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Rollup merge of #105317 - RalfJung:retag-rework, r=oli-obk
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make retagging work even with 'unstable' places

This is based on top of #105301. Only the last two commits are new.

While investigating rust-lang/unsafe-code-guidelines#381 I realized that we would have caught this issue much earlier if the add_retag pass wouldn't bail out on assignments of the form `*ptr = ...`.

So this PR changes our retag strategy:
- When a new reference is created via `Rvalue::Ref` (or a raw ptr via `Rvalue::AddressOf`), we do the retagging as part of just executing that address-taking operation.
- For everything else, we still insert retags -- these retags basically serve to ensure that references stored in local variables (and their fields) are always freshly tagged, so skipping this for assignments like `*ptr = ...` is less egregious.
r? ```@oli-obk```
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matthiaskrgr authored Dec 8, 2022
2 parents e826a9a + 34c58e8 commit f1f7560
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Showing 17 changed files with 302 additions and 275 deletions.
16 changes: 14 additions & 2 deletions compiler/rustc_const_eval/src/interpret/machine.rs
Original file line number Diff line number Diff line change
Expand Up @@ -373,9 +373,21 @@ pub trait Machine<'mir, 'tcx>: Sized {
Ok(())
}

/// Executes a retagging operation.
/// Executes a retagging operation for a single pointer.
/// Returns the possibly adjusted pointer.
#[inline]
fn retag(
fn retag_ptr_value(
_ecx: &mut InterpCx<'mir, 'tcx, Self>,
_kind: mir::RetagKind,
val: &ImmTy<'tcx, Self::Provenance>,
) -> InterpResult<'tcx, ImmTy<'tcx, Self::Provenance>> {
Ok(val.clone())
}

/// Executes a retagging operation on a compound value.
/// Replaces all pointers stored in the given place.
#[inline]
fn retag_place_contents(
_ecx: &mut InterpCx<'mir, 'tcx, Self>,
_kind: mir::RetagKind,
_place: &PlaceTy<'tcx, Self::Provenance>,
Expand Down
39 changes: 35 additions & 4 deletions compiler/rustc_const_eval/src/interpret/step.rs
Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ use rustc_middle::mir;
use rustc_middle::mir::interpret::{InterpResult, Scalar};
use rustc_middle::ty::layout::LayoutOf;

use super::{InterpCx, Machine};
use super::{ImmTy, InterpCx, Machine};

/// Classify whether an operator is "left-homogeneous", i.e., the LHS has the
/// same type as the result.
Expand Down Expand Up @@ -108,7 +108,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
// Stacked Borrows.
Retag(kind, place) => {
let dest = self.eval_place(**place)?;
M::retag(self, *kind, &dest)?;
M::retag_place_contents(self, *kind, &dest)?;
}

Intrinsic(box ref intrinsic) => self.emulate_nondiverging_intrinsic(intrinsic)?,
Expand Down Expand Up @@ -247,10 +247,41 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
self.write_scalar(Scalar::from_machine_usize(len, self), &dest)?;
}

AddressOf(_, place) | Ref(_, _, place) => {
Ref(_, borrow_kind, place) => {
let src = self.eval_place(place)?;
let place = self.force_allocation(&src)?;
self.write_immediate(place.to_ref(self), &dest)?;
let val = ImmTy::from_immediate(place.to_ref(self), dest.layout);
// A fresh reference was created, make sure it gets retagged.
let val = M::retag_ptr_value(
self,
if borrow_kind.allows_two_phase_borrow() {
mir::RetagKind::TwoPhase
} else {
mir::RetagKind::Default
},
&val,
)?;
self.write_immediate(*val, &dest)?;
}

AddressOf(_, place) => {
// Figure out whether this is an addr_of of an already raw place.
let place_base_raw = if place.has_deref() {
let ty = self.frame().body.local_decls[place.local].ty;
ty.is_unsafe_ptr()
} else {
// Not a deref, and thus not raw.
false
};

let src = self.eval_place(place)?;
let place = self.force_allocation(&src)?;
let mut val = ImmTy::from_immediate(place.to_ref(self), dest.layout);
if !place_base_raw {
// If this was not already raw, it needs retagging.
val = M::retag_ptr_value(self, mir::RetagKind::Raw, &val)?;
}
self.write_immediate(*val, &dest)?;
}

