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llvm: add safety-check for Wasm memcpy

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When lowering the `memcpy` instruction, LLVM will lower it to WebAssembly's
`memory.copy` instruction when the bulk-memory feature is enabled. This
instruction will trap when the destination or source pointer is out-of-bounds.
By Zig's semantics, it is valid to have an invalid pointer when the length is 0.
To prevent runtimes from trapping, we add a safety-check for slices to only
lower to a memcpy instruction when the length is larger than 0.
This commit is contained in:
Luuk de Gram 2023-07-03 20:08:13 +02:00
parent 89396ff02b
commit 836f9fceab
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1 changed files with 35 additions and 0 deletions
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35
src/codegen/llvm.zig
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@ -8634,6 +8634,41 @@ pub const FuncGen = struct {
const len = self.sliceOrArrayLenInBytes(dest_slice, dest_ptr_ty);
const dest_ptr = self.sliceOrArrayPtr(dest_slice, dest_ptr_ty);
const is_volatile = src_ptr_ty.isVolatilePtr(mod) or dest_ptr_ty.isVolatilePtr(mod);
// When bulk-memory is enabled, this will be lowered to WebAssembly's memory.copy instruction.
// This instruction will trap on an invalid address, regardless of the length.
// For this reason we must add a safety-check for 0-sized slices as its pointer field can be undefined.
// We only have to do this for slices as arrays will have a valid pointer.
if (o.target.isWasm() and
std.Target.wasm.featureSetHas(o.target.cpu.features, .bulk_memory) and
(src_ptr_ty.isSlice(mod) or dest_ptr_ty.isSlice(mod)))
{
const parent_block = self.context.createBasicBlock("Block");
const llvm_usize_ty = self.context.intType(o.target.ptrBitWidth());
const cond = try self.cmp(len, llvm_usize_ty.constInt(0, .False), Type.usize, .eq);
const then_block = self.context.appendBasicBlock(self.llvm_func, "Then");
const else_block = self.context.appendBasicBlock(self.llvm_func, "Else");
_ = self.builder.buildCondBr(cond, then_block, else_block);
self.builder.positionBuilderAtEnd(then_block);
_ = self.builder.buildBr(parent_block);
self.builder.positionBuilderAtEnd(else_block);
_ = self.builder.buildMemCpy(
dest_ptr,
dest_ptr_ty.ptrAlignment(mod),
src_ptr,
src_ptr_ty.ptrAlignment(mod),
len,
is_volatile,
);
_ = self.builder.buildBr(parent_block);
self.llvm_func.appendExistingBasicBlock(parent_block);
self.builder.positionBuilderAtEnd(parent_block);
return null;
}
_ = self.builder.buildMemCpy(
dest_ptr,
dest_ptr_ty.ptrAlignment(mod),