Re: [RFC PATCH bpf-next] bpf: Make actual max tail call count as MAX_TAIL_CALL_CNT

On 9/8/21 7:36 PM, Johan Almbladh wrote:

On Wed, Sep 8, 2021 at 12:56 PM Tiezhu Yang [off-list ref] wrote:

quoted

On 09/08/2021 04:47 PM, Daniel Borkmann wrote:

quoted

[ You have a huge Cc list, but forgot to add Paul and Johan who recently
   looked into this. Added here. ]

On 9/8/21 10:20 AM, Tiezhu Yang wrote:

quoted

In the current code, the actual max tail call count is 33 which is
greater
than MAX_TAIL_CALL_CNT, this is not consistent with the intended meaning
in the commit 04fd61ab36ec ("bpf: allow bpf programs to tail-call other
bpf programs"):

"The chain of tail calls can form unpredictable dynamic loops therefore
tail_call_cnt is used to limit the number of calls and currently is set
to 32."

Additionally, 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

with this patch, make the actual max tail call count as
MAX_TAIL_CALL_CNT,
at the same time, the above failed testcase can be fixed.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---

Hi all,

This is a RFC patch, if I am wrong or I missed something,
please let me know, thank you!

Yes, the original commit from 04fd61ab36ec ("bpf: allow bpf programs
to tail-call
other bpf programs") got the counting wrong, but please also check
f9dabe016b63
("bpf: Undo off-by-one in interpreter tail call count limit") where we
agreed to
align everything to 33 in order to avoid changing existing behavior,
and if we
intend to ever change the count, then only in terms of increasing but
not decreasing
since that ship has sailed.

Thank you, understood.

But I still think there is some confusion about the macro MAX_TAIL_CALL_CNT
which is 32 and the actual value 33, I spent some time to understand it
at the first glance.

Is it impossible to keep the actual max tail call count consistent with
the value 32 of MAX_TAIL_CALL_CNT now?

Yes. If the limit is 32 or 33 does not really matter, but there has to
be a limit. Since the actual limit has been 33, we don't want to break
any user space program relying on this value.

OK, I see, it is very clear now, thank you.

Since the actual limit is 33 now, can we change the value of
MAX_TAIL_CALL_CNT from 32 to 33 and then do some small changes
of the related code?

It will not change the current status but make the actual max
tail call count consistent with the value of MAX_TAIL_CALL_CNT.

If it is OK, I will send a patch later.

Thanks,
Tiezhu

quoted

At least, maybe we need to modify the testcase?

Before making any changes in the test or the BPF code, we need to
understand what the current behaviour actually is. Then we can make
the necessary changes to make everything consistent with 33.

quoted

quoted

Tiezhu, do you still see any arch that is not on 33
from your testing?

If the testcase "Tail call error path, max count reached" in test_bpf is
right,
it seems that the tail call count limit is 32 on x86, because the testcase
passed on x86 jited.

When I run the test_bpf.ko suite I get the following limits.

Interpreter: 33
JIT for arm{32,64}, mips, s390x, powerpc{32,64}, sparc: 33
JIT for x86-{32,64}: 32

However, there are also tail call tests in the selftests suite.
According to Paul those tests show that everything is consistent with
33 now. So, there seem to be a discrepancy between the test_bpf.ko
tests and the selftests.

I am trying to investigate this matter further, but so far I have only
been able to run the test_bpf.ko module for various archs in QEMU.
Link: https://github.com/almbladh/test-linux

I am currently working on getting the selftests to run in QEMU too,
using Buildroot. If you have a working setup for this, it would be
great if you could share that.

Thanks,
Johan

quoted

quoted

Last time Paul fixed the remaining ones in 96bc4432f5ad ("bpf,
riscv: Limit to 33 tail calls") and e49e6f6db04e ("bpf, mips: Limit to
33 tail calls").

Thanks,
Daniel