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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Wed, 8 Sep 2021 22:50:24 -0700
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Subject: Re: [PATCH bpf-next] bpf: Change value of MAX_TAIL_CALL_CNT from 32
 to 33
To:     Tiezhu Yang <yangtiezhu@loongson.cn>
Cc:     Shubham Bansal <illusionist.neo@gmail.com>,
        Russell King <linux@armlinux.org.uk>,
        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>,
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        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Paul Burton <paulburton@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        naveen.n.rao@linux.ibm.com, Michael Ellerman <mpe@ellerman.id.au>,
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        Paul Mackerras <paulus@samba.org>,
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        Albert Ou <aou@eecs.berkeley.edu>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
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On Wed, Sep 8, 2021 at 8:33 PM Tiezhu Yang <yangtiezhu@loongson.cn> wrote:
>
> In the current code, the actual max tail call count is 33 which is greater
> than MAX_TAIL_CALL_CNT (defined as 32), the actual limit is not consistent
> with the meaning of MAX_TAIL_CALL_CNT, there is some confusion and need to
> spend some time to think the reason at the first glance.

think *about* the reason

>
> We can see the historical evolution from commit 04fd61ab36ec ("bpf: allow
> bpf programs to tail-call other bpf programs") and commit f9dabe016b63
> ("bpf: Undo off-by-one in interpreter tail call count limit").
>
> In order to avoid changing existing behavior, the actual limit is 33 now,
> this is resonable.

typo: reasonable

>
> After commit 874be05f525e ("bpf, tests: Add tail call test suite"), we can
> see there exists failed testcase.
>
> On all archs when CONFIG_BPF_JIT_ALWAYS_ON is not set:
>  # echo 0 > /proc/sys/net/core/bpf_jit_enable
>  # modprobe test_bpf
>  # dmesg | grep -w FAIL
>  Tail call error path, max count reached jited:0 ret 34 != 33 FAIL
>
> On some archs:
>  # echo 1 > /proc/sys/net/core/bpf_jit_enable
>  # modprobe test_bpf
>  # dmesg | grep -w FAIL
>  Tail call error path, max count reached jited:1 ret 34 != 33 FAIL
>
> So it is necessary to change the value of MAX_TAIL_CALL_CNT from 32 to 33,
> then do some small changes of the related code.
>
> With this patch, it does not change the current limit, MAX_TAIL_CALL_CNT
> can reflect the actual max tail call count, and the above failed testcase
> can be fixed.
>
> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
> ---

This change breaks selftests ([0]), please fix them at the same time
as you are changing the kernel behavior:

  test_tailcall_2:PASS:tailcall 128 nsec
  test_tailcall_2:PASS:tailcall 128 nsec
  test_tailcall_2:FAIL:tailcall err 0 errno 2 retval 4
  #135/2 tailcalls/tailcall_2:FAIL
  test_tailcall_3:PASS:tailcall 128 nsec
  test_tailcall_3:FAIL:tailcall count err 0 errno 2 count 34
  test_tailcall_3:PASS:tailcall 128 nsec
  #135/3 tailcalls/tailcall_3:FAIL
  #135/4 tailcalls/tailcall_4:OK
  #135/5 tailcalls/tailcall_5:OK
  #135/6 tailcalls/tailcall_bpf2bpf_1:OK
  test_tailcall_bpf2bpf_2:PASS:tailcall 128 nsec
  test_tailcall_bpf2bpf_2:FAIL:tailcall count err 0 errno 2 count 34
  test_tailcall_bpf2bpf_2:PASS:tailcall 128 nsec
  #135/7 tailcalls/tailcall_bpf2bpf_2:FAIL
  #135/8 tailcalls/tailcall_bpf2bpf_3:OK
  test_tailcall_bpf2bpf_4:PASS:tailcall 54 nsec
  test_tailcall_bpf2bpf_4:FAIL:tailcall count err 0 errno 2 count 32
  #135/9 tailcalls/tailcall_bpf2bpf_4:FAIL
  test_tailcall_bpf2bpf_4:PASS:tailcall 54 nsec
  test_tailcall_bpf2bpf_4:FAIL:tailcall count err 0 errno 2 count 32
  #135/10 tailcalls/tailcall_bpf2bpf_5:FAIL
  #135 tailcalls:FAIL


  [0] https://github.com/kernel-patches/bpf/pull/1747/checks?check_run_id=3552002906

>  arch/arm/net/bpf_jit_32.c         | 11 ++++++-----
>  arch/arm64/net/bpf_jit_comp.c     |  7 ++++---
>  arch/mips/net/ebpf_jit.c          |  4 ++--
>  arch/powerpc/net/bpf_jit_comp32.c |  4 ++--
>  arch/powerpc/net/bpf_jit_comp64.c | 12 ++++++------
>  arch/riscv/net/bpf_jit_comp32.c   |  4 ++--
>  arch/riscv/net/bpf_jit_comp64.c   |  4 ++--
>  arch/sparc/net/bpf_jit_comp_64.c  |  8 ++++----
>  include/linux/bpf.h               |  2 +-
>  kernel/bpf/core.c                 |  4 ++--
>  10 files changed, 31 insertions(+), 29 deletions(-)
>

[...]