = {
> >>       {
> >>           "Tail call error path, max count reached",
> >>           .insns = {
> >> -            BPF_ALU64_IMM(BPF_ADD, R1, 1),
> >> -            BPF_ALU64_REG(BPF_MOV, R0, R1),
> >> +            BPF_LDX_MEM(BPF_W, R2, R1, 0),
> >> +            BPF_ALU64_IMM(BPF_ADD, R2, 1),
> >> +            BPF_STX_MEM(BPF_W, R1, R2, 0),
> >>               TAIL_CALL(0),
> >>               BPF_EXIT_INSN(),
> >>           },
> >> -        .result = MAX_TAIL_CALL_CNT + 1,
> >> +        .flags = FLAG_NEED_STATE | FLAG_RESULT_IN_STATE,
> >> +        .result = (MAX_TAIL_CALL_CNT + 1 + 1) * MAX_TESTRUNS,
> >
> > Hi Johan,
> >
> > I have tested this patch,
> > It should be "MAX_TAIL_CALL_CNT + 1" instead of "MAX_TAIL_CALL_CNT + 1
> > + 1"?
>
> Oh, sorry, it is right when MAX_TAIL_CALL_CNT is 32,
> I have tested it based on MAX_TAIL_CALL_CNT is 33,
> so I need to modify here if MAX_TAIL_CALL_CNT is 33 in my v3 patch.

No worries! I wrote it that way to indicate that there are two +1s.
The first is from the behaviour that actual count (33) = configured
count (32) + 1. The second is for the initial BPF program call, which
increments the counter but is not in itself a tail call.

>
> Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>

Thanks!
Johan

>
> >
> > [...]
> >
> > Thanks,
> > Tiezhu
>