Re: [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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
>
> 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(-)
>
> [...]