check_unsafety.rs

use rustc::hir::map::Map;
use rustc::lint::builtin::{SAFE_PACKED_BORROWS, UNUSED_UNSAFE};
use rustc::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
use rustc::mir::*;
use rustc::ty::cast::CastTy;
use rustc::ty::query::Providers;
use rustc::ty::{self, TyCtxt};
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::struct_span_err;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit;
use rustc_hir::Node;
use rustc_span::symbol::{sym, Symbol};

use std::ops::Bound;

use crate::const_eval::{is_const_fn, is_min_const_fn};
use crate::util;

pub struct UnsafetyChecker<'a, 'tcx> {
    body: &'a Body<'tcx>,
    const_context: bool,
    min_const_fn: bool,
    violations: Vec<UnsafetyViolation>,
    source_info: SourceInfo,
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    /// Mark an `unsafe` block as used, so we don't lint it.
    used_unsafe: FxHashSet<hir::HirId>,
    inherited_blocks: Vec<(hir::HirId, bool)>,
}

impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
    fn new(
        const_context: bool,
        min_const_fn: bool,
        body: &'a Body<'tcx>,
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
    ) -> Self {
        // sanity check
        if min_const_fn {
            assert!(const_context);
        }
        Self {
            body,
            const_context,
            min_const_fn,
            violations: vec![],
            source_info: SourceInfo { span: body.span, scope: OUTERMOST_SOURCE_SCOPE },
            tcx,
            param_env,
            used_unsafe: Default::default(),
            inherited_blocks: vec![],
        }
    }
}

impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
        self.source_info = terminator.source_info;
        match terminator.kind {
            TerminatorKind::Goto { .. }
            | TerminatorKind::SwitchInt { .. }
            | TerminatorKind::Drop { .. }
            | TerminatorKind::Yield { .. }
            | TerminatorKind::Assert { .. }
            | TerminatorKind::DropAndReplace { .. }
            | TerminatorKind::GeneratorDrop
            | TerminatorKind::Resume
            | TerminatorKind::Abort
            | TerminatorKind::Return
            | TerminatorKind::Unreachable
            | TerminatorKind::FalseEdges { .. }
            | TerminatorKind::FalseUnwind { .. } => {
                // safe (at least as emitted during MIR construction)
            }

            TerminatorKind::Call { ref func, .. } => {
                let func_ty = func.ty(self.body, self.tcx);
                let sig = func_ty.fn_sig(self.tcx);
                if let hir::Unsafety::Unsafe = sig.unsafety() {
                    self.require_unsafe(
                        "call to unsafe function",
                        "consult the function's documentation for information on how to avoid \
                         undefined behavior",
                        UnsafetyViolationKind::GeneralAndConstFn,
                    )
                }
            }
        }
        self.super_terminator(terminator, location);
    }

    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
        self.source_info = statement.source_info;
        match statement.kind {
            StatementKind::Assign(..)
            | StatementKind::FakeRead(..)
            | StatementKind::SetDiscriminant { .. }
            | StatementKind::StorageLive(..)
            | StatementKind::StorageDead(..)
            | StatementKind::Retag { .. }
            | StatementKind::AscribeUserType(..)
            | StatementKind::Nop => {
                // safe (at least as emitted during MIR construction)
            }

            StatementKind::InlineAsm { .. } => self.require_unsafe(
                "use of inline assembly",
                "inline assembly is entirely unchecked and can cause undefined behavior",
                UnsafetyViolationKind::General,
            ),
        }
        self.super_statement(statement, location);
    }

    fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
        match rvalue {
            Rvalue::Aggregate(box ref aggregate, _) => match aggregate {
                &AggregateKind::Array(..) | &AggregateKind::Tuple => {}
                &AggregateKind::Adt(ref def, ..) => {
                    match self.tcx.layout_scalar_valid_range(def.did) {
                        (Bound::Unbounded, Bound::Unbounded) => {}
                        _ => self.require_unsafe(
                            "initializing type with `rustc_layout_scalar_valid_range` attr",
                            "initializing a layout restricted type's field with a value \
                                outside the valid range is undefined behavior",
                            UnsafetyViolationKind::GeneralAndConstFn,
                        ),
                    }
                }
                &AggregateKind::Closure(def_id, _) | &AggregateKind::Generator(def_id, _, _) => {
                    let UnsafetyCheckResult { violations, unsafe_blocks } =
                        self.tcx.unsafety_check_result(def_id);
                    self.register_violations(&violations, &unsafe_blocks);
                }
            },
            // casting pointers to ints is unsafe in const fn because the const evaluator cannot
            // possibly know what the result of various operations like `address / 2` would be
            // pointers during const evaluation have no integral address, only an abstract one
            Rvalue::Cast(CastKind::Misc, ref operand, cast_ty)
                if self.const_context && self.tcx.features().const_raw_ptr_to_usize_cast =>
            {
                let operand_ty = operand.ty(self.body, self.tcx);
                let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
                let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
                match (cast_in, cast_out) {
                    (CastTy::Ptr(_), CastTy::Int(_)) | (CastTy::FnPtr, CastTy::Int(_)) => {
                        self.require_unsafe(
                            "cast of pointer to int",
                            "casting pointers to integers in constants",
                            UnsafetyViolationKind::General,
                        );
                    }
                    _ => {}
                }
            }
            // raw pointer and fn pointer operations are unsafe as it is not clear whether one
            // pointer would be "less" or "equal" to another, because we cannot know where llvm
            // or the linker will place various statics in memory. Without this information the
            // result of a comparison of addresses would differ between runtime and compile-time.
            Rvalue::BinaryOp(_, ref lhs, _)
                if self.const_context && self.tcx.features().const_compare_raw_pointers =>
            {
                if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind {
                    self.require_unsafe(
                        "pointer operation",
                        "operations on pointers in constants",
                        UnsafetyViolationKind::General,
                    );
                }
            }
            _ => {}
        }
        self.super_rvalue(rvalue, location);
    }

    fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
        // prevent
        // * `&mut x.field`
        // * `x.field = y;`
        // * `&x.field` if `field`'s type has interior mutability
        // because either of these would allow modifying the layout constrained field and
        // insert values that violate the layout constraints.
        if context.is_mutating_use() || context.is_borrow() {
            self.check_mut_borrowing_layout_constrained_field(place, context.is_mutating_use());
        }

        for (i, elem) in place.projection.iter().enumerate() {
            let proj_base = &place.projection[..i];

            if context.is_borrow() {
                if util::is_disaligned(self.tcx, self.body, self.param_env, place) {
                    let source_info = self.source_info;
                    let lint_root = self.body.source_scopes[source_info.scope]
                        .local_data
                        .as_ref()
                        .assert_crate_local()
                        .lint_root;
                    self.require_unsafe(
                        "borrow of packed field",
                        "fields of packed structs might be misaligned: dereferencing a \
                        misaligned pointer or even just creating a misaligned reference \
                        is undefined behavior",
                        UnsafetyViolationKind::BorrowPacked(lint_root),
                    );
                }
            }
            let old_source_info = self.source_info;
            if let [] = proj_base {
                let decl = &self.body.local_decls[place.local];
                if decl.internal {
                    if let LocalInfo::StaticRef { def_id, .. } = decl.local_info {
                        if self.tcx.is_mutable_static(def_id) {
                            self.require_unsafe(
                                "use of mutable static",
                                "mutable statics can be mutated by multiple threads: aliasing \
                            violations or data races will cause undefined behavior",
                                UnsafetyViolationKind::General,
                            );
                            return;
                        } else if self.tcx.is_foreign_item(def_id) {
                            self.require_unsafe(
                                "use of extern static",
                                "extern statics are not controlled by the Rust type system: \
                            invalid data, aliasing violations or data races will cause \
                            undefined behavior",
                                UnsafetyViolationKind::General,
                            );
                            return;
                        }
                    } else {
                        // Internal locals are used in the `move_val_init` desugaring.
                        // We want to check unsafety against the source info of the
                        // desugaring, rather than the source info of the RHS.
                        self.source_info = self.body.local_decls[place.local].source_info;
                    }
                }
            }
            let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
            match base_ty.kind {
                ty::RawPtr(..) => self.require_unsafe(
                    "dereference of raw pointer",
                    "raw pointers may be NULL, dangling or unaligned; they can violate \
                         aliasing rules and cause data races: all of these are undefined \
                         behavior",
                    UnsafetyViolationKind::General,
                ),
                ty::Adt(adt, _) => {
                    if adt.is_union() {
                        if context == PlaceContext::MutatingUse(MutatingUseContext::Store)
                            || context == PlaceContext::MutatingUse(MutatingUseContext::Drop)
                            || context == PlaceContext::MutatingUse(MutatingUseContext::AsmOutput)
                        {
                            let elem_ty = match elem {
                                ProjectionElem::Field(_, ty) => ty,
                                _ => span_bug!(
                                    self.source_info.span,
                                    "non-field projection {:?} from union?",
                                    place
                                ),
                            };
                            if !elem_ty.is_copy_modulo_regions(
                                self.tcx,
                                self.param_env,
                                self.source_info.span,
                            ) {
                                self.require_unsafe(
                                    "assignment to non-`Copy` union field",
                                    "the previous content of the field will be dropped, which \
                                     causes undefined behavior if the field was not properly \
                                     initialized",
                                    UnsafetyViolationKind::GeneralAndConstFn,
                                )
                            } else {
                                // write to non-move union, safe
                            }
                        } else {
                            self.require_unsafe(
                                "access to union field",
                                "the field may not be properly initialized: using \
                                 uninitialized data will cause undefined behavior",
                                UnsafetyViolationKind::GeneralAndConstFn,
                            )
                        }
                    }
                }
                _ => {}
            }
            self.source_info = old_source_info;
        }
    }
}

impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
    fn require_unsafe(
        &mut self,
        description: &'static str,
        details: &'static str,
        kind: UnsafetyViolationKind,
    ) {
        let source_info = self.source_info;
        self.register_violations(
            &[UnsafetyViolation {
                source_info,
                description: Symbol::intern(description),
                details: Symbol::intern(details),
                kind,
            }],
            &[],
        );
    }

    fn register_violations(
        &mut self,
        violations: &[UnsafetyViolation],
        unsafe_blocks: &[(hir::HirId, bool)],
    ) {
        let safety = self.body.source_scopes[self.source_info.scope]
            .local_data
            .as_ref()
            .assert_crate_local()
            .safety;
        let within_unsafe = match safety {
            // `unsafe` blocks are required in safe code
            Safety::Safe => {
                for violation in violations {
                    let mut violation = *violation;
                    match violation.kind {
                        UnsafetyViolationKind::GeneralAndConstFn
                        | UnsafetyViolationKind::General => {}
                        UnsafetyViolationKind::BorrowPacked(_) => {
                            if self.min_const_fn {
                                // const fns don't need to be backwards compatible and can
                                // emit these violations as a hard error instead of a backwards
                                // compat lint
                                violation.kind = UnsafetyViolationKind::General;
                            }
                        }
                    }
                    if !self.violations.contains(&violation) {
                        self.violations.push(violation)
                    }
                }
                false
            }
            // `unsafe` function bodies allow unsafe without additional unsafe blocks
            Safety::BuiltinUnsafe | Safety::FnUnsafe => true,
            Safety::ExplicitUnsafe(hir_id) => {
                // mark unsafe block as used if there are any unsafe operations inside
                if !violations.is_empty() {
                    self.used_unsafe.insert(hir_id);
                }
                // only some unsafety is allowed in const fn
                if self.min_const_fn {
                    for violation in violations {
                        match violation.kind {
                            // these unsafe things are stable in const fn
                            UnsafetyViolationKind::GeneralAndConstFn => {}
                            // these things are forbidden in const fns
                            UnsafetyViolationKind::General
                            | UnsafetyViolationKind::BorrowPacked(_) => {
                                let mut violation = *violation;
                                // const fns don't need to be backwards compatible and can
                                // emit these violations as a hard error instead of a backwards
                                // compat lint
                                violation.kind = UnsafetyViolationKind::General;
                                if !self.violations.contains(&violation) {
                                    self.violations.push(violation)
                                }
                            }
                        }
                    }
                }
                true
            }
        };
        self.inherited_blocks.extend(
            unsafe_blocks.iter().map(|&(hir_id, is_used)| (hir_id, is_used && !within_unsafe)),
        );
    }
    fn check_mut_borrowing_layout_constrained_field(
        &mut self,
        place: &Place<'tcx>,
        is_mut_use: bool,
    ) {
        let mut cursor = place.projection.as_ref();
        while let &[ref proj_base @ .., elem] = cursor {
            cursor = proj_base;

            match elem {
                // Modifications behind a dereference don't affect the value of
                // the pointer.
                ProjectionElem::Deref => return,
                ProjectionElem::Field(..) => {
                    let ty =
                        Place::ty_from(place.local, proj_base, &self.body.local_decls, self.tcx).ty;
                    match ty.kind {
                        ty::Adt(def, _) => match self.tcx.layout_scalar_valid_range(def.did) {
                            (Bound::Unbounded, Bound::Unbounded) => {}
                            _ => {
                                let (description, details) = if is_mut_use {
                                    (
                                        "mutation of layout constrained field",
                                        "mutating layout constrained fields cannot statically be \
                                        checked for valid values",
                                    )

