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Refactor PointerFinder into a separate module

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This also parameterize the "excluded pointee types" and exposes a
general method for inserting checks on pointers.

This is a preparation for adding a NullCheck that makes use of the same
code.
This commit is contained in:
Bastian Kersting 2024-12-16 14:42:49 +00:00
parent e6f12c8b7d
commit 851322b74d
3 changed files with 271 additions and 159 deletions
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196
compiler/rustc_mir_transform/src/check_alignment.rs
View file

@ -1,11 +1,10 @@
use rustc_hir::lang_items::LangItem;
use rustc_index::IndexVec;
use rustc_middle::mir::interpret::Scalar;
use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::ty::{Ty, TyCtxt};
use rustc_session::Session;
use tracing::{debug, trace};
use crate::check_pointers::{BorrowCheckMode, PointerCheck, check_pointers};
pub(super) struct CheckAlignment;
@ -19,46 +18,19 @@ impl<'tcx> crate::MirPass<'tcx> for CheckAlignment {
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
// This pass emits new panics. If for whatever reason we do not have a panic
// implementation, running this pass may cause otherwise-valid code to not compile.
if tcx.lang_items().get(LangItem::PanicImpl).is_none() {
return;
}
// Skip trivially aligned place types.
let excluded_pointees = [tcx.types.bool, tcx.types.i8, tcx.types.u8];
let typing_env = body.typing_env(tcx);
let basic_blocks = body.basic_blocks.as_mut();
let local_decls = &mut body.local_decls;
// This pass inserts new blocks. Each insertion changes the Location for all
// statements/blocks after. Iterating or visiting the MIR in order would require updating
// our current location after every insertion. By iterating backwards, we dodge this issue:
// The only Locations that an insertion changes have already been handled.
for block in (0..basic_blocks.len()).rev() {
let block = block.into();
for statement_index in (0..basic_blocks[block].statements.len()).rev() {
let location = Location { block, statement_index };
let statement = &basic_blocks[block].statements[statement_index];
let source_info = statement.source_info;
let mut finder =
PointerFinder { tcx, local_decls, typing_env, pointers: Vec::new() };
finder.visit_statement(statement, location);
for (local, ty) in finder.pointers {
debug!("Inserting alignment check for {:?}", ty);
let new_block = split_block(basic_blocks, location);
insert_alignment_check(
tcx,
local_decls,
&mut basic_blocks[block],
local,
ty,
source_info,
new_block,
);
}
}
}
// We have to exclude borrows here: in `&x.field`, the exact
// requirement is that the final reference must be aligned, but
// `check_pointers` would check that `x` is aligned, which would be wrong.
check_pointers(
tcx,
body,
&excluded_pointees,
insert_alignment_check,
BorrowCheckMode::ExcludeBorrows,
);
}
fn is_required(&self) -> bool {
@ -66,119 +38,33 @@ impl<'tcx> crate::MirPass<'tcx> for CheckAlignment {
}
}
struct PointerFinder<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
local_decls: &'a mut LocalDecls<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
pointers: Vec<(Place<'tcx>, Ty<'tcx>)>,
}
impl<'a, 'tcx> Visitor<'tcx> for PointerFinder<'a, 'tcx> {
fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, location: Location) {
// We want to only check reads and writes to Places, so we specifically exclude
// Borrow and RawBorrow.
match context {
PlaceContext::MutatingUse(
MutatingUseContext::Store
| MutatingUseContext::AsmOutput
| MutatingUseContext::Call
| MutatingUseContext::Yield
| MutatingUseContext::Drop,
) => {}
PlaceContext::NonMutatingUse(
NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
) => {}
_ => {
return;
}
}
if !place.is_indirect() {
return;
}
// Since Deref projections must come first and only once, the pointer for an indirect place
// is the Local that the Place is based on.
let pointer = Place::from(place.local);
let pointer_ty = self.local_decls[place.local].ty;
// We only want to check places based on unsafe pointers
if !pointer_ty.is_unsafe_ptr() {
trace!("Indirect, but not based on an unsafe ptr, not checking {:?}", place);
return;
}
let pointee_ty =
pointer_ty.builtin_deref(true).expect("no builtin_deref for an unsafe pointer");
// Ideally we'd support this in the future, but for now we are limited to sized types.
if !pointee_ty.is_sized(self.tcx, self.typing_env) {
debug!("Unsafe pointer, but pointee is not known to be sized: {:?}", pointer_ty);
return;
}
// Try to detect types we are sure have an alignment of 1 and skip the check