NullaryOp(null_op, ty) => {
Expand Down
2 changes: 1 addition & 1 deletion compiler/rustc_middle/src/mir/syntax.rs
Original file line number Diff line number Diff line change
Expand Up @@ -400,7 +400,7 @@ impl std::fmt::Display for NonDivergingIntrinsic<'_> {
#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, Hash, HashStable)]
#[rustc_pass_by_value]
pub enum RetagKind {
/// The initial retag when entering a function.
/// The initial retag of arguments when entering a function.
FnEntry,
/// Retag preparing for a two-phase borrow.
TwoPhase,
Expand Down
53 changes: 13 additions & 40 deletions compiler/rustc_mir_transform/src/add_retag.rs
Original file line number Diff line number Diff line change
Expand Up @@ -10,16 +10,6 @@ use rustc_middle::ty::{self, Ty, TyCtxt};

pub struct AddRetag;

/// Determines whether this place is "stable": Whether, if we evaluate it again
/// after the assignment, we can be sure to obtain the same place value.
/// (Concurrent accesses by other threads are no problem as these are anyway non-atomic
/// copies. Data races are UB.)
fn is_stable(place: PlaceRef<'_>) -> bool {
// Which place this evaluates to can change with any memory write,
// so cannot assume deref to be stable.
!place.has_deref()
}

/// Determine whether this type may contain a reference (or box), and thus needs retagging.
/// We will only recurse `depth` times into Tuples/ADTs to bound the cost of this.
fn may_contain_reference<'tcx>(ty: Ty<'tcx>, depth: u32, tcx: TyCtxt<'tcx>) -> bool {
Expand Down Expand Up @@ -69,22 +59,10 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
let basic_blocks = body.basic_blocks.as_mut();
let local_decls = &body.local_decls;
let needs_retag = |place: &Place<'tcx>| {
// FIXME: Instead of giving up for unstable places, we should introduce
// a temporary and retag on that.
is_stable(place.as_ref())
!place.has_deref() // we're not eally interested in stores to "outside" locations, they are hard to keep track of anyway
&& may_contain_reference(place.ty(&*local_decls, tcx).ty, /*depth*/ 3, tcx)
&& !local_decls[place.local].is_deref_temp()
};
let place_base_raw = |place: &Place<'tcx>| {
// If this is a `Deref`, get the type of what we are deref'ing.
if place.has_deref() {
let ty = &local_decls[place.local].ty;
ty.is_unsafe_ptr()
} else {
// Not a deref, and thus not raw.
false
}
};

// PART 1
// Retag arguments at the beginning of the start block.
Expand All @@ -108,7 +86,7 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
}

// PART 2
// Retag return values of functions. Also escape-to-raw the argument of `drop`.
// Retag return values of functions.
// We collect the return destinations because we cannot mutate while iterating.
let returns = basic_blocks
.iter_mut()
Expand Down Expand Up @@ -140,30 +118,25 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
}

// PART 3
// Add retag after assignment.
// Add retag after assignments where data "enters" this function: the RHS is behind a deref and the LHS is not.
for block_data in basic_blocks {
// We want to insert statements as we iterate. To this end, we
// iterate backwards using indices.
for i in (0..block_data.statements.len()).rev() {
let (retag_kind, place) = match block_data.statements[i].kind {
// Retag-as-raw after escaping to a raw pointer, if the referent
// is not already a raw pointer.
StatementKind::Assign(box (lplace, Rvalue::AddressOf(_, ref rplace)))
if !place_base_raw(rplace) =>
{
(RetagKind::Raw, lplace)
}
// Retag after assignments of reference type.
StatementKind::Assign(box (ref place, ref rvalue)) if needs_retag(place) => {
let kind = match rvalue {
Rvalue::Ref(_, borrow_kind, _)
if borrow_kind.allows_two_phase_borrow() =>
{
RetagKind::TwoPhase
}
_ => RetagKind::Default,
let add_retag = match rvalue {
// Ptr-creating operations already do their own internal retagging, no
// need to also add a retag statement.
Rvalue::Ref(..) | Rvalue::AddressOf(..) => false,
_ => true,
};
(kind, *place)
if add_retag {
(RetagKind::Default, *place)
} else {
continue;
}
}
// Do nothing for the rest
_ => continue,
Expand Down
10 changes: 0 additions & 10 deletions compiler/rustc_mir_transform/src/generator.rs
Original file line number Diff line number Diff line change
Expand Up @@ -985,16 +985,6 @@ fn create_generator_drop_shim<'tcx>(
tcx.mk_ptr(ty::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::Mut }),
source_info,
);
if tcx.sess.opts.unstable_opts.mir_emit_retag {
// Alias tracking must know we changed the type
body.basic_blocks_mut()[START_BLOCK].statements.insert(
0,
Statement {
source_info,
kind: StatementKind::Retag(RetagKind::Raw, Box::new(Place::from(SELF_ARG))),
},
)
}

