miri/shims/native_lib/

mod.rs

//! Implements calling functions from a native library.

use std::cell::Cell;
use std::marker::PhantomData;
use std::ops::Deref;
use std::os::raw::c_void;
use std::ptr;
use std::sync::atomic::AtomicBool;

use libffi::low::CodePtr;
use libffi::middle::Type as FfiType;
use rustc_abi::{HasDataLayout, Size};
use rustc_data_structures::either;
use rustc_middle::ty::layout::TyAndLayout;
use rustc_middle::ty::{self, Ty};
use rustc_span::Symbol;
use serde::{Deserialize, Serialize};

use crate::*;

#[cfg_attr(
    not(all(
        target_os = "linux",
        target_env = "gnu",
        any(target_arch = "x86", target_arch = "x86_64")
    )),
    path = "trace/stub.rs"
)]
pub mod trace;

/// An argument for an FFI call.
#[derive(Debug, Clone)]
pub struct OwnedArg {
    /// The type descriptor for this argument.
    ty: Option<FfiType>,
    /// Corresponding bytes for the value.
    bytes: Box<[u8]>,
}

impl OwnedArg {
    /// Instantiates an argument from a type descriptor and bytes.
    pub fn new(ty: FfiType, bytes: Box<[u8]>) -> Self {
        Self { ty: Some(ty), bytes }
    }
}

/// The final results of an FFI trace, containing every relevant event detected
/// by the tracer.
#[derive(Serialize, Deserialize, Debug)]
pub struct MemEvents {
    /// An list of memory accesses that occurred, in the order they occurred in.
    pub acc_events: Vec<AccessEvent>,
}

/// A single memory access.
#[derive(Serialize, Deserialize, Clone, Debug)]
pub enum AccessEvent {
    /// A read occurred on this memory range.
    Read(AccessRange),
    /// A write may have occurred on this memory range.
    /// Some instructions *may* write memory without *always* doing that,
    /// so this can be an over-approximation.
    /// The range info, however, is reliable if the access did happen.
    /// If the second field is true, the access definitely happened.
    Write(AccessRange, bool),
}

impl AccessEvent {
    fn get_range(&self) -> AccessRange {
        match self {
            AccessEvent::Read(access_range) => access_range.clone(),
            AccessEvent::Write(access_range, _) => access_range.clone(),
        }
    }
}

/// The memory touched by a given access.
#[derive(Serialize, Deserialize, Clone, Debug)]
pub struct AccessRange {
    /// The base address in memory where an access occurred.
    pub addr: usize,
    /// The number of bytes affected from the base.
    pub size: usize,
}

impl AccessRange {
    fn end(&self) -> usize {
        self.addr.strict_add(self.size)
    }
}

impl<'tcx> EvalContextExtPriv<'tcx> for crate::MiriInterpCx<'tcx> {}
trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
    /// Call native host function and return the output and the memory accesses
    /// that occurred during the call.
    fn call_native_raw(
        &mut self,
        fun: CodePtr,
        args: &mut [OwnedArg],
        ret: (FfiType, Size),
    ) -> InterpResult<'tcx, (Box<[u8]>, Option<MemEvents>)> {
        let ecx = self.eval_context_mut();
        #[cfg(target_os = "linux")]
        let alloc = ecx.machine.allocator.as_ref().unwrap().clone();
        #[cfg(not(target_os = "linux"))]
        // Placeholder value.
        let alloc = ();

        // Expose InterpCx for use by closure callbacks.
        ecx.machine.native_lib_ecx_interchange.set(ptr::from_mut(ecx).expose_provenance());

        let res = trace::Supervisor::do_ffi(&alloc, || {
            use libffi::middle::{Arg, Cif, Ret};

            let cif = Cif::new(args.iter_mut().map(|arg| arg.ty.take().unwrap()), ret.0);
            let arg_ptrs: Vec<_> = args.iter().map(|arg| Arg::new(&*arg.bytes)).collect();
            let mut ret = vec![0u8; ret.1.bytes_usize()];

            unsafe { cif.call_return_into(fun, &arg_ptrs, Ret::new::<[u8]>(&mut *ret)) };
            ret.into()
        });

        ecx.machine.native_lib_ecx_interchange.set(0);