// We don't need to look for str and slices, we already rejected unsized types above
let element_ty = match pointee_ty.kind() {
ty::Array(ty, _) => *ty,
_ => pointee_ty,
};
if [self.tcx.types.bool, self.tcx.types.i8, self.tcx.types.u8].contains(&element_ty) {
debug!("Trivially aligned place type: {:?}", pointee_ty);
return;
}
// Ensure that this place is based on an aligned pointer.
self.pointers.push((pointer, pointee_ty));
self.super_place(place, context, location);
}
}
fn split_block(
basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>,
location: Location,
) -> BasicBlock {
let block_data = &mut basic_blocks[location.block];
// Drain every statement after this one and move the current terminator to a new basic block
let new_block = BasicBlockData {
statements: block_data.statements.split_off(location.statement_index),
terminator: block_data.terminator.take(),
is_cleanup: block_data.is_cleanup,
};
basic_blocks.push(new_block)
}
/// Inserts the actual alignment check's logic. Returns a
/// [AssertKind::MisalignedPointerDereference] on failure.
fn insert_alignment_check<'tcx>(
tcx: TyCtxt<'tcx>,
local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
block_data: &mut BasicBlockData<'tcx>,
pointer: Place<'tcx>,
pointee_ty: Ty<'tcx>,
local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
stmts: &mut Vec<Statement<'tcx>>,
source_info: SourceInfo,
new_block: BasicBlock,
) {
// Cast the pointer to a *const ()
) -> PointerCheck<'tcx> {
// Cast the pointer to a *const ().
let const_raw_ptr = Ty::new_imm_ptr(tcx, tcx.types.unit);
let rvalue = Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(pointer), const_raw_ptr);
let thin_ptr = local_decls.push(LocalDecl::with_source_info(const_raw_ptr, source_info)).into();
block_data
.statements
stmts
.push(Statement { source_info, kind: StatementKind::Assign(Box::new((thin_ptr, rvalue))) });
// Transmute the pointer to a usize (equivalent to `ptr.addr()`)
// Transmute the pointer to a usize (equivalent to `ptr.addr()`).
let rvalue = Rvalue::Cast(CastKind::Transmute, Operand::Copy(thin_ptr), tcx.types.usize);
let addr = local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
block_data
.statements
.push(Statement { source_info, kind: StatementKind::Assign(Box::new((addr, rvalue))) });
stmts.push(Statement { source_info, kind: StatementKind::Assign(Box::new((addr, rvalue))) });
// Get the alignment of the pointee
let alignment =
local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
let rvalue = Rvalue::NullaryOp(NullOp::AlignOf, pointee_ty);
block_data.statements.push(Statement {
stmts.push(Statement {
source_info,
kind: StatementKind::Assign(Box::new((alignment, rvalue))),
});
@ -191,7 +77,7 @@ fn insert_alignment_check<'tcx>(
user_ty: None,
const_: Const::Val(ConstValue::Scalar(Scalar::from_target_usize(1, &tcx)), tcx.types.usize),
}));
block_data.statements.push(Statement {
stmts.push(Statement {
source_info,
kind: StatementKind::Assign(Box::new((
alignment_mask,
@ -202,7 +88,7 @@ fn insert_alignment_check<'tcx>(
// BitAnd the alignment mask with the pointer
let alignment_bits =
local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
block_data.statements.push(Statement {
stmts.push(Statement {
source_info,
kind: StatementKind::Assign(Box::new((
alignment_bits,
@ -220,7 +106,7 @@ fn insert_alignment_check<'tcx>(
user_ty: None,
const_: Const::Val(ConstValue::Scalar(Scalar::from_target_usize(0, &tcx)), tcx.types.usize),
}));
block_data.statements.push(Statement {
stmts.push(Statement {
source_info,
kind: StatementKind::Assign(Box::new((
is_ok,
@ -228,21 +114,13 @@ fn insert_alignment_check<'tcx>(
))),
});
// Set this block's terminator to our assert, continuing to new_block if we pass
block_data.terminator = Some(Terminator {
source_info,
kind: TerminatorKind::Assert {
cond: Operand::Copy(is_ok),
expected: true,
target: new_block,
msg: Box::new(AssertKind::MisalignedPointerDereference {
required: Operand::Copy(alignment),
found: Operand::Copy(addr),
}),
// This calls panic_misaligned_pointer_dereference, which is #[rustc_nounwind].
// We never want to insert an unwind into unsafe code, because unwinding could
// make a failing UB check turn into much worse UB when we start unwinding.
unwind: UnwindAction::Unreachable,
},
});
// Emit a check that asserts on the alignment and otherwise triggers a
// AssertKind::MisalignedPointerDereference.
PointerCheck {
cond: Operand::Copy(is_ok),
assert_kind: Box::new(AssertKind::MisalignedPointerDereference {
required: Operand::Copy(alignment),
found: Operand::Copy(addr),
}),
}
}

