hopper_runtime/

foreign.rs

//! Manifest-backed foreign-account lenses.
//!
//! The Hopper Safety Audit (page 14, "Manifest-backed foreign account
//! lenses") proposed a verifiable cross-program read API as the next
//! step beyond ad-hoc offset-based foreign reads. This module
//! implements it.
//!
//! # Problem
//!
//! Today, reading a field from an account owned by a *different* program
//! either imports the foreign program's crate (tight coupling, forces
//! version-lock) or reads raw bytes by hand-maintained offset
//! (no ABI-drift detection. if the foreign program changes its layout,
//! silent misreads result).
//!
//! # Design
//!
//! A `ForeignManifest` is an opaque witness (supplied by the caller)
//! that carries the foreign program's `wire_fp64` hash plus the layout
//! discriminator it expects for a particular `T: AccountLayout`. When
//! `ctx.foreign::<T>(idx, &manifest)?` is called:
//!
//! 1. The account's owner must match `manifest.program_id`
//! 2. The account's header discriminator must match `T::DISC` and
//!    `manifest.expected_disc`
//! 3. The header's `wire_fp64` must match `T::WIRE_FINGERPRINT` and
//!    `manifest.expected_wire_fp`
//! 4. `schema_epoch` must fall in `manifest.supported_epochs`
//!
//! Only after all four pass does the lens expose field access. Any
//! mismatch returns `ProgramError::InvalidAccountData`. never silent
//! mis-reads, never UB.
//!
//! # Manifest sourcing
//!
//! Hopper does not fetch manifests from RPC inside a program (that
//! would be round-trip CPI with no caching story). Manifests are
//! caller-supplied, typically from:
//!
//! - An embedded `const ForeignManifest` authored when the program was
//!   built (works when the foreign program's ABI is known at build time)
//! - A manifest account located at the canonical manifest PDA
//!   (`find_program_address(&[MANIFEST_SEED], &foreign_program_id)`)
//!   whose payload has already been verified by a prior instruction
//! - A Hopper-authored IDL that emits manifest constants as part of
//!   its client-generation output

use crate::account::AccountView;
use crate::address::Address;
use crate::borrow::Ref;
use crate::error::ProgramError;
use crate::layout::{HopperHeader, LayoutContract};
use crate::zerocopy::{AccountLayout, ZeroCopy};

/// Opaque witness to a foreign program's layout ABI.
///
/// Callers construct this once per foreign program they want to read
/// from, typically as a `const` from build-time-embedded metadata or
/// from the foreign program's Hopper manifest account.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct ForeignManifest {
    /// Owner program that authored the layout. The account's owner
    /// must match this address exactly.
    pub program_id: Address,
    /// Discriminator byte the foreign layout expects.
    pub expected_disc: u8,
    /// Canonical wire-fingerprint hash from the foreign program's
    /// schema manifest. Matches `AccountLayout::WIRE_FINGERPRINT` on
    /// the reader side.
    pub expected_wire_fp: u64,
    /// Inclusive range of `schema_epoch` values the reader supports.
    /// Accounts outside this range fail verification. the caller can
    /// then fall back to a migration path or a different manifest.
    pub supported_epochs: core::ops::RangeInclusive<u32>,
}

impl ForeignManifest {
    /// Build a single-epoch manifest covering `expected_wire_fp` for
    /// `program_id` at exactly the given schema epoch.
    pub const fn single_epoch(
        program_id: Address,
        expected_disc: u8,
        expected_wire_fp: u64,
        epoch: u32,
    ) -> Self {
        Self {
            program_id,
            expected_disc,
            expected_wire_fp,
            supported_epochs: epoch..=epoch,
        }
    }
}

/// A verified read-only handle into a foreign account.
///
/// `ForeignLens<'a, T>` borrows the underlying account data for its
/// lifetime. Field access (`.get()`, `.field::<F, OFFSET>()`) performs
/// only pointer arithmetic. no further verification, because all
/// cross-program invariants were pinned at construction.
pub struct ForeignLens<'a, T: AccountLayout + LayoutContract> {
    inner: Ref<'a, T>,
}

impl<'a, T: AccountLayout + LayoutContract> ForeignLens<'a, T> {
    /// Verify a foreign account against the supplied manifest and, on
    /// success, return a read-only lens into its body.
    ///
    /// The four verification steps correspond one-to-one with the
    /// audit's page-14 requirements:
    ///
    /// 1. owner match
    /// 2. discriminator match (both `T::DISC` *and* `manifest.expected_disc`)
    /// 3. wire-fingerprint match
    /// 4. schema_epoch in supported range
    #[inline]
    pub fn open(
        account: &'a AccountView,
        manifest: &ForeignManifest,
    ) -> Result<Self, ProgramError> {
        // 1. Owner match. `check_owned_by` compares address bytes.
        account.check_owned_by(&manifest.program_id)?;

