Messages in this thread | | | Subject | Re: [PATCH bpf-next] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33 | From | Daniel Borkmann <> | Date | Thu, 9 Sep 2021 09:57:02 +0200 |
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On 9/9/21 7:50 AM, Andrii Nakryiko wrote: > On Wed, Sep 8, 2021 at 8:33 PM Tiezhu Yang <yangtiezhu@loongson.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@loongson.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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