233
compiler/rustc_mir_transform/src/check_pointers.rs Normal file
View file

@ -0,0 +1,233 @@
use rustc_hir::lang_items::LangItem;
use rustc_index::IndexVec;
use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::{self, Ty, TyCtxt};
use tracing::{debug, trace};
/// Details of a pointer check, the condition on which we decide whether to
/// fail the assert and an [AssertKind] that defines the behavior on failure.
pub(crate) struct PointerCheck<'tcx> {
pub(crate) cond: Operand<'tcx>,
pub(crate) assert_kind: Box<AssertKind<Operand<'tcx>>>,
}
/// Indicates whether we insert the checks for borrow places of a raw pointer.
/// Concretely places with [MutatingUseContext::Borrow] or
/// [NonMutatingUseContext::SharedBorrow].
#[derive(Copy, Clone)]
pub(crate) enum BorrowCheckMode {
ExcludeBorrows,
}
/// Utility for adding a check for read/write on every sized, raw pointer.
///
/// Visits every read/write access to a [Sized], raw pointer and inserts a
/// new basic block directly before the pointer access. (Read/write accesses
/// are determined by the `PlaceContext` of the MIR visitor.) Then calls
/// `on_finding` to insert the actual logic for a pointer check (e.g. check for
/// alignment). A check can choose to be inserted for (mutable) borrows of
/// raw pointers via the `borrow_check_mode` parameter.
///
/// This utility takes care of the right order of blocks, the only thing a
/// caller must do in `on_finding` is:
/// - Append [Statement]s to `stmts`.
/// - Append [LocalDecl]s to `local_decls`.
/// - Return a [PointerCheck] that contains the condition and an [AssertKind].
/// The AssertKind must be a panic with `#[rustc_nounwind]`. The condition
/// should always return the boolean `is_ok`, so evaluate to true in case of
/// success and fail the check otherwise.
/// This utility will insert a terminator block that asserts on the condition
/// and panics on failure.
pub(crate) fn check_pointers<'a, 'tcx, F>(
tcx: TyCtxt<'tcx>,
body: &mut Body<'tcx>,
excluded_pointees: &'a [Ty<'tcx>],
on_finding: F,
borrow_check_mode: BorrowCheckMode,
) where
F: Fn(
/* tcx: */ TyCtxt<'tcx>,
/* pointer: */ Place<'tcx>,
/* pointee_ty: */ Ty<'tcx>,
/* local_decls: */ &mut IndexVec<Local, LocalDecl<'tcx>>,
/* stmts: */ &mut Vec<Statement<'tcx>>,
/* source_info: */ SourceInfo,
) -> PointerCheck<'tcx>,
{
// This pass emits new panics. If for whatever reason we do not have a panic
// implementation, running this pass may cause otherwise-valid code to not compile.
if tcx.lang_items().get(LangItem::PanicImpl).is_none() {
return;
}
let typing_env = body.typing_env(tcx);
let basic_blocks = body.basic_blocks.as_mut();
let local_decls = &mut body.local_decls;
// This operation inserts new blocks. Each insertion changes the Location for all
// statements/blocks after. Iterating or visiting the MIR in order would require updating
// our current location after every insertion. By iterating backwards, we dodge this issue:
// The only Locations that an insertion changes have already been handled.
for block in (0..basic_blocks.len()).rev() {
let block = block.into();
for statement_index in (0..basic_blocks[block].statements.len()).rev() {
let location = Location { block, statement_index };
let statement = &basic_blocks[block].statements[statement_index];
let source_info = statement.source_info;
let mut finder = PointerFinder::new(
tcx,
local_decls,
typing_env,
excluded_pointees,
borrow_check_mode,
);
finder.visit_statement(statement, location);
for (local, ty) in finder.into_found_pointers() {
debug!("Inserting check for {:?}", ty);
let new_block = split_block(basic_blocks, location);
// Invoke `on_finding` which appends to `local_decls` and the
// blocks statements. It returns information about the assert
// we're performing in the Terminator.
let block_data = &mut basic_blocks[block];
let pointer_check = on_finding(
tcx,
local,
ty,
local_decls,
&mut block_data.statements,