        // 2–4. Header inspection. must happen behind a byte borrow
        //     so the data can't mutate underneath us. We use the same
        //     load path authored accounts use, which verifies the
        //     discriminator too. That closes #2.
        let loaded: Ref<'a, T> = account.load::<T>()?;
        if <T as AccountLayout>::DISC != manifest.expected_disc {
            return Err(ProgramError::InvalidAccountData);
        }

        // Re-read the header bytes directly so we can match the
        // manifest's wire-fingerprint and epoch fields. The load
        // above already verified disc/version, so this step only
        // checks the manifest-specific fields. HopperHeader is
        // `#[repr(C, packed)]` at 16 bytes. `from_bytes` returns a
        // properly bounds-checked reference without touching unaligned
        // primitives (we copy packed fields out by value below).
        let data = account.try_borrow()?;
        let header = HopperHeader::from_bytes(&data)
            .ok_or(ProgramError::AccountDataTooSmall)?;
        // Packed-field reads must go through a local copy.
        let layout_id = header.layout_id;
        let schema_epoch = header.schema_epoch;
        let actual_wire_fp = u64::from_le_bytes(layout_id);
        if actual_wire_fp != manifest.expected_wire_fp {
            return Err(ProgramError::InvalidAccountData);
        }
        if actual_wire_fp != <T as AccountLayout>::WIRE_FINGERPRINT {
            return Err(ProgramError::InvalidAccountData);
        }
        if !manifest.supported_epochs.contains(&schema_epoch) {
            return Err(ProgramError::InvalidAccountData);
        }

        // Explicit drop so the re-borrow guard releases before we
        // hand out `loaded`, which already pins its own guard.
        drop(data);

        Ok(Self { inner: loaded })
    }

    /// The full verified layout. Field access through this path is
    /// zero-cost; no further checks fire.
    #[inline(always)]
    pub fn get(&self) -> &T {
        &self.inner
    }

    /// Project a typed field by byte offset. Returns a pointer-cast
    /// reference with the lens's lifetime.
    ///
    /// `OFFSET` must be the field's offset *within the layout body*
    /// (i.e. already past the 16-byte Hopper header). Callers should
    /// prefer the auto-emitted `{FIELD}_OFFSET` constants from
    /// `#[hopper::state]`.
    #[inline(always)]
    pub fn field<F: ZeroCopy, const OFFSET: usize>(&self) -> Result<&F, ProgramError> {
        let body_size = core::mem::size_of::<T>();
        let field_size = core::mem::size_of::<F>();
        if OFFSET.checked_add(field_size).map(|end| end > body_size).unwrap_or(true) {
            return Err(ProgramError::AccountDataTooSmall);
        }
        // SAFETY: We checked the byte range lies entirely inside the
        // body. The layout is `Pod` (from `T: AccountLayout: ZeroCopy`),
        // so every byte pattern is valid for `F: ZeroCopy`. The
        // returned reference inherits the lens's lifetime and thus
        // cannot outlive the underlying borrow guard.
        // `Ref<T>` derefs to `T`; take the address via `&*`.
        let layout_ref: &T = &*self.inner;
        unsafe {
            let base = layout_ref as *const T as *const u8;
            let field_ptr = base.add(OFFSET) as *const F;
            Ok(&*field_ptr)
        }
    }
}


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

    #[test]
    fn manifest_single_epoch_is_inclusive_single_value() {
        let program = Address::new_from_array([7u8; 32]);
        let m = ForeignManifest::single_epoch(program, 42, 0xDEAD_BEEF_1234_5678, 3);
        assert!(m.supported_epochs.contains(&3));
        assert!(!m.supported_epochs.contains(&2));
        assert!(!m.supported_epochs.contains(&4));
        assert_eq!(m.expected_disc, 42);
        assert_eq!(m.expected_wire_fp, 0xDEAD_BEEF_1234_5678);
    }

    #[test]
    fn manifest_range_spans_inclusive() {
        let program = Address::new_from_array([0u8; 32]);
        let m = ForeignManifest {
            program_id: program,
            expected_disc: 1,
            expected_wire_fp: 0,
            supported_epochs: 2..=5,
        };
        for ok in [2u32, 3, 4, 5] {
            assert!(m.supported_epochs.contains(&ok), "{ok}");
        }
        for fail in [0u32, 1, 6, 100] {
            assert!(!m.supported_epochs.contains(&fail), "{fail}");
        }
    }
}