intptrcast.rs

use std::cell::RefCell;
use std::cmp::max;
use std::collections::hash_map::Entry;

use log::trace;
use rand::Rng;

use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_span::Span;
use rustc_target::abi::{HasDataLayout, Size};

use crate::*;

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum ProvenanceMode {
    /// We support `expose_addr`/`from_exposed_addr` via "wildcard" provenance.
    /// However, we want on `from_exposed_addr` to alert the user of the precision loss.
    Default,
    /// Like `Default`, but without the warning.
    Permissive,
    /// We error on `from_exposed_addr`, ensuring no precision loss.
    Strict,
}

pub type GlobalState = RefCell<GlobalStateInner>;

#[derive(Clone, Debug)]
pub struct GlobalStateInner {
    /// This is used as a map between the address of each allocation and its `AllocId`. It is always
    /// sorted by address. We cannot use a `HashMap` since we can be given an address that is offset
    /// from the base address, and we need to find the `AllocId` it belongs to. This is not the
    /// *full* inverse of `base_addr`; dead allocations have been removed.
    int_to_ptr_map: Vec<(u64, AllocId)>,
    /// The base address for each allocation.  We cannot put that into
    /// `AllocExtra` because function pointers also have a base address, and
    /// they do not have an `AllocExtra`.
    /// This is the inverse of `int_to_ptr_map`.
    base_addr: FxHashMap<AllocId, u64>,
    /// Whether an allocation has been exposed or not. This cannot be put
    /// into `AllocExtra` for the same reason as `base_addr`.
    exposed: FxHashSet<AllocId>,
    /// This is used as a memory address when a new pointer is casted to an integer. It
    /// is always larger than any address that was previously made part of a block.
    next_base_addr: u64,
    /// The provenance to use for int2ptr casts
    provenance_mode: ProvenanceMode,
}

impl VisitProvenance for GlobalStateInner {
    fn visit_provenance(&self, _visit: &mut VisitWith<'_>) {
        let GlobalStateInner {
            int_to_ptr_map: _,
            base_addr: _,
            exposed: _,
            next_base_addr: _,
            provenance_mode: _,
        } = self;
        // Though base_addr, int_to_ptr_map, and exposed contain AllocIds, we do not want to visit them.
        // int_to_ptr_map and exposed must contain only live allocations, and those were already handled
        // by visiting all AllocIds in our Machine::MemoryMap.
        // base_addr is only relevant if we have a pointer to an AllocId and need to look up its
        // base address; so if an AllocId is not reachable from somewhere else we can remove it
        // here.
    }
}

impl GlobalStateInner {
    pub fn new(config: &MiriConfig, stack_addr: u64) -> Self {
        GlobalStateInner {
            int_to_ptr_map: Vec::default(),
            base_addr: FxHashMap::default(),
            exposed: FxHashSet::default(),
            next_base_addr: stack_addr,
            provenance_mode: config.provenance_mode,
        }
    }

    pub fn remove_unreachable_allocs(&mut self, reachable_allocs: &FxHashSet<AllocId>) {
        // `exposed` and `int_to_ptr_map` are cleared immediately when an allocation
        // is freed, so `base_addr` is the only one we have to clean up based on the GC.
        self.base_addr.retain(|id, _| reachable_allocs.contains(id));
    }
}

/// Shifts `addr` to make it aligned with `align` by rounding `addr` to the smallest multiple
/// of `align` that is larger or equal to `addr`
fn align_addr(addr: u64, align: u64) -> u64 {
    match addr % align {
        0 => addr,
        rem => addr.checked_add(align).unwrap() - rem,
    }
}

impl<'mir, 'tcx: 'mir> EvalContextExtPriv<'mir, 'tcx> for crate::MiriInterpCx<'mir, 'tcx> {}
trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
    // Returns the exposed `AllocId` that corresponds to the specified addr,
    // or `None` if the addr is out of bounds
    fn alloc_id_from_addr(&self, addr: u64) -> Option<AllocId> {
        let ecx = self.eval_context_ref();
        let global_state = ecx.machine.intptrcast.borrow();
        assert!(global_state.provenance_mode != ProvenanceMode::Strict);

        let pos = global_state.int_to_ptr_map.binary_search_by_key(&addr, |(addr, _)| *addr);

        // Determine the in-bounds provenance for this pointer.
        // (This is only called on an actual access, so in-bounds is the only possible kind of provenance.)
        let alloc_id = match pos {
            Ok(pos) => Some(global_state.int_to_ptr_map[pos].1),
            Err(0) => None,
            Err(pos) => {
                // This is the largest of the addresses smaller than `int`,
                // i.e. the greatest lower bound (glb)
                let (glb, alloc_id) = global_state.int_to_ptr_map[pos - 1];
                // This never overflows because `addr >= glb`
                let offset = addr - glb;
                // We require this to be strict in-bounds of the allocation. This arm is only
                // entered for addresses that are not the base address, so even zero-sized
                // allocations will get recognized at their base address -- but all other
                // allocations will *not* be recognized at their "end" address.
                let size = ecx.get_alloc_info(alloc_id).0;
                if offset < size.bytes() { Some(alloc_id) } else { None }
            }
        }?;