// Make sure we remove dead blocks to remove
// unrelated code from the resume part of the function
Expand Down
10 changes: 0 additions & 10 deletions compiler/rustc_mir_transform/src/shim.rs
Original file line number Diff line number Diff line change
Expand Up @@ -177,16 +177,6 @@ fn build_drop_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, ty: Option<Ty<'tcx>>)
if ty.is_some() {
// The first argument (index 0), but add 1 for the return value.
let dropee_ptr = Place::from(Local::new(1 + 0));
if tcx.sess.opts.unstable_opts.mir_emit_retag {
// Function arguments should be retagged, and we make this one raw.
body.basic_blocks_mut()[START_BLOCK].statements.insert(
0,
Statement {
source_info,
kind: StatementKind::Retag(RetagKind::Raw, Box::new(dropee_ptr)),
},
);
}
let patch = {
let param_env = tcx.param_env_reveal_all_normalized(def_id);
let mut elaborator =
Expand Down
4 changes: 0 additions & 4 deletions src/test/mir-opt/inline/inline_retag.bar.Inline.after.mir
Original file line number Diff line number Diff line change
Expand Up @@ -38,9 +38,7 @@ fn bar() -> bool {
// + literal: Const { ty: &i32, val: Unevaluated(bar, [], Some(promoted[1])) }
Retag(_10); // scope 1 at $DIR/inline_retag.rs:+2:7: +2:9
_4 = &(*_10); // scope 1 at $DIR/inline_retag.rs:+2:7: +2:9
Retag(_4); // scope 1 at $DIR/inline_retag.rs:+2:7: +2:9
_3 = &(*_4); // scope 1 at $DIR/inline_retag.rs:+2:7: +2:9
Retag(_3); // scope 1 at $DIR/inline_retag.rs:+2:7: +2:9
StorageLive(_6); // scope 1 at $DIR/inline_retag.rs:+2:11: +2:14
StorageLive(_7); // scope 1 at $DIR/inline_retag.rs:+2:11: +2:14
_9 = const _; // scope 1 at $DIR/inline_retag.rs:+2:11: +2:14
Expand All @@ -49,9 +47,7 @@ fn bar() -> bool {
// + literal: Const { ty: &i32, val: Unevaluated(bar, [], Some(promoted[0])) }
Retag(_9); // scope 1 at $DIR/inline_retag.rs:+2:11: +2:14
_7 = &(*_9); // scope 1 at $DIR/inline_retag.rs:+2:11: +2:14
Retag(_7); // scope 1 at $DIR/inline_retag.rs:+2:11: +2:14
_6 = &(*_7); // scope 1 at $DIR/inline_retag.rs:+2:11: +2:14
Retag(_6); // scope 1 at $DIR/inline_retag.rs:+2:11: +2:14
Retag(_3); // scope 2 at $DIR/inline_retag.rs:16:8: 16:9
Retag(_6); // scope 2 at $DIR/inline_retag.rs:16:17: 16:18
StorageLive(_11); // scope 2 at $DIR/inline_retag.rs:17:5: 17:7
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -68,9 +68,7 @@ fn array_casts() -> () {
StorageLive(_3); // scope 1 at $DIR/retag.rs:+2:13: +2:19
StorageLive(_4); // scope 1 at $DIR/retag.rs:+2:13: +2:19
_4 = &mut _1; // scope 1 at $DIR/retag.rs:+2:13: +2:19
Retag(_4); // scope 1 at $DIR/retag.rs:+2:13: +2:19
_3 = &raw mut (*_4); // scope 1 at $DIR/retag.rs:+2:13: +2:19
Retag([raw] _3); // scope 1 at $DIR/retag.rs:+2:13: +2:19
_2 = move _3 as *mut usize (Pointer(ArrayToPointer)); // scope 1 at $DIR/retag.rs:+2:13: +2:33
StorageDead(_3); // scope 1 at $DIR/retag.rs:+2:32: +2:33
StorageDead(_4); // scope 1 at $DIR/retag.rs:+2:33: +2:34
Expand All @@ -96,9 +94,7 @@ fn array_casts() -> () {
StorageLive(_10); // scope 4 at $DIR/retag.rs:+6:13: +6:15
StorageLive(_11); // scope 4 at $DIR/retag.rs:+6:13: +6:15
_11 = &_8; // scope 4 at $DIR/retag.rs:+6:13: +6:15
Retag(_11); // scope 4 at $DIR/retag.rs:+6:13: +6:15
_10 = &raw const (*_11); // scope 4 at $DIR/retag.rs:+6:13: +6:15
