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C: avoid undefined behavior for bit shifts >= 32 #54

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C: avoid undefined behavior for bit shifts >= 32 #54

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5 changes: 5 additions & 0 deletions c.go
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Original file line number Diff line number Diff line change
Expand Up @@ -187,6 +187,11 @@ func insnToC(insn instruction, blk *block) ([]string, error) {
case bpf.ALUOpConstant:
return stat("a %s= %d;", aluToCOp[i.Op], i.Val)
case bpf.ALUOpX:
// Shifts >=32 are undefined behavior in C.
// https://elixir.bootlin.com/linux/v7.2/source/kernel/bpf/core.c#L1899
if i.Op == bpf.ALUOpShiftLeft || i.Op == bpf.ALUOpShiftRight {
return stat("a %s= (x & 31);", aluToCOp[i.Op])
}
return stat("a %s= x;", aluToCOp[i.Op])
case bpf.NegateA:
return stat("a = -a;")
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9 changes: 8 additions & 1 deletion insn_test.go
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Expand Up @@ -526,10 +526,15 @@ func checkAlu(t *testing.T, op bpf.ALUOp, a, b, res uint32) {
}

checkBackends(t, constFilter, []byte{}, match)
checkAluX(t, op, a, b, res)
}

func checkAluX(t *testing.T, op bpf.ALUOp, a, x, res uint32) {
t.Helper()

xFilter := []bpf.Instruction{
bpf.LoadConstant{Dst: bpf.RegA, Val: a},
bpf.LoadConstant{Dst: bpf.RegX, Val: b},
bpf.LoadConstant{Dst: bpf.RegX, Val: x},

bpf.ALUOpX{Op: op},

Expand Down Expand Up @@ -623,6 +628,7 @@ func TestALUShiftLeft(t *testing.T) {
checkAlu(t, bpf.ALUOpShiftLeft, 1, 0, 1)
checkAlu(t, bpf.ALUOpShiftLeft, 1, 4, 0x10)
checkAlu(t, bpf.ALUOpShiftLeft, 1, 31, 0x80000000)
checkAluX(t, bpf.ALUOpShiftLeft, 1, 32, 1)
}

func TestALUShiftRight(t *testing.T) {
Expand All @@ -631,6 +637,7 @@ func TestALUShiftRight(t *testing.T) {
checkAlu(t, bpf.ALUOpShiftRight, 0xF0, 4, 0x0F)
checkAlu(t, bpf.ALUOpShiftRight, 0xF0, 8, 0)
checkAlu(t, bpf.ALUOpShiftRight, 0x80000000, 31, 1)
checkAluX(t, bpf.ALUOpShiftRight, 0x80000000, 32, 0x80000000)
}

func TestALUMod(t *testing.T) {
Expand Down
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