        // We only use this provenance if it has been exposed.
        if global_state.exposed.contains(&alloc_id) {
            // This must still be live, since we remove allocations from `int_to_ptr_map` when they get freed.
            debug_assert!(!matches!(ecx.get_alloc_info(alloc_id).2, AllocKind::Dead));
            Some(alloc_id)
        } else {
            None
        }
    }

    fn addr_from_alloc_id(&self, alloc_id: AllocId) -> InterpResult<'tcx, u64> {
        let ecx = self.eval_context_ref();
        let mut global_state = ecx.machine.intptrcast.borrow_mut();
        let global_state = &mut *global_state;

        Ok(match global_state.base_addr.entry(alloc_id) {
            Entry::Occupied(entry) => *entry.get(),
            Entry::Vacant(entry) => {
                let (size, align, kind) = ecx.get_alloc_info(alloc_id);
                // This is either called immediately after allocation (and then cached), or when
                // adjusting `tcx` pointers (which never get freed). So assert that we are looking
                // at a live allocation. This also ensures that we never re-assign an address to an
                // allocation that previously had an address, but then was freed and the address
                // information was removed.
                assert!(!matches!(kind, AllocKind::Dead));

                // This allocation does not have a base address yet, pick one.
                // Leave some space to the previous allocation, to give it some chance to be less aligned.
                let slack = {
                    let mut rng = ecx.machine.rng.borrow_mut();
                    // This means that `(global_state.next_base_addr + slack) % 16` is uniformly distributed.
                    rng.gen_range(0..16)
                };
                // From next_base_addr + slack, round up to adjust for alignment.
                let base_addr = global_state
                    .next_base_addr
                    .checked_add(slack)
                    .ok_or_else(|| err_exhaust!(AddressSpaceFull))?;
                let base_addr = align_addr(base_addr, align.bytes());
                entry.insert(base_addr);
                trace!(
                    "Assigning base address {:#x} to allocation {:?} (size: {}, align: {}, slack: {})",
                    base_addr,
                    alloc_id,
                    size.bytes(),
                    align.bytes(),
                    slack,
                );

                // Remember next base address.  If this allocation is zero-sized, leave a gap
                // of at least 1 to avoid two allocations having the same base address.
                // (The logic in `alloc_id_from_addr` assumes unique addresses, and different
                // function/vtable pointers need to be distinguishable!)
                global_state.next_base_addr = base_addr
                    .checked_add(max(size.bytes(), 1))
                    .ok_or_else(|| err_exhaust!(AddressSpaceFull))?;
                // Even if `Size` didn't overflow, we might still have filled up the address space.
                if global_state.next_base_addr > ecx.target_usize_max() {
                    throw_exhaust!(AddressSpaceFull);
                }
                // Also maintain the opposite mapping in `int_to_ptr_map`.
                // Given that `next_base_addr` increases in each allocation, pushing the
                // corresponding tuple keeps `int_to_ptr_map` sorted
                global_state.int_to_ptr_map.push((base_addr, alloc_id));

                base_addr
            }
        })
    }
}

impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriInterpCx<'mir, 'tcx> {}
pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
    fn expose_ptr(&mut self, alloc_id: AllocId, tag: BorTag) -> InterpResult<'tcx> {
        let ecx = self.eval_context_mut();
        let global_state = ecx.machine.intptrcast.get_mut();
        // In strict mode, we don't need this, so we can save some cycles by not tracking it.
        if global_state.provenance_mode == ProvenanceMode::Strict {
            return Ok(());
        }
        // Exposing a dead alloc is a no-op, because it's not possible to get a dead allocation
        // via int2ptr.
        if matches!(ecx.get_alloc_info(alloc_id).2, AllocKind::Dead) {
            return Ok(());
        }
        trace!("Exposing allocation id {alloc_id:?}");
        let ecx = self.eval_context_mut();
        let global_state = ecx.machine.intptrcast.get_mut();
        global_state.exposed.insert(alloc_id);
        if ecx.machine.borrow_tracker.is_some() {
            ecx.expose_tag(alloc_id, tag)?;
        }
        Ok(())
    }

    fn ptr_from_addr_cast(&self, addr: u64) -> InterpResult<'tcx, Pointer<Option<Provenance>>> {
        trace!("Casting {:#x} to a pointer", addr);

        let ecx = self.eval_context_ref();
        let global_state = ecx.machine.intptrcast.borrow();