        res
    }

    /// Get the pointer to the function of the specified name in the shared object file,
    /// if it exists. The function must be in one of the shared object files specified:
    /// we do *not* return pointers to functions in dependencies of libraries.
    fn get_func_ptr_explicitly_from_lib(&mut self, link_name: Symbol) -> Option<CodePtr> {
        let ecx = self.eval_context_mut();
        // Try getting the function from one of the shared libraries.
        for (lib, lib_path) in &ecx.machine.native_lib {
            let Ok(func): Result<libloading::Symbol<'_, unsafe extern "C" fn()>, _> =
                (unsafe { lib.get(link_name.as_str().as_bytes()) })
            else {
                continue;
            };
            #[expect(clippy::as_conversions)] // fn-ptr to raw-ptr cast needs `as`.
            let fn_ptr = *func.deref() as *mut std::ffi::c_void;

            // FIXME: this is a hack!
            // The `libloading` crate will automatically load system libraries like `libc`.
            // On linux `libloading` is based on `dlsym`: https://docs.rs/libloading/0.7.3/src/libloading/os/unix/mod.rs.html#202
            // and `dlsym`(https://linux.die.net/man/3/dlsym) looks through the dependency tree of the
            // library if it can't find the symbol in the library itself.
            // So, in order to check if the function was actually found in the specified
            // `machine.external_so_lib` we need to check its `dli_fname` and compare it to
            // the specified SO file path.
            // This code is a reimplementation of the mechanism for getting `dli_fname` in `libloading`,
            // from: https://docs.rs/libloading/0.7.3/src/libloading/os/unix/mod.rs.html#411
            // using the `libc` crate where this interface is public.
            let mut info = std::mem::MaybeUninit::<libc::Dl_info>::zeroed();
            unsafe {
                let res = libc::dladdr(fn_ptr, info.as_mut_ptr());
                assert!(res != 0, "failed to load info about function we already loaded");
                let info = info.assume_init();
                #[cfg(target_os = "cygwin")]
                let fname_ptr = info.dli_fname.as_ptr();
                #[cfg(not(target_os = "cygwin"))]
                let fname_ptr = info.dli_fname;
                assert!(!fname_ptr.is_null());
                if std::ffi::CStr::from_ptr(fname_ptr).to_str().unwrap()
                    != lib_path.to_str().unwrap()
                {
                    // The function is not actually in this .so, check the next one.
                    continue;
                }
            }

            // Return a pointer to the function.
            return Some(CodePtr(fn_ptr));
        }
        None
    }

    /// Applies the `events` to Miri's internal state. The event vector must be
    /// ordered sequentially by when the accesses happened, and the sizes are
    /// assumed to be exact.
    fn tracing_apply_accesses(&mut self, events: MemEvents) -> InterpResult<'tcx> {
        let ecx = self.eval_context_mut();

        for evt in events.acc_events {
            let evt_rg = evt.get_range();
            // LLVM at least permits vectorising accesses to adjacent allocations,
            // so we cannot assume 1 access = 1 allocation. :(
            let mut rg = evt_rg.addr..evt_rg.end();
            while let Some(curr) = rg.next() {
                let Some(alloc_id) =
                    ecx.alloc_id_from_addr(curr.to_u64(), rg.len().try_into().unwrap())
                else {
                    throw_ub_format!("Foreign code did an out-of-bounds access!")
                };
                let alloc = ecx.get_alloc_raw(alloc_id)?;
                // The logical and physical address of the allocation coincide, so we can use
                // this instead of `addr_from_alloc_id`.
                let alloc_addr = alloc.get_bytes_unchecked_raw().addr();