                                // Check `is_freeze` as late as possible to avoid cycle errors
                                // with opaque types.
                                } else if !place.ty(self.body, self.tcx).ty.is_freeze(
                                    self.tcx,
                                    self.param_env,
                                    self.source_info.span,
                                ) {
                                    (
                                        "borrow of layout constrained field with interior \
                                        mutability",
                                        "references to fields of layout constrained fields \
                                        lose the constraints. Coupled with interior mutability, \
                                        the field can be changed to invalid values",
                                    )
                                } else {
                                    continue;
                                };
                                self.require_unsafe(
                                    description,
                                    details,
                                    UnsafetyViolationKind::GeneralAndConstFn,
                                );
                            }
                        },
                        _ => {}
                    }
                }
                _ => {}
            }
        }
    }
}

pub(crate) fn provide(providers: &mut Providers<'_>) {
    *providers = Providers { unsafety_check_result, unsafe_derive_on_repr_packed, ..*providers };
}

struct UnusedUnsafeVisitor<'a> {
    used_unsafe: &'a FxHashSet<hir::HirId>,
    unsafe_blocks: &'a mut Vec<(hir::HirId, bool)>,
}

impl<'a, 'tcx> intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'a> {
    type Map = Map<'tcx>;

    fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<'_, Self::Map> {
        intravisit::NestedVisitorMap::None
    }

    fn visit_block(&mut self, block: &'tcx hir::Block<'tcx>) {
        intravisit::walk_block(self, block);

        if let hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::UserProvided) = block.rules {
            self.unsafe_blocks.push((block.hir_id, self.used_unsafe.contains(&block.hir_id)));
        }
    }
}

fn check_unused_unsafe(
    tcx: TyCtxt<'_>,
    def_id: DefId,
    used_unsafe: &FxHashSet<hir::HirId>,
    unsafe_blocks: &mut Vec<(hir::HirId, bool)>,
) {
    let body_id =
        tcx.hir().as_local_hir_id(def_id).and_then(|hir_id| tcx.hir().maybe_body_owned_by(hir_id));

    let body_id = match body_id {
        Some(body) => body,
        None => {
            debug!("check_unused_unsafe({:?}) - no body found", def_id);
            return;
        }
    };
    let body = tcx.hir().body(body_id);
    debug!("check_unused_unsafe({:?}, body={:?}, used_unsafe={:?})", def_id, body, used_unsafe);

    let mut visitor = UnusedUnsafeVisitor { used_unsafe, unsafe_blocks };
    intravisit::Visitor::visit_body(&mut visitor, body);
}

fn unsafety_check_result(tcx: TyCtxt<'_>, def_id: DefId) -> UnsafetyCheckResult {
    debug!("unsafety_violations({:?})", def_id);

    // N.B., this borrow is valid because all the consumers of
    // `mir_built` force this.
    let body = &tcx.mir_built(def_id).borrow();

    let param_env = tcx.param_env(def_id);

    let id = tcx.hir().as_local_hir_id(def_id).unwrap();
    let (const_context, min_const_fn) = match tcx.hir().body_owner_kind(id) {
        hir::BodyOwnerKind::Closure => (false, false),
        hir::BodyOwnerKind::Fn => (is_const_fn(tcx, def_id), is_min_const_fn(tcx, def_id)),
        hir::BodyOwnerKind::Const | hir::BodyOwnerKind::Static(_) => (true, false),
    };
    let mut checker = UnsafetyChecker::new(const_context, min_const_fn, body, tcx, param_env);
    // mir_built ensures that body has a computed cache, so we don't (and can't) attempt to
    // recompute it here.
    let body = body.unwrap_read_only();
    checker.visit_body(body);

    check_unused_unsafe(tcx, def_id, &checker.used_unsafe, &mut checker.inherited_blocks);
    UnsafetyCheckResult {
        violations: checker.violations.into(),
        unsafe_blocks: checker.inherited_blocks.into(),
    }
}

fn unsafe_derive_on_repr_packed(tcx: TyCtxt<'_>, def_id: DefId) {
    let lint_hir_id = tcx
        .hir()
        .as_local_hir_id(def_id)
        .unwrap_or_else(|| bug!("checking unsafety for non-local def id {:?}", def_id));

    tcx.struct_span_lint_hir(SAFE_PACKED_BORROWS, lint_hir_id, tcx.def_span(def_id), |lint| {
        // FIXME: when we make this a hard error, this should have its
        // own error code.
        let message = if tcx.generics_of(def_id).own_requires_monomorphization() {
            "`#[derive]` can't be used on a `#[repr(packed)]` struct with \
             type or const parameters (error E0133)"
                .to_string()
        } else {
            "`#[derive]` can't be used on a `#[repr(packed)]` struct that \
             does not derive Copy (error E0133)"
                .to_string()
        };
        lint.build(&message).emit()
    });
}

