Implement map_file_cow so that files can be mapped

before a VM is initialised

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>

Author: simongdavies

src/hyperlight_host/src/hypervisor/hyperlight_vm/x86_64.rs

@@ -79,6 +79,7 @@ impl HyperlightVm {
         _pml4_addr: u64,
         entrypoint: NextAction,
         rsp_gva: u64,
+        page_size: usize,
         #[cfg_attr(target_os = "windows", allow(unused_variables))] config: &SandboxConfiguration,
         #[cfg(gdb)] gdb_conn: Option<DebugCommChannel<DebugResponse, DebugMsg>>,
         #[cfg(crashdump)] rt_cfg: SandboxRuntimeConfig,
@@ -145,7 +146,7 @@ impl HyperlightVm {
             entrypoint,
             rsp_gva,
             interrupt_handle,
-            page_size: 0, // Will be set in `initialise`
+            page_size,
 
             next_slot: scratch_slot + 1,
             freed_slots: Vec::new(),
@@ -207,8 +208,6 @@ impl HyperlightVm {
             return Ok(());
         };
 
-        self.page_size = page_size as usize;
-
         let regs = CommonRegisters {
             rip: initialise,
             // We usually keep the top of the stack 16-byte
@@ -1505,6 +1504,7 @@ mod tests {
             gshm,
             &config,
             stack_top_gva,
+            page_size::get(),
             #[cfg(any(crashdump, gdb))]
             rt_cfg,
             crate::mem::exe::LoadInfo::dummy(),

src/hyperlight_host/src/hypervisor/mod.rs

@@ -493,6 +493,7 @@ pub(crate) mod tests {
             gshm,
             &config,
             exn_stack_top_gva,
+            page_size::get(),
             #[cfg(any(crashdump, gdb))]
             rt_cfg,
             sandbox.load_info,

