lkml.org 
[lkml]   [2021]   [Sep]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH bpf-next v2] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33
Date
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@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--;

[...]

\
 
 \ /
  Last update: 2021-09-14 14:38    [W:0.036 / U:0.352 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site