/// Returns the `HirId` for an enclosing scope that is also `unsafe`.
fn is_enclosed(
    tcx: TyCtxt<'_>,
    used_unsafe: &FxHashSet<hir::HirId>,
    id: hir::HirId,
) -> Option<(String, hir::HirId)> {
    let parent_id = tcx.hir().get_parent_node(id);
    if parent_id != id {
        if used_unsafe.contains(&parent_id) {
            Some(("block".to_string(), parent_id))
        } else if let Some(Node::Item(&hir::Item {
            kind: hir::ItemKind::Fn(ref sig, _, _), ..
        })) = tcx.hir().find(parent_id)
        {
            match sig.header.unsafety {
                hir::Unsafety::Unsafe => Some(("fn".to_string(), parent_id)),
                hir::Unsafety::Normal => None,
            }
        } else {
            is_enclosed(tcx, used_unsafe, parent_id)
        }
    } else {
        None
    }
}

fn report_unused_unsafe(tcx: TyCtxt<'_>, used_unsafe: &FxHashSet<hir::HirId>, id: hir::HirId) {
    let span = tcx.sess.source_map().def_span(tcx.hir().span(id));
    tcx.struct_span_lint_hir(UNUSED_UNSAFE, id, span, |lint| {
        let msg = "unnecessary `unsafe` block";
        let mut db = lint.build(msg);
        db.span_label(span, msg);
        if let Some((kind, id)) = is_enclosed(tcx, used_unsafe, id) {
            db.span_label(
                tcx.sess.source_map().def_span(tcx.hir().span(id)),
                format!("because it's nested under this `unsafe` {}", kind),
            );
        }
        db.emit();
    });
}

fn builtin_derive_def_id(tcx: TyCtxt<'_>, def_id: DefId) -> Option<DefId> {
    debug!("builtin_derive_def_id({:?})", def_id);
    if let Some(impl_def_id) = tcx.impl_of_method(def_id) {
        if tcx.has_attr(impl_def_id, sym::automatically_derived) {
            debug!("builtin_derive_def_id({:?}) - is {:?}", def_id, impl_def_id);
            Some(impl_def_id)
        } else {
            debug!("builtin_derive_def_id({:?}) - not automatically derived", def_id);
            None
        }
    } else {
        debug!("builtin_derive_def_id({:?}) - not a method", def_id);
        None
    }
}

pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: DefId) {
    debug!("check_unsafety({:?})", def_id);

    // closures are handled by their parent fn.
    if tcx.is_closure(def_id) {
        return;
    }

    let UnsafetyCheckResult { violations, unsafe_blocks } = tcx.unsafety_check_result(def_id);

    for &UnsafetyViolation { source_info, description, details, kind } in violations.iter() {
        // Report an error.
        match kind {
            UnsafetyViolationKind::GeneralAndConstFn | UnsafetyViolationKind::General => {
                struct_span_err!(
                    tcx.sess,
                    source_info.span,
                    E0133,
                    "{} is unsafe and requires unsafe function or block",
                    description
                )
                .span_label(source_info.span, &*description.as_str())
                .note(&details.as_str())
                .emit();
            }
            UnsafetyViolationKind::BorrowPacked(lint_hir_id) => {
                if let Some(impl_def_id) = builtin_derive_def_id(tcx, def_id) {
                    tcx.unsafe_derive_on_repr_packed(impl_def_id);
                } else {
                    tcx.struct_span_lint_hir(
                        SAFE_PACKED_BORROWS,
                        lint_hir_id,
                        source_info.span,
                        |lint| {
                            lint.build(&format!(
                                "{} is unsafe and requires unsafe function or block (error E0133)",
                                description
                            ))
                            .note(&details.as_str())
                            .emit()
                        },
                    )
                }
            }
        }
    }

    let mut unsafe_blocks: Vec<_> = unsafe_blocks.iter().collect();
    unsafe_blocks.sort_by_cached_key(|(hir_id, _)| tcx.hir().hir_to_node_id(*hir_id));
    let used_unsafe: FxHashSet<_> =
        unsafe_blocks.iter().flat_map(|&&(id, used)| used.then_some(id)).collect();
    for &(block_id, is_used) in unsafe_blocks {
        if !is_used {
            report_unused_unsafe(tcx, &used_unsafe, block_id);
        }
    }
}