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riscv64: Fix br-table segfault with zero sized jump tables (#5508)

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We had a off-by-one bounds check error when checking if we should
jump to the default block in a br-table. Instead of always jumping
to the default block when we have a jump table with 0 targets we
would try to compute an offset past the end of the table.

This sometimes would not crash, but it would crash if the there was
no block after the br_table, thus adding a cold block would cause a
segfault.

The actual fix is quite simple, do not count the default block
as a jump table entry when computing the limits.

This commit also does a bunch of cleanup and adding some comments
to the br_table emission code.
This commit is contained in:
Afonso Bordado
2023-01-03 18:22:48 +00:00
committed by GitHub
parent 0043f8e17a
commit c9c7d4991c
2 changed files with 112 additions and 50 deletions
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126
cranelift/codegen/src/isa/riscv64/inst/emit.rs
View File

@@ -1143,10 +1143,41 @@ impl MachInstEmit for Inst {
} => {
let index = allocs.next(index);
let tmp1 = allocs.next_writable(tmp1);
let tmp2 = writable_spilltmp_reg();
Inst::load_constant_u32(writable_spilltmp_reg(), targets.len() as u64, &mut |_| {
writable_spilltmp_reg()
})
// The default target is passed in as the 0th element of `targets`
// separate it here for clarity.
let default_target = targets[0];
let targets = &targets[1..];
// We emit a bounds check on the index, if the index is larger than the number of
// jump table entries, we jump to the default block. Otherwise we compute a jump
// offset by multiplying the index by 8 (the size of each entry) and then jump to
// that offset. Each jump table entry is a regular auipc+jalr which we emit sequentially.
//
// Build the following sequence:
// bounds_check:
// li tmp, n_labels
// bltu index, tmp, compute_target
// jump_to_default_block:
// auipc pc, 0
// jalr zero, pc, default_block
// compute_target:
// auipc pc, 0
// slli tmp, index, 3
// add pc, pc, tmp
// jalr zero, pc, 0x10
// jump_table:
// ; This repeats for each entry in the jumptable
// auipc pc, 0
// jalr zero, pc, block_target
// Bounds check.
//
// Check if the index passed in is larger than the number of jumptable
// entries that we have. If it is, we fallthrough to a jump into the
// default block.
Inst::load_constant_u32(tmp2, targets.len() as u64, &mut |_| tmp2)
.iter()
.for_each(|i| i.emit(&[], sink, emit_info, state));
Inst::CondBr {
@@ -1155,74 +1186,85 @@ impl MachInstEmit for Inst {
kind: IntegerCompare {
kind: IntCC::UnsignedLessThan,
rs1: index,
rs2: spilltmp_reg(),
rs2: tmp2.to_reg(),
},
}
.emit(&[], sink, emit_info, state);
sink.use_label_at_offset(
sink.cur_offset(),
targets[0].as_label().unwrap(),
default_target.as_label().unwrap(),
LabelUse::PCRel32,
);
Inst::construct_auipc_and_jalr(None, writable_spilltmp_reg(), 0)
Inst::construct_auipc_and_jalr(None, tmp2, 0)
.iter()
.for_each(|i| i.emit(&[], sink, emit_info, state));
let mut insts = SmallInstVec::new();
// get current pc.
insts.push(Inst::Auipc {
// Compute the jump table offset.
// We need to emit a PC relative offset,
// Get the current PC.
Inst::Auipc {
rd: tmp1,
imm: Imm20::from_bits(0),
});
// t *= 8; very jump that I emit is 8 byte size.
insts.push(Inst::AluRRImm12 {
}
.emit(&[], sink, emit_info, state);
// Multiply the index by 8, since that is the size in
// bytes of each jump table entry
Inst::AluRRImm12 {
alu_op: AluOPRRI::Slli,
rd: writable_spilltmp_reg(),
rd: tmp2,
rs: index,
imm12: Imm12::from_bits(3),
});
// tmp1 += t
insts.push(Inst::AluRRR {
}
.emit(&[], sink, emit_info, state);
// Calculate the base of the jump, PC + the offset from above.
Inst::AluRRR {
alu_op: AluOPRRR::Add,
rd: tmp1,
rs1: tmp1.to_reg(),
rs2: spilltmp_reg(),
});
insts.push(Inst::Jalr {
rs2: tmp2.to_reg(),
}
.emit(&[], sink, emit_info, state);
// Jump to the middle of the jump table.
// We add a 16 byte offset here, since we used 4 instructions
// since the AUIPC that was used to get the PC.
Inst::Jalr {
rd: writable_zero_reg(),
base: tmp1.to_reg(),
offset: Imm12::from_bits(16),
});
// here is all the jumps.
let mut need_label_use = vec![];
for t in targets.iter().skip(1) {
need_label_use.push((insts.len(), t.clone()));
insts.extend(Inst::construct_auipc_and_jalr(
None,
writable_spilltmp_reg(),
0,
));
offset: Imm12::from_bits((4 * Inst::INSTRUCTION_SIZE) as i16),
}
// emit island if need.
let distance = (insts.len() * 4) as u32;
.emit(&[], sink, emit_info, state);
// Emit the jump table.
//
// Each entry is a aupc + jalr to the target block. We also start with a island
// if necessary.
// Each entry in the jump table is 2 instructions, so 8 bytes. Check if
// we need to emit a jump table here to support that jump.
let distance = (targets.len() * 2 * Inst::INSTRUCTION_SIZE as usize) as u32;
if sink.island_needed(distance) {
sink.emit_island(distance);
}
let mut need_label_use = &need_label_use[..];
insts.into_iter().enumerate().for_each(|(index, inst)| {
if !need_label_use.is_empty() && need_label_use[0].0 == index {
// Emit the jumps back to back
for target in targets.iter() {
sink.use_label_at_offset(
sink.cur_offset(),
need_label_use[0].1.as_label().unwrap(),
target.as_label().unwrap(),
LabelUse::PCRel32,
);
need_label_use = &need_label_use[1..];
Inst::construct_auipc_and_jalr(None, tmp2, 0)
.iter()
.for_each(|i| i.emit(&[], sink, emit_info, state));
}
inst.emit(&[], sink, emit_info, state);
});
// emit the island before, so we can safely
// disable the worst-case-size check in this case.
// We've just emitted an island that is safe up to *here*.
// Mark it as such so that we don't needlessly emit additional islands.
start_off = sink.cur_offset();
}

20
cranelift/filetests/filetests/runtests/br_table.clif
View File

@@ -40,3 +40,23 @@ block5(v5: i32):
; run: %br_table_i32(5) == 9
; run: %br_table_i32(6) == 10
; run: %br_table_i32(-1) == 3
; RISC-V had a bug where having a br_table on a cold block would cause a segfault
; See #5496 for more details.
function %br_table_cold_block(i32) -> i32 system_v {
jt0 = jump_table []
block0(v0: i32):
jump block1
block1 cold:
br_table v0, block2, jt0
block2:
v1 = iconst.i32 0
return v1
}
; run: %br_table_cold_block(0) == 0
; run: %br_table_cold_block(1) == 0