source_info,
);
block_data.terminator = Some(Terminator {
source_info,
kind: TerminatorKind::Assert {
cond: pointer_check.cond,
expected: true,
target: new_block,
msg: pointer_check.assert_kind,
// This calls a panic function associated with the pointer check, which
// is #[rustc_nounwind]. We never want to insert an unwind into unsafe
// code, because unwinding could make a failing UB check turn into much
// worse UB when we start unwinding.
unwind: UnwindAction::Unreachable,
},
});
}
}
}
}
struct PointerFinder<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
local_decls: &'a mut LocalDecls<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
pointers: Vec<(Place<'tcx>, Ty<'tcx>)>,
excluded_pointees: &'a [Ty<'tcx>],
borrow_check_mode: BorrowCheckMode,
}
impl<'a, 'tcx> PointerFinder<'a, 'tcx> {
fn new(
tcx: TyCtxt<'tcx>,
local_decls: &'a mut LocalDecls<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
excluded_pointees: &'a [Ty<'tcx>],
borrow_check_mode: BorrowCheckMode,
) -> Self {
PointerFinder {
tcx,
local_decls,
typing_env,
excluded_pointees,
pointers: Vec::new(),
borrow_check_mode,
}
}
fn into_found_pointers(self) -> Vec<(Place<'tcx>, Ty<'tcx>)> {
self.pointers
}
/// Whether or not we should visit a [Place] with [PlaceContext].
///
/// We generally only visit Reads/Writes to a place and only Borrows if
/// requested.
fn should_visit_place(&self, context: PlaceContext) -> bool {
match context {
PlaceContext::MutatingUse(
MutatingUseContext::Store
| MutatingUseContext::Call
| MutatingUseContext::Yield
| MutatingUseContext::Drop,
) => true,
PlaceContext::NonMutatingUse(
NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
) => true,
PlaceContext::MutatingUse(MutatingUseContext::Borrow)
| PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) => {
!matches!(self.borrow_check_mode, BorrowCheckMode::ExcludeBorrows)
}
_ => false,
}
}
}
impl<'a, 'tcx> Visitor<'tcx> for PointerFinder<'a, 'tcx> {
fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, location: Location) {
if !self.should_visit_place(context) || !place.is_indirect() {
return;
}
// Since Deref projections must come first and only once, the pointer for an indirect place
// is the Local that the Place is based on.
let pointer = Place::from(place.local);
let pointer_ty = self.local_decls[place.local].ty;
// We only want to check places based on raw pointers
if !pointer_ty.is_unsafe_ptr() {
trace!("Indirect, but not based on an raw ptr, not checking {:?}", place);
return;
}
let pointee_ty =
pointer_ty.builtin_deref(true).expect("no builtin_deref for an raw pointer");
// Ideally we'd support this in the future, but for now we are limited to sized types.
if !pointee_ty.is_sized(self.tcx, self.typing_env) {
trace!("Raw pointer, but pointee is not known to be sized: {:?}", pointer_ty);
return;
}
// We don't need to look for slices, we already rejected unsized types above.
let element_ty = match pointee_ty.kind() {
ty::Array(ty, _) => *ty,
_ => pointee_ty,
};
if self.excluded_pointees.contains(&element_ty) {
trace!("Skipping pointer for type: {:?}", pointee_ty);
return;
}
self.pointers.push((pointer, pointee_ty));
self.super_place(place, context, location);
}
}
fn split_block(
basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>,
location: Location,
) -> BasicBlock {
let block_data = &mut basic_blocks[location.block];
// Drain every statement after this one and move the current terminator to a new basic block.
let new_block = BasicBlockData {
statements: block_data.statements.split_off(location.statement_index),
terminator: block_data.terminator.take(),
is_cleanup: block_data.is_cleanup,
};
basic_blocks.push(new_block)
}

1
compiler/rustc_mir_transform/src/lib.rs
View file

@ -44,6 +44,7 @@ use std::sync::LazyLock;
use pass_manager::{self as pm, Lint, MirLint, MirPass, WithMinOptLevel};
mod check_pointers;
mod cost_checker;
mod cross_crate_inline;
mod deduce_param_attrs;