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

[Daniel Borkmann]

On 9/9/21 7:50 AM, Andrii Nakryiko wrote:
> On Wed, Sep 8, 2021 at 8:33 PM Tiezhu Yang <yangtiezhu at 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 at loongson.cn>
>> ---
> 
> This change breaks selftests ([0]), please fix them at the same time
> as you are changing the kernel behavior:

The below selftests shouldn't have to change given there is no change in
behavior intended (MAX_TAIL_CALL_CNT is bumped to 33 but counter inc'ed
prior to the comparison). It just means that /all/ JITs must be changed
and in particular properly _tested_.

>    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(-)
>>
> 
> [...]
>