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Disable almost certainly unsound early otherwise branch MIR opt #95161

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merged 1 commit into from
Mar 22, 2022
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Disable almost certainly unsound early otherwise branch MIR opt #95161

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33 changes: 32 additions & 1 deletion compiler/rustc_mir_transform/src/early_otherwise_branch.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -95,7 +95,7 @@ pub struct EarlyOtherwiseBranch;

impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 2
sess.mir_opt_level() >= 3 && sess.opts.debugging_opts.unsound_mir_opts
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}

fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
Expand Down Expand Up @@ -226,6 +226,37 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {

/// Returns true if computing the discriminant of `place` may be hoisted out of the branch
fn may_hoist<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, place: Place<'tcx>) -> bool {
// FIXME(JakobDegen): This is unsound. Someone could write code like this:
// ```rust
// let Q = val;
// if discriminant(P) == otherwise {
// let ptr = &mut Q as *mut _ as *mut u8;
// unsafe { *ptr = 10; } // Any invalid value for the type
// }
//
// match P {
// A => match Q {
// A => {
// // code
// }
// _ => {
// // don't use Q
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This does use Q. The match does tmp = discriminant(Q) and then switchInt(tmp) to jump to the right arm.

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@JakobDegen JakobDegen Mar 21, 2022 •
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It's not actually important what happens in this branch because we don't take the arm of the outer match that includes match q anyway (in the code path we miscompile). It's only important that this branch is trivially the same as the other "don't use q" branch - the optimization will see that they do the same thing, and replace the outer match with

if discrim(p) == discrim(q)

I suppose this should really read "do the same thing as in the other fall through branch"

// }
// }
// _ => {
// // don't use Q
// }
// };
// ```
//
// Hoisting the `discriminant(Q)` out of the `A` arm causes us to compute the discriminant of an
// invalid value, which is UB.
//
// In order to fix this, we would either need to show that the discriminant computation of
// `place` is computed in all branches, including the `otherwise` branch, or we would need
// another analysis pass to determine that the place is fully initialized. It might even be best
// to have the hoisting be performed in a different pass and just do the CFG changing in this
// pass.
for (place, proj) in place.iter_projections() {
match proj {
// Dereferencing in the computation of `place` might cause issues from one of two
Expand Down