                // Determine the range inside the allocation that this access covers. This range is
                // in terms of offsets from the start of `alloc`. The start of the overlap range
                // will be `curr`; the end will be the minimum of the end of the allocation and the
                // end of the access' range.
                let overlap = curr.strict_sub(alloc_addr)
                    ..std::cmp::min(alloc.len(), rg.end.strict_sub(alloc_addr));
                // Skip forward however many bytes of the access are contained in the current
                // allocation, subtracting 1 since the overlap range includes the current addr
                // that was already popped off of the range.
                rg.advance_by(overlap.len().strict_sub(1)).unwrap();

                match evt {
                    AccessEvent::Read(_) => {
                        // If a provenance was read by the foreign code, expose it.
                        for (_prov_range, prov) in alloc.provenance().get_range(overlap.into(), ecx)
                        {
                            ecx.expose_provenance(prov)?;
                        }
                    }
                    AccessEvent::Write(_, certain) => {
                        // Sometimes we aren't certain if a write happened, in which case we
                        // only initialise that data if the allocation is mutable.
                        if certain || alloc.mutability.is_mut() {
                            let (alloc, cx) = ecx.get_alloc_raw_mut(alloc_id)?;
                            alloc.process_native_write(
                                &cx.tcx,
                                Some(AllocRange {
                                    start: Size::from_bytes(overlap.start),
                                    size: Size::from_bytes(overlap.len()),
                                }),
                            )
                        }
                    }
                }
            }
        }

        interp_ok(())
    }

    /// Extract the value from the result of reading an operand from the machine
    /// and convert it to a `OwnedArg`.
    fn op_to_ffi_arg(&self, v: &OpTy<'tcx>, tracing: bool) -> InterpResult<'tcx, OwnedArg> {
        let ecx = self.eval_context_ref();

        // This should go first so that we emit unsupported before doing a bunch
        // of extra work for types that aren't supported yet.
        let ty = ecx
            .ty_to_ffitype(v.layout)
            .map_err(|ty| err_unsup_format!("unsupported argument type for native call: {ty}"))?;

        // Helper to print a warning when a pointer is shared with the native code.
        let expose = |prov: Provenance| -> InterpResult<'tcx> {
            static DEDUP: AtomicBool = AtomicBool::new(false);
            if !DEDUP.swap(true, std::sync::atomic::Ordering::Relaxed) {
                // Newly set, so first time we get here.
                ecx.emit_diagnostic(NonHaltingDiagnostic::NativeCallSharedMem { tracing });
            }

            ecx.expose_provenance(prov)?;
            interp_ok(())
        };

        // Compute the byte-level representation of the argument. If there's a pointer in there, we
        // expose it inside the AM. Later in `visit_reachable_allocs`, the "meta"-level provenance
        // for accessing the pointee gets exposed; this is crucial to justify the C code effectively
        // casting the integer in `byte` to a pointer and using that.
        let bytes = match v.as_mplace_or_imm() {
            either::Either::Left(mplace) => {
                // Get the alloc id corresponding to this mplace, alongside
                // a pointer that's offset to point to this particular
                // mplace (not one at the base addr of the allocation).
                let sz = mplace.layout.size.bytes_usize();
                if sz == 0 {
                    throw_unsup_format!("attempting to pass a ZST over FFI");
                }
                let (id, ofs, _) = ecx.ptr_get_alloc_id(mplace.ptr(), sz.try_into().unwrap())?;
                let ofs = ofs.bytes_usize();
                let range = ofs..ofs.strict_add(sz);
                // Expose all provenances in the allocation within the byte range of the struct, if
                // any. These pointers are being directly passed to native code by-value.
                let alloc = ecx.get_alloc_raw(id)?;
                for (_prov_range, prov) in alloc.provenance().get_range(range.clone().into(), ecx) {
                    expose(prov)?;
                }
                // Read the bytes that make up this argument. We cannot use the normal getter as
                // those would fail if any part of the argument is uninitialized. Native code
                // is kind of outside the interpreter, after all...
                Box::from(alloc.inspect_with_uninit_and_ptr_outside_interpreter(range))
            }
            either::Either::Right(imm) => {
                let mut bytes: Box<[u8]> = vec![0; imm.layout.size.bytes_usize()].into();

                // A little helper to write scalars to our byte array.
                let mut write_scalar = |ecx: &MiriInterpCx<'tcx>, sc: Scalar, pos: usize| {
                    // If a scalar is a pointer, then expose its provenance.
                    if let interpret::Scalar::Ptr(p, _) = sc {
                        expose(p.provenance)?;
                    }
                    write_target_uint(
                        ecx.data_layout().endian,
                        &mut bytes[pos..][..sc.size().bytes_usize()],
                        sc.to_scalar_int()?.to_bits_unchecked(),
                    )
                    .unwrap();
                    interp_ok(())
                };

