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x/net/bpf: NegateA is not implemented for VM #16109

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mdlayher opened this issue Jun 18, 2016 · 1 comment
Closed

x/net/bpf: NegateA is not implemented for VM #16109

mdlayher opened this issue Jun 18, 2016 · 1 comment
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@mdlayher
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Using this as a tracking issue to see if it's feasible to allow bpf.NegateA to be processed by the bpf.VM type.

My research thus far shows that the BPF VM treats any negative result as "return all bytes".

Here's the test I'm writing:

func TestVMNegateA(t *testing.T) {
        vm, done, err := testVM(t, []bpf.Instruction{
                bpf.LoadAbsolute{
                        Off:  8,
                        Size: 1,
                },
                bpf.NegateA{},
                bpf.RetA{},
        })
        if err != nil {
                t.Fatalf("failed to load BPF program: %v", err)
        }
        defer done()

        out, err := vm.Run([]byte{
                0xff, 0xff, 0xff, 0xff,
                0xff, 0xff, 0xff, 0xff,
                1, 2, 3, 4, 5,
        })
        if err != nil {
                t.Fatalf("unexpected error while running program: %v", err)
        }
        if want, got := 5, out; want != got {
                t.Fatalf("unexpected number of output bytes:\n- want: %d\n-  got: %d",
                        want, got)
        }
}

And the results using the current VM:

$ go test -v -run=TestVMNegateA .
=== RUN   TestVMNegateA
--- FAIL: TestVMNegateA (0.00s)
        vm_bpf_test.go:120: output byte count does not match:
                - go: 0
                - os: 5
        vm_bpf_test.go:124: Go BPF and OS BPF packet outputs do not match
FAIL
exit status 1
FAIL    golang.org/x/net/bpf    0.002s

Same logic seems to apply when subtraction brings the register results below zero. Will keep working on this and try to submit a CL to enable this functionality in the near future.

/cc @danderson
@ianlancetaylor ianlancetaylor added this to the Unreleased milestone Jun 20, 2016
@mdlayher
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I'm going to close this out as unnecessary.

@golang golang locked and limited conversation to collaborators Mar 17, 2020
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@mdlayher @ianlancetaylor @gopherbot