Retag([raw] _10); // scope 4 at $DIR/retag.rs:+6:13: +6:15
_9 = move _10 as *const usize (Pointer(ArrayToPointer)); // scope 4 at $DIR/retag.rs:+6:13: +6:31
StorageDead(_10); // scope 4 at $DIR/retag.rs:+6:30: +6:31
StorageDead(_11); // scope 4 at $DIR/retag.rs:+6:31: +6:32
Expand All @@ -119,15 +115,13 @@ fn array_casts() -> () {
StorageDead(_17); // scope 6 at $DIR/retag.rs:+7:33: +7:34
_15 = (*_16); // scope 6 at $DIR/retag.rs:+7:25: +7:34
_14 = &_15; // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_14); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageLive(_18); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_35 = const _; // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
// mir::Constant
// + span: $SRC_DIR/core/src/macros/mod.rs:LL:COL
// + literal: Const { ty: &usize, val: Unevaluated(array_casts, [], Some(promoted[0])) }
Retag(_35); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_18 = &(*_35); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_18); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Deinit(_13); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
(_13.0: &usize) = move _14; // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
(_13.1: &usize) = move _18; // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Expand Down Expand Up @@ -164,15 +158,11 @@ fn array_casts() -> () {
StorageLive(_30); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageLive(_31); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_31 = &(*_20); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_31); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_30 = &(*_31); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_30); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageLive(_32); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageLive(_33); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_33 = &(*_21); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_33); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_32 = &(*_33); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_32); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageLive(_34); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Deinit(_34); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
discriminant(_34) = 0; // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,6 @@ fn std::ptr::drop_in_place(_1: *mut Test) -> () {
let mut _3: (); // in scope 0 at $SRC_DIR/core/src/ptr/mod.rs:+0:1: +0:56

bb0: {
Retag([raw] _1); // scope 0 at $SRC_DIR/core/src/ptr/mod.rs:+0:1: +0:56
_2 = &mut (*_1); // scope 0 at $SRC_DIR/core/src/ptr/mod.rs:+0:1: +0:56
_3 = <Test as Drop>::drop(move _2) -> bb1; // scope 0 at $SRC_DIR/core/src/ptr/mod.rs:+0:1: +0:56
// mir::Constant
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,6 @@ fn main::{closure#0}(_1: &[closure@main::{closure#0}], _2: &i32) -> &i32 {
_3 = _2; // scope 0 at $DIR/retag.rs:+1:18: +1:19
Retag(_3); // scope 0 at $DIR/retag.rs:+1:18: +1:19
_0 = &(*_2); // scope 1 at $DIR/retag.rs:+2:9: +2:10
Retag(_0); // scope 1 at $DIR/retag.rs:+2:9: +2:10
StorageDead(_3); // scope 0 at $DIR/retag.rs:+3:5: +3:6
return; // scope 0 at $DIR/retag.rs:+3:6: +3:6
}
Expand Down
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