cranelift_fuzzgen/passes/
int_divz.rs

1use crate::FuzzGen;
2use anyhow::Result;
3use cranelift::codegen::cursor::{Cursor, FuncCursor};
4use cranelift::codegen::ir::{Function, Inst, Opcode};
5use cranelift::prelude::{InstBuilder, IntCC};
6
7pub fn do_int_divz_pass(fuzz: &mut FuzzGen, func: &mut Function) -> Result<()> {
8    // Insert this per function, otherwise the actual rate of int_divz doesn't go down that much
9    // Experimentally if we decide this per instruction with a 0.1% allow rate, we get 4.4% of runs
10    // trapping. Doing this per function decreases the number of runs that trap. It also consumes
11    // fewer fuzzer input bytes which is nice.
12    let ratio = fuzz.config.allowed_int_divz_ratio;
13    let insert_seq = !fuzz.u.ratio(ratio.0, ratio.1)?;
14    if !insert_seq {
15        return Ok(());
16    }
17
18    let mut pos = FuncCursor::new(func);
19    while let Some(_block) = pos.next_block() {
20        while let Some(inst) = pos.next_inst() {
21            if can_int_divz(&pos, inst) {
22                insert_int_divz_sequence(&mut pos, inst);
23            }
24        }
25    }
26    Ok(())
27}
28
29/// Returns true/false if this instruction can cause a `int_divz` trap
30fn can_int_divz(pos: &FuncCursor, inst: Inst) -> bool {
31    let opcode = pos.func.dfg.insts[inst].opcode();
32
33    matches!(
34        opcode,
35        Opcode::Sdiv | Opcode::Udiv | Opcode::Srem | Opcode::Urem
36    )
37}
38
39/// Prepend instructions to inst to avoid `int_divz` traps
40fn insert_int_divz_sequence(pos: &mut FuncCursor, inst: Inst) {
41    let opcode = pos.func.dfg.insts[inst].opcode();
42    let inst_args = pos.func.dfg.inst_args(inst);
43    let (lhs, rhs) = (inst_args[0], inst_args[1]);
44    assert_eq!(pos.func.dfg.value_type(lhs), pos.func.dfg.value_type(rhs));
45    let ty = pos.func.dfg.value_type(lhs);
46
47    // All of these instructions can trap if the denominator is zero
48    let zero = pos.ins().iconst(ty, 0);
49    let one = pos.ins().iconst(ty, 1);
50    let denominator_is_zero = pos.ins().icmp(IntCC::Equal, rhs, zero);
51
52    let replace_denominator = if matches!(opcode, Opcode::Srem | Opcode::Sdiv) {
53        // Srem and Sdiv can also trap on INT_MIN / -1. So we need to check for the second one
54
55        // 1 << (ty bits - 1) to get INT_MIN
56        let int_min = pos.ins().ishl_imm(one, ty.lane_bits() as i64 - 1);
57
58        // Get a -1 const
59        // TODO: A iconst -1 would be clearer, but #2906 makes this impossible for i128
60        let neg_one = pos.ins().isub(zero, one);
61
62        let lhs_check = pos.ins().icmp(IntCC::Equal, lhs, int_min);
63        let rhs_check = pos.ins().icmp(IntCC::Equal, rhs, neg_one);
64        let is_invalid = pos.ins().band(lhs_check, rhs_check);
65
66        // These also crash if the denominator is zero, so we still need to check for that.
67        pos.ins().bor(denominator_is_zero, is_invalid)
68    } else {
69        denominator_is_zero
70    };
71
72    // If we have a trap we replace the denominator with a 1
73    let new_rhs = pos.ins().select(replace_denominator, one, rhs);
74
75    // Replace the previous rhs with the new one
76    let args = pos.func.dfg.inst_args_mut(inst);
77    args[1] = new_rhs;
78}
cranelift_fuzzgen/passes/int_divz.rs

cranelift_fuzzgen/passes/
int_divz.rs