                // Write the scalar into the `bytes` buffer.
                match *imm {
                    Immediate::Scalar(sc) => write_scalar(ecx, sc, 0)?,
                    Immediate::ScalarPair(sc_first, sc_second) => {
                        // The first scalar has an offset of zero; compute the offset of the 2nd.
                        let ofs_second = {
                            let rustc_abi::BackendRepr::ScalarPair { a: _, b: _, b_offset } =
                                imm.layout.backend_repr
                            else {
                                span_bug!(
                                    ecx.cur_span(),
                                    "op_to_ffi_arg: invalid scalar pair layout: {:#?}",
                                    imm.layout
                                )
                            };
                            b_offset.bytes_usize()
                        };

                        write_scalar(ecx, sc_first, 0)?;
                        write_scalar(ecx, sc_second, ofs_second)?;
                    }
                    Immediate::Uninit => {
                        // Nothing to write.
                    }
                }

                bytes
            }
        };
        interp_ok(OwnedArg::new(ty, bytes))
    }

    fn ffi_ret_to_mem(&mut self, v: Box<[u8]>, dest: &MPlaceTy<'tcx>) -> InterpResult<'tcx> {
        let ecx = self.eval_context_mut();
        let len = v.len();
        ecx.write_bytes_ptr(dest.ptr(), v)?;
        if len == 0 {
            return interp_ok(());
        }
        // We have no idea which provenance these bytes have, so we reset it to wildcard.
        let tcx = ecx.tcx;
        let (alloc_id, offset, _) = ecx.ptr_try_get_alloc_id(dest.ptr(), 0).unwrap();
        let alloc = ecx.get_alloc_raw_mut(alloc_id)?.0;
        alloc.process_native_write(&tcx, Some(alloc_range(offset, dest.layout.size)));
        // Run the validation that would usually be part of `return`, also to reset
        // any provenance and padding that would not survive the return.
        if MiriMachine::enforce_validity(ecx, dest.layout) {
            ecx.validate_place(
                &dest.clone().into(),
                MiriMachine::enforce_validity_recursively(ecx, dest.layout),
                /*reset_provenance_and_padding*/ true,
            )?;
        }
        interp_ok(())
    }

    /// Parses an ADT to construct the matching libffi type.
    fn adt_to_ffitype(
        &self,
        orig_ty: Ty<'tcx>,
        adt_def: ty::AdtDef<'tcx>,
        args: ty::GenericArgs<'tcx>,
    ) -> Result<FfiType, Ty<'tcx>> {
        let ecx = self.eval_context_ref();
        // TODO: unions, etc.
        if !adt_def.is_struct() {
            return Err(orig_ty);
        }
        // TODO: Certain non-C reprs should be okay also.
        if !adt_def.repr().c() {
            return Err(orig_ty);
        }

        let mut fields = vec![];
        for field in &adt_def.non_enum_variant().fields {
            let layout =
                ecx.layout_of(field.ty(*ecx.tcx, args).skip_norm_wip()).map_err(|_err| orig_ty)?;
            fields.push(ecx.ty_to_ffitype(layout)?);
        }

        Ok(FfiType::structure(fields))
    }

    /// Gets the matching libffi type for a given Ty.
    fn ty_to_ffitype(&self, layout: TyAndLayout<'tcx>) -> Result<FfiType, Ty<'tcx>> {
        use rustc_abi::{AddressSpace, BackendRepr, Float, Integer, Primitive};

