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Date: Thu, 9 Sep 2021 09:57:02 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>,
Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Shubham Bansal <illusionist.neo@...il.com>,
Russell King <linux@...linux.org.uk>,
Alexei Starovoitov <ast@...nel.org>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Zi Shen Lim <zlim.lnx@...il.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Paul Burton <paulburton@...nel.org>,
Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
naveen.n.rao@...ux.ibm.com, Michael Ellerman <mpe@...erman.id.au>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>,
Luke Nelson <luke.r.nels@...il.com>,
Xi Wang <xi.wang@...il.com>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
Björn Töpel <bjorn@...nel.org>,
"David S. Miller" <davem@...emloft.net>,
Johan Almbladh <johan.almbladh@...finetworks.com>,
Paul Chaignon <paul@...ium.io>,
Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
open list <linux-kernel@...r.kernel.org>,
linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
linux-riscv@...ts.infradead.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Change value of MAX_TAIL_CALL_CNT from 32
to 33
On 9/9/21 7:50 AM, Andrii Nakryiko wrote:
> On Wed, Sep 8, 2021 at 8:33 PM Tiezhu Yang <yangtiezhu@...ngson.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@...ngson.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(-)
>>
>
> [...]
>
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