• Tiezhu Yang's avatar
    bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33 · ebf7f6f0
    Tiezhu Yang authored
    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 and thus confusing at first glance.
    We can see the historical evolution from commit 04fd61ab ("bpf: allow
    bpf programs to tail-call other bpf programs") and commit f9dabe01
    ("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
    reasonable.
    
    After commit 874be05f ("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
    
    Although the above failed testcase has been fixed in commit 18935a72
    ("bpf/tests: Fix error in tail call limit tests"), it would still be good
    to change the value of MAX_TAIL_CALL_CNT from 32 to 33 to make the code
    more readable.
    
    The 32-bit x86 JIT was using a limit of 32, just fix the wrong comments and
    limit to 33 tail calls as the constant MAX_TAIL_CALL_CNT updated. For the
    mips64 JIT, use "ori" instead of "addiu" as suggested by Johan Almbladh.
    For the riscv JIT, use RV_REG_TCC directly to save one register move as
    suggested by Björn Töpel. For the other implementations, no function changes,
    it does not change the current limit 33, the new value of MAX_TAIL_CALL_CNT
    can reflect the actual max tail call count, the related tail call testcases
    in test_bpf module and selftests can work well for the interpreter and the
    JIT.
    
    Here are the test results on x86_64:
    
     # uname -m
     x86_64
     # echo 0 > /proc/sys/net/core/bpf_jit_enable
     # modprobe test_bpf test_suite=test_tail_calls
     # dmesg | tail -1
     test_bpf: test_tail_calls: Summary: 8 PASSED, 0 FAILED, [0/8 JIT'ed]
     # rmmod test_bpf
     # echo 1 > /proc/sys/net/core/bpf_jit_enable
     # modprobe test_bpf test_suite=test_tail_calls
     # dmesg | tail -1
     test_bpf: test_tail_calls: Summary: 8 PASSED, 0 FAILED, [8/8 JIT'ed]
     # rmmod test_bpf
     # ./test_progs -t tailcalls
     #142 tailcalls:OK
     Summary: 1/11 PASSED, 0 SKIPPED, 0 FAILED
    Signed-off-by: default avatarTiezhu Yang <yangtiezhu@loongson.cn>
    Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    Tested-by: default avatarJohan Almbladh <johan.almbladh@anyfinetworks.com>
    Tested-by: default avatarIlya Leoshkevich <iii@linux.ibm.com>
    Acked-by: default avatarBjörn Töpel <bjorn@kernel.org>
    Acked-by: default avatarJohan Almbladh <johan.almbladh@anyfinetworks.com>
    Acked-by: default avatarIlya Leoshkevich <iii@linux.ibm.com>
    Link: https://lore.kernel.org/bpf/1636075800-3264-1-git-send-email-yangtiezhu@loongson.cn
    ebf7f6f0
bpf_jit_comp64.c 31.5 KB