        // Potentially emit a warning.
        match global_state.provenance_mode {
            ProvenanceMode::Default => {
                // The first time this happens at a particular location, print a warning.
                thread_local! {
                    // `Span` is non-`Send`, so we use a thread-local instead.
                    static PAST_WARNINGS: RefCell<FxHashSet<Span>> = RefCell::default();
                }
                PAST_WARNINGS.with_borrow_mut(|past_warnings| {
                    let first = past_warnings.is_empty();
                    if past_warnings.insert(ecx.cur_span()) {
                        // Newly inserted, so first time we see this span.
                        ecx.emit_diagnostic(NonHaltingDiagnostic::Int2Ptr { details: first });
                    }
                });
            }
            ProvenanceMode::Strict => {
                throw_machine_stop!(TerminationInfo::Int2PtrWithStrictProvenance);
            }
            ProvenanceMode::Permissive => {}
        }

        // We do *not* look up the `AllocId` here! This is a `ptr as usize` cast, and it is
        // completely legal to do a cast and then `wrapping_offset` to another allocation and only
        // *then* do a memory access. So the allocation that the pointer happens to point to on a
        // cast is fairly irrelevant. Instead we generate this as a "wildcard" pointer, such that
        // *every time the pointer is used*, we do an `AllocId` lookup to find the (exposed)
        // allocation it might be referencing.
        Ok(Pointer::new(Some(Provenance::Wildcard), Size::from_bytes(addr)))
    }

    /// Convert a relative (tcx) pointer to a Miri pointer.
    fn ptr_from_rel_ptr(
        &self,
        ptr: Pointer<AllocId>,
        tag: BorTag,
    ) -> InterpResult<'tcx, Pointer<Provenance>> {
        let ecx = self.eval_context_ref();

        let (alloc_id, offset) = ptr.into_parts(); // offset is relative (AllocId provenance)
        let base_addr = ecx.addr_from_alloc_id(alloc_id)?;

        // Add offset with the right kind of pointer-overflowing arithmetic.
        let dl = ecx.data_layout();
        let absolute_addr = dl.overflowing_offset(base_addr, offset.bytes()).0;
        Ok(Pointer::new(Provenance::Concrete { alloc_id, tag }, Size::from_bytes(absolute_addr)))
    }

    /// When a pointer is used for a memory access, this computes where in which allocation the
    /// access is going.
    fn ptr_get_alloc(&self, ptr: Pointer<Provenance>) -> Option<(AllocId, Size)> {
        let ecx = self.eval_context_ref();

        let (tag, addr) = ptr.into_parts(); // addr is absolute (Tag provenance)

        let alloc_id = if let Provenance::Concrete { alloc_id, .. } = tag {
            alloc_id
        } else {
            // A wildcard pointer.
            ecx.alloc_id_from_addr(addr.bytes())?
        };

        // This cannot fail: since we already have a pointer with that provenance, rel_ptr_to_addr
        // must have been called in the past, so we can just look up the address in the map.
        let base_addr = ecx.addr_from_alloc_id(alloc_id).unwrap();

        // Wrapping "addr - base_addr"
        #[allow(clippy::cast_possible_wrap)] // we want to wrap here
        let neg_base_addr = (base_addr as i64).wrapping_neg();
        Some((
            alloc_id,
            Size::from_bytes(ecx.overflowing_signed_offset(addr.bytes(), neg_base_addr).0),
        ))
    }
}

impl GlobalStateInner {
    pub fn free_alloc_id(&mut self, dead_id: AllocId) {
        // We can *not* remove this from `base_addr`, since the interpreter design requires that we
        // be able to retrieve an AllocId + offset for any memory access *before* we check if the
        // access is valid. Specifically, `ptr_get_alloc` is called on each attempt at a memory
        // access to determine the allocation ID and offset -- and there can still be pointers with
        // `dead_id` that one can attempt to use for a memory access. `ptr_get_alloc` may return
        // `None` only if the pointer truly has no provenance (this ensures consistent error
        // messages).
        // However, we *can* remove it from `int_to_ptr_map`, since any wildcard pointers that exist
        // can no longer actually be accessing that address. This ensures `alloc_id_from_addr` never
        // returns a dead allocation.
        // To avoid a linear scan we first look up the address in `base_addr`, and then find it in
        // `int_to_ptr_map`.
        let addr = *self.base_addr.get(&dead_id).unwrap();
        let pos = self.int_to_ptr_map.binary_search_by_key(&addr, |(addr, _)| *addr).unwrap();
        let removed = self.int_to_ptr_map.remove(pos);
        assert_eq!(removed, (addr, dead_id)); // double-check that we removed the right thing
        // We can also remove it from `exposed`, since this allocation can anyway not be returned by
        // `alloc_id_from_addr` any more.
        self.exposed.remove(&dead_id);
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_align_addr() {
        assert_eq!(align_addr(37, 4), 40);
        assert_eq!(align_addr(44, 4), 44);
    }
}