[PATCH bpf-next v2] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33

[Tiezhu Yang]

On 09/14/2021 03:30 PM, Daniel Borkmann wrote:
> On 9/11/21 3:56 AM, Tiezhu Yang wrote:
>>
[...]
>> With this patch, it does not change the current limit 33, 
>> MAX_TAIL_CALL_CNT
>> can reflect the actual max tail call count, the tailcall selftests 
>> can work
>> well, and also the above failed testcase in test_bpf can be fixed for 
>> the
>> interpreter (all archs) and the JIT (all archs except for x86).
>>
>>   # uname -m
>>   x86_64
>>   # 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 33 != 34 FAIL
>
> Could you also state in here which archs you have tested with this 
> change? I
> presume /every/ arch which has a JIT?

OK, will do it in v3.
I have tested on x86 and mips.

>
>> Signed-off-by: Tiezhu Yang <yangtiezhu at loongson.cn>
>> ---
>>
>> v2:
>>    -- fix the typos in the commit message and update the commit message.
>>    -- fix the failed tailcall selftests for x86 jit.
>>       I am not quite sure the change on x86 is proper, with this change,
>>       tailcall selftests passed, but tailcall limit test in test_bpf.ko
>>       failed, I do not know the reason now, I think this is another 
>> issue,
>>       maybe someone more versed in x86 jit could take a look.
>
> There should be a series from Johan coming today with regards to 
> test_bpf.ko
> that will fix the "tail call error path, max count reached" test which 
> had an
> assumption in that R0 would always be valid for the fall-through and 
> could be
> passed to the bpf_exit insn whereas it is not guaranteed and verifier, 
> for
> example, forbids a subsequent access to R0 w/o reinit. For your 
> testing, I
> would suggested to recheck once this series is out.

I will test the following patch on x86 and mips:

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

[...]

>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index 0fe6aac..74a9e61 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -402,7 +402,7 @@ static int get_pop_bytes(bool *callee_regs_used)
>>    * ... bpf_tail_call(void *ctx, struct bpf_array *array, u64 index) 
>> ...
>>    *   if (index >= array->map.max_entries)
>>    *     goto out;
>> - *   if (++tail_call_cnt > MAX_TAIL_CALL_CNT)
>> + *   if (tail_call_cnt++ == MAX_TAIL_CALL_CNT)
>
> Why such inconsistency to e.g. above with arm64 case but also compared to
> x86 32 bit which uses JAE? If so, we should cleanly follow the reference
> implementation (== interpreter) _everywhere_ and _not_ introduce 
> additional
> variants/implementations across JITs.

In order tokeep consistencyand make as few changes as possible,
<javascript:void(0);>I will modify the check condition as follows:

#define MAX_TAIL_CALL_CNT 33
(1) for x86, arm64, ... (0 ~ 32)
tcc = 0;
if (tcc == MAX_TAIL_CALL_CNT)
     goto out;
tcc++;

(2) for mips, riscv (33 ~ 1)
tcc = MAX_TAIL_CALL_CNT;
if (tcc == 0)
     goto out;
tcc--;

[...]