Re: [PATCH bpf-next] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33

On Wed, Sep 8, 2021 at 8:33 PM Tiezhu Yang [off-list ref] 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(-)

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