        // `BackendRepr::Scalar` is also a signal to pass this type as a scalar in the ABI. This
        // matches what codegen does. This does mean that we support some types whose ABI is not
        // stable, but that's fine -- we are anyway quite conservative in native-lib mode.
        if let BackendRepr::Scalar(s) = layout.backend_repr {
            // Simple sanity-check: this cannot be a `repr(C)` struct or union. (It could be a
            // repr(C) enum. Those indeed behave like integers in the ABI.)
            assert!(!layout.ty.ty_adt_def().is_some_and(|adt| !adt.is_enum() && adt.repr().c()));
            return Ok(match s.primitive() {
                Primitive::Int(Integer::I8, /* signed */ true) => FfiType::i8(),
                Primitive::Int(Integer::I16, /* signed */ true) => FfiType::i16(),
                Primitive::Int(Integer::I32, /* signed */ true) => FfiType::i32(),
                Primitive::Int(Integer::I64, /* signed */ true) => FfiType::i64(),
                Primitive::Int(Integer::I8, /* signed */ false) => FfiType::u8(),
                Primitive::Int(Integer::I16, /* signed */ false) => FfiType::u16(),
                Primitive::Int(Integer::I32, /* signed */ false) => FfiType::u32(),
                Primitive::Int(Integer::I64, /* signed */ false) => FfiType::u64(),
                Primitive::Float(Float::F32) => FfiType::f32(),
                Primitive::Float(Float::F64) => FfiType::f64(),
                Primitive::Pointer(AddressSpace::ZERO) => FfiType::pointer(),
                _ => return Err(layout.ty),
            });
        }
        Ok(match layout.ty.kind() {
            // Scalar types have already been handled above.
            ty::Adt(adt_def, args) => self.adt_to_ffitype(layout.ty, *adt_def, *args)?,
            // Rust uses `()` as return type for `void` function, which becomes `Tuple([])`.
            ty::Tuple(t_list) if t_list.len() == 0 => FfiType::void(),
            _ => return Err(layout.ty),
        })
    }
}

/// The data passed to the closure shim function used to intercept function pointer calls from
/// native code.
struct LibffiClosureData<'tcx> {
    ecx_interchange: &'static Cell<usize>,
    unsupported: bool,
    marker: PhantomData<MiriInterpCx<'tcx>>,
}

/// This function sets up a new libffi closure to intercept
/// calls to rust code via function pointers passed to native code.
///
/// Calling this function leaks the data passed into the libffi closure as
/// these need to be available until the execution terminates as the native
/// code side could store a function pointer and only call it at a later point.
pub fn build_libffi_closure<'tcx>(
    ecx: &MiriInterpCx<'tcx>,
    fn_sig: rustc_middle::ty::FnSig<'tcx>,
) -> InterpResult<'tcx, unsafe extern "C" fn()> {
    // Compute argument and return types in libffi representation.
    let closure_builder = try {
        let mut closure_builder = libffi::middle::Builder::new();
        for &input in fn_sig.inputs().iter() {
            let layout = ecx.layout_of(input).map_err(|_| input)?;
            let ty = ecx.ty_to_ffitype(layout)?;
            closure_builder = closure_builder.arg(ty);
        }
        let res_type = fn_sig.output();
        let res_type = {
            let layout = ecx.layout_of(res_type).map_err(|_| res_type)?;
            ecx.ty_to_ffitype(layout)?
        };
        closure_builder.res(res_type)
    };
    let mut unsupported = false;
    let closure_builder = closure_builder.unwrap_or_else(|_| {
        unsupported = true;
        // We hope that a closure which aborts execution is works correctly even if we don't
        // set its signature.
        libffi::middle::Builder::new()
    });

    // Build the actual closure.
    let data = LibffiClosureData {
        ecx_interchange: ecx.machine.native_lib_ecx_interchange,
        unsupported,
        marker: PhantomData,
    };
    let data = Box::leak(Box::new(data));
    let closure = closure_builder.into_closure(libffi_closure_callback, data);
    let closure = Box::leak(Box::new(closure));

    // The actual argument/return type doesn't matter.
    let fn_ptr = unsafe { closure.instantiate_code_ptr::<unsafe extern "C" fn()>() };
    // Libffi returns a **reference** to a function ptr here.
    // Therefore we need to dereference the reference to get the actual function pointer.
    interp_ok(*fn_ptr)
}

