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interpret: unify offset_from check with offset check

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This commit is contained in:
Ralf Jung 2022-06-09 20:47:06 -04:00
parent be16c6166f
commit e5245ef1eb
5 changed files with 87 additions and 84 deletions
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137
compiler/rustc_const_eval/src/interpret/intrinsics.rs
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@ -313,78 +313,82 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
let a = self.read_pointer(&args[0])?;
let b = self.read_pointer(&args[1])?;
// Special case: if both scalars are *equal integers*
// and not null, we pretend there is an allocation of size 0 right there,
// and their offset is 0. (There's never a valid object at null, making it an
// exception from the exception.)
// This is the dual to the special exception for offset-by-0
// in the inbounds pointer offset operation (see `ptr_offset_inbounds` below).
match (self.ptr_try_get_alloc_id(a), self.ptr_try_get_alloc_id(b)) {
(Err(a), Err(b)) if a == b && a != 0 => {
// Both are the same non-null integer.
self.write_scalar(Scalar::from_machine_isize(0, self), dest)?;
}
(Err(offset), _) | (_, Err(offset)) => {
throw_ub!(DanglingIntPointer(offset, CheckInAllocMsg::OffsetFromTest));
}
(Ok((a_alloc_id, a_offset, _)), Ok((b_alloc_id, b_offset, _))) => {
// Both are pointers. They must be into the same allocation.
if a_alloc_id != b_alloc_id {
let usize_layout = self.layout_of(self.tcx.types.usize)?;
let isize_layout = self.layout_of(self.tcx.types.isize)?;
// Get offsets for both that are at least relative to the same base.
let (a_offset, b_offset) =
match (self.ptr_try_get_alloc_id(a), self.ptr_try_get_alloc_id(b)) {
(Err(a), Err(b)) => {
// Neither poiner points to an allocation.
// If these are inequal or null, this *will* fail the deref check below.
(a, b)
}
(Err(_), _) | (_, Err(_)) => {
// We managed to find a valid allocation for one pointer, but not the other.
// That means they are definitely not pointing to the same allocation.
throw_ub_format!(
"{} cannot compute offset of pointers into different allocations.",
intrinsic_name,
"{} called on pointers into different allocations",
intrinsic_name
);
}
// And they must both be valid for zero-sized accesses ("in-bounds or one past the end").
self.check_ptr_access_align(
a,
Size::ZERO,
Align::ONE,
CheckInAllocMsg::OffsetFromTest,
)?;
self.check_ptr_access_align(
b,
Size::ZERO,
Align::ONE,
CheckInAllocMsg::OffsetFromTest,
)?;
if intrinsic_name == sym::ptr_offset_from_unsigned && a_offset < b_offset {
throw_ub_format!(
"{} cannot compute a negative offset, but {} < {}",
intrinsic_name,
a_offset.bytes(),
b_offset.bytes(),
);
(Ok((a_alloc_id, a_offset, _)), Ok((b_alloc_id, b_offset, _))) => {
// Found allocation for both. They must be into the same allocation.
if a_alloc_id != b_alloc_id {
throw_ub_format!(
"{} called on pointers into different allocations",
intrinsic_name
);
}
// Use these offsets for distance calculation.
(a_offset.bytes(), b_offset.bytes())
}
};
// Compute offset.
let usize_layout = self.layout_of(self.tcx.types.usize)?;
let isize_layout = self.layout_of(self.tcx.types.isize)?;
let ret_layout = if intrinsic_name == sym::ptr_offset_from {
isize_layout
} else {
usize_layout
};
// The subtraction is always done in `isize` to enforce
// the "no more than `isize::MAX` apart" requirement.
let a_offset = ImmTy::from_uint(a_offset.bytes(), isize_layout);
let b_offset = ImmTy::from_uint(b_offset.bytes(), isize_layout);
let (val, overflowed, _ty) =
self.overflowing_binary_op(BinOp::Sub, &a_offset, &b_offset)?;
if overflowed {
throw_ub_format!("Pointers were too far apart for {}", intrinsic_name);
}
let pointee_layout = self.layout_of(substs.type_at(0))?;
// This re-interprets an isize at ret_layout, but we already checked
// that if ret_layout is usize, then the result must be non-negative.
let val = ImmTy::from_scalar(val, ret_layout);
let size = ImmTy::from_int(pointee_layout.size.bytes(), ret_layout);
self.exact_div(&val, &size, dest)?;
// Compute distance.
let distance = {
// The subtraction is always done in `isize` to enforce
// the "no more than `isize::MAX` apart" requirement.
let a_offset = ImmTy::from_uint(a_offset, isize_layout);
let b_offset = ImmTy::from_uint(b_offset, isize_layout);
let (val, overflowed, _ty) =
self.overflowing_binary_op(BinOp::Sub, &a_offset, &b_offset)?;
if overflowed {
throw_ub_format!("pointers were too far apart for {}", intrinsic_name);
}
val.to_machine_isize(self)?
};
// Check that the range between them is dereferenceable ("in-bounds or one past the
// end of the same allocation"). This is like the check in ptr_offset_inbounds.
let min_ptr = if distance >= 0 { b } else { a };
self.check_ptr_access_align(
min_ptr,
Size::from_bytes(distance.unsigned_abs()),
Align::ONE,
CheckInAllocMsg::OffsetFromTest,
)?;
if intrinsic_name == sym::ptr_offset_from_unsigned && distance < 0 {
throw_ub_format!(
"{} called when first pointer has smaller offset than second: {} < {}",
intrinsic_name,
a_offset,
b_offset,
);
}
// Perform division by size to compute return value.
let ret_layout = if intrinsic_name == sym::ptr_offset_from_unsigned {
usize_layout
} else {
isize_layout
};
let pointee_layout = self.layout_of(substs.type_at(0))?;
// If ret_layout is unsigned, we checked that so is the distance, so we are good.
let val = ImmTy::from_int(distance, ret_layout);
let size = ImmTy::from_int(pointee_layout.size.bytes(), ret_layout);
self.exact_div(&val, &size, dest)?;
}
sym::transmute => {
@ -575,11 +579,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
// memory between these pointers must be accessible. Note that we do not require the
// pointers to be properly aligned (unlike a read/write operation).
let min_ptr = if offset_bytes >= 0 { ptr } else { offset_ptr };
let size = offset_bytes.unsigned_abs();
// This call handles checking for integer/null pointers.
self.check_ptr_access_align(
min_ptr,
Size::from_bytes(size),
Size::from_bytes(offset_bytes.unsigned_abs()),
Align::ONE,
CheckInAllocMsg::PointerArithmeticTest,
)?;