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(-)
[...]