/// A shim function to intercept calls back from native code into the interpreter
/// via function pointers passed to the native code.
///
/// For now this shim only reports that such constructs are not supported by miri.
/// As future improvement we might continue execution in the interpreter here.
unsafe extern "C" fn libffi_closure_callback<'tcx>(
    _cif: &libffi::low::ffi_cif,
    _result: &mut c_void,
    _args: *const *const c_void,
    data: &LibffiClosureData<'tcx>,
) {
    let ecx = unsafe {
        ptr::with_exposed_provenance_mut::<MiriInterpCx<'tcx>>(data.ecx_interchange.get())
            .as_mut()
            .expect("libffi closure called while no FFI call is active")
    };
    let err = if data.unsupported {
        err_unsup_format!(
            "calling a function pointer with unsupported argument/return type through the FFI boundary"
        )
    } else {
        err_unsup_format!("calling a function pointer through the FFI boundary")
    };

    crate::diagnostics::report_result(ecx, err.into());
    // We abort the execution at this point as we cannot return the
    // expected value here.
    std::process::exit(1);
}

impl<'tcx> EvalContextExt<'tcx> for crate::MiriInterpCx<'tcx> {}
pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
    /// Call the native host function, with supplied arguments.
    /// Needs to convert all the arguments from their Miri representations to
    /// a native form (through `libffi` call).
    /// Then, convert the return value from the native form into something that
    /// can be stored in Miri's internal memory.
    ///
    /// Returns `true` if a call has been made, `false` if no functions of this name was found.
    fn call_native_fn(
        &mut self,
        link_name: Symbol,
        dest: &MPlaceTy<'tcx>,
        args: &[OpTy<'tcx>],
    ) -> InterpResult<'tcx, bool> {
        let ecx = self.eval_context_mut();
        // Get the pointer to the function in the shared object file if it exists.
        let Some(code_ptr) = ecx.get_func_ptr_explicitly_from_lib(link_name) else {
            // Shared object file does not export this function -- try the shims next.
            return interp_ok(false);
        };

        // Do we have ptrace?
        let tracing = trace::Supervisor::is_enabled();

        // Get the function arguments, copy them, and prepare the type descriptions.
        let mut libffi_args = Vec::<OwnedArg>::with_capacity(args.len());
        for arg in args.iter() {
            libffi_args.push(ecx.op_to_ffi_arg(arg, tracing)?);
        }
        let ret_ty = ecx
            .ty_to_ffitype(dest.layout)
            .map_err(|ty| err_unsup_format!("unsupported return type for native call: {ty}"))?;

        // Prepare all exposed memory (both previously exposed, and just newly exposed since a
        // pointer was passed as argument). Uninitialised memory is left as-is, but any data
        // exposed this way is garbage anyway.
        ecx.visit_reachable_allocs(ecx.exposed_allocs(), |ecx, alloc_id, info| {
            // If there is no data behind this pointer, skip this.
            if !matches!(info.kind, AllocKind::LiveData) {
                return interp_ok(());
            }
            // It's okay to get raw access, what we do does not correspond to any actual
            // AM operation, it just approximates the state to account for the native call.
            let alloc = ecx.get_alloc_raw(alloc_id)?;
            // Also expose the provenance of the interpreter-level allocation, so it can
            // be read by FFI. The `black_box` is defensive programming as LLVM likes
            // to (incorrectly) optimize away ptr2int casts whose result is unused.
            std::hint::black_box(alloc.get_bytes_unchecked_raw().expose_provenance());

            if !tracing {
                // Expose all provenances in this allocation, since the native code can do
                // $whatever. Can be skipped when tracing; in that case we'll expose just the
                // actually-read parts later.
                for prov in alloc.provenance().provenances() {
                    ecx.expose_provenance(prov)?;
                }
            }

            // Prepare for possible write from native code if mutable.
            if info.mutbl.is_mut() {
                let (alloc, cx) = ecx.get_alloc_raw_mut(alloc_id)?;
                // These writes could initialize everything and wreck havoc with the pointers.
                // We can skip that when tracing; in that case we'll later do that only for the
                // memory that got actually written.
                if !tracing {
                    alloc.process_native_write(&cx.tcx, None);
                }
                // Also expose *mutable* provenance for the interpreter-level allocation.
                std::hint::black_box(alloc.get_bytes_unchecked_raw_mut().expose_provenance());
            }

            interp_ok(())
        })?;

        // Call the function and store its output.
        let (ret, maybe_memevents) =
            ecx.call_native_raw(code_ptr, &mut libffi_args, (ret_ty, dest.layout.size))?;
        if tracing {
            ecx.tracing_apply_accesses(maybe_memevents.unwrap())?;
        }
        ecx.ffi_ret_to_mem(ret, dest)?;
        interp_ok(true)
    }
}