src/hyperlight_host/src/sandbox/file_mapping.rs

@@ -0,0 +1,294 @@
+/*
+Copyright 2025  The Hyperlight Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+//! Host-side file mapping preparation for [`map_file_cow`].
+//!
+//! This module splits the file mapping operation into two phases:
+//! - **Prepare** ([`prepare_file_cow`]): performs host-side OS calls
+//!   (open file, create mapping) without requiring a VM.
+//! - **Apply**: performed by the caller (either [`MultiUseSandbox::map_file_cow`]
+//!   or [`evolve_impl_multi_use`]) to map the prepared region into
+//!   the guest via [`HyperlightVm::map_region`].
+//!
+//! This separation allows [`UninitializedSandbox`] to accept
+//! `map_file_cow` calls before the VM exists, deferring the VM-side
+//! work until [`evolve()`].
+
+use std::ffi::c_void;
+use std::path::Path;
+
+use tracing::{Span, instrument};
+
+#[cfg(target_os = "windows")]
+use crate::HyperlightError;
+#[cfg(target_os = "windows")]
+use crate::hypervisor::wrappers::HandleWrapper;
+#[cfg(target_os = "windows")]
+use crate::mem::memory_region::{HostRegionBase, MemoryRegionKind};
+use crate::mem::memory_region::{MemoryRegion, MemoryRegionFlags, MemoryRegionType};
+use crate::{Result, log_then_return};
+
+/// A prepared (host-side) file mapping ready to be applied to a VM.
+///
+/// Created by [`prepare_file_cow`]. The host-side OS resources (file
+/// mapping handle + view on Windows, mmap on Linux) are held here
+/// until consumed by the VM-side apply step.
+///
+/// If dropped without being consumed, the `Drop` impl releases all
+/// host-side resources — preventing leaks when an
+/// [`UninitializedSandbox`] is dropped without evolving or when
+/// apply fails.
+#[must_use = "holds OS resources that leak if discarded — apply to a VM or let Drop clean up"]
+pub(crate) struct PreparedFileMapping {
+    /// The guest address where this file should be mapped.
+    pub(crate) guest_base: u64,
+    /// The page-aligned size of the mapping in bytes.
+    pub(crate) size: usize,
+    /// Host-side OS resources. `None` after successful consumption
+    /// by the apply step (ownership transferred to the VM layer).
+    pub(crate) host_resources: Option<HostFileResources>,
+}
+
+/// Platform-specific host-side file mapping resources.
+pub(crate) enum HostFileResources {
+    /// Windows: `CreateFileMappingW` handle + `MapViewOfFile` view.
+    #[cfg(target_os = "windows")]
+    Windows {
+        mapping_handle: HandleWrapper,
+        view_base: *mut c_void,
+    },
+    /// Linux: `mmap` base pointer.
+    #[cfg(unix)]
+    Unix {
+        mmap_base: *mut c_void,
+        mmap_size: usize,
+    },
+}
+
+impl Drop for PreparedFileMapping {
+    fn drop(&mut self) {
+        // Clean up host resources if they haven't been consumed.
+        if let Some(resources) = self.host_resources.take() {
+            match resources {
+                #[cfg(target_os = "windows")]
+                HostFileResources::Windows {
+                    mapping_handle,
+                    view_base,
+                } => unsafe {
+                    use windows::Win32::Foundation::CloseHandle;
+                    use windows::Win32::System::Memory::{
+                        MEMORY_MAPPED_VIEW_ADDRESS, UnmapViewOfFile,
+                    };
+                    if let Err(e) = UnmapViewOfFile(MEMORY_MAPPED_VIEW_ADDRESS { Value: view_base })
+                    {
+                        tracing::error!(
+                            "PreparedFileMapping::drop: UnmapViewOfFile failed: {:?}",
+                            e
+                        );
+                    }
+                    if let Err(e) = CloseHandle(mapping_handle.into()) {
+                        tracing::error!("PreparedFileMapping::drop: CloseHandle failed: {:?}", e);
+                    }
+                },
+                #[cfg(unix)]
+                HostFileResources::Unix {
+                    mmap_base,
+                    mmap_size,
+                } => unsafe {
+                    libc::munmap(mmap_base, mmap_size);
+                },
+            }
+        }
+    }
+}
+
+// SAFETY: The raw pointers in HostFileResources point to kernel-managed
+// mappings (Windows file mapping views / Linux mmap regions), not aliased
+// user-allocated heap memory. Ownership is fully contained within the
+// struct, and cleanup APIs (UnmapViewOfFile, CloseHandle, munmap) are
+// thread-safe.
+unsafe impl Send for PreparedFileMapping {}
+
+impl PreparedFileMapping {
+    /// Build the [`MemoryRegion`] that describes this mapping for the
+    /// VM layer. The host resources must still be present (not yet
+    /// consumed).
+    pub(crate) fn to_memory_region(&self) -> Result<MemoryRegion> {
+        let resources = self.host_resources.as_ref().ok_or_else(|| {
+            crate::HyperlightError::Error(
+                "PreparedFileMapping resources already consumed".to_string(),
+            )
+        })?;
+
+        match resources {
+            #[cfg(target_os = "windows")]
+            HostFileResources::Windows {
+                mapping_handle,
+                view_base,
+            } => {
+                let host_base = HostRegionBase {
+                    from_handle: *mapping_handle,
+                    handle_base: *view_base as usize,
+                    handle_size: self.size,
+                    offset: 0,
+                };
+                let host_end =
+                    <crate::mem::memory_region::HostGuestMemoryRegion as MemoryRegionKind>::add(
+                        host_base, self.size,
+                    );
+                Ok(MemoryRegion {
+                    host_region: host_base..host_end,
+                    guest_region: self.guest_base as usize..self.guest_base as usize + self.size,
+                    flags: MemoryRegionFlags::READ | MemoryRegionFlags::EXECUTE,
+                    region_type: MemoryRegionType::MappedFile,
+                })
+            }
+            #[cfg(unix)]
+            HostFileResources::Unix {
+                mmap_base,
+                mmap_size,
+            } => Ok(MemoryRegion {
+                host_region: *mmap_base as usize..(*mmap_base as usize).wrapping_add(*mmap_size),
+                guest_region: self.guest_base as usize..self.guest_base as usize + self.size,
+                flags: MemoryRegionFlags::READ | MemoryRegionFlags::EXECUTE,
+                region_type: MemoryRegionType::MappedFile,
+            }),
+        }
+    }
+
+    /// Mark the host resources as consumed — ownership has been
+    /// transferred to the VM layer. After this call, `Drop` will
+    /// not release them.
+    pub(crate) fn mark_consumed(&mut self) {
+        self.host_resources = None;
+    }
+}
+
+/// Perform host-side file mapping preparation without requiring a VM.
+///
+/// Opens the file, creates a read-only mapping in the host process,
+/// and returns a [`PreparedFileMapping`] that can be applied to the
+/// VM later.
+///
+/// # Errors
+///
+/// Returns an error if the file cannot be opened, is empty, or the
+/// OS mapping calls fail.
+#[instrument(err(Debug), skip(file_path, guest_base), parent = Span::current())]
+pub(crate) fn prepare_file_cow(file_path: &Path, guest_base: u64) -> Result<PreparedFileMapping> {
+    // Validate alignment eagerly to fail fast before allocating OS resources.
+    let page_size = page_size::get();
+    if guest_base as usize % page_size != 0 {
+        log_then_return!(
+            "map_file_cow: guest_base {:#x} is not page-aligned (page size: {:#x})",
+            guest_base,
+            page_size
+        );
+    }
+
+    #[cfg(target_os = "windows")]
+    {
+        use std::os::windows::io::AsRawHandle;
+
+        use windows::Win32::Foundation::HANDLE;
+        use windows::Win32::System::Memory::{
+            CreateFileMappingW, FILE_MAP_READ, MapViewOfFile, PAGE_READONLY,
+        };
+
+        let file = std::fs::File::options().read(true).open(file_path)?;
+        let file_size = file.metadata()?.len();
+        if file_size == 0 {
+            log_then_return!("map_file_cow: cannot map an empty file: {:?}", file_path);
+        }
+        let size = usize::try_from(file_size).map_err(|_| {
+            HyperlightError::Error(format!(
+                "File size {file_size} exceeds addressable range on this platform"
+            ))
+        })?;
+        let size = size.div_ceil(page_size) * page_size;
+
+        let file_handle = HANDLE(file.as_raw_handle());
+
+        // Create a read-only file mapping object backed by the actual file.
+        // Pass 0,0 for size to use the file's actual size — Windows will
+        // NOT extend a read-only file, so requesting page-aligned size
+        // would fail for files smaller than one page.
+        let mapping_handle =
+            unsafe { CreateFileMappingW(file_handle, None, PAGE_READONLY, 0, 0, None) }
+                .map_err(|e| HyperlightError::Error(format!("CreateFileMappingW failed: {e}")))?;
+
+        // Map a read-only view into the host process.
+        // Passing 0 for dwNumberOfBytesToMap maps the entire file; the OS
+        // rounds up to the next page boundary and zero-fills the remainder.
+        let view = unsafe { MapViewOfFile(mapping_handle, FILE_MAP_READ, 0, 0, 0) };
+        if view.Value.is_null() {
+            unsafe {
+                let _ = windows::Win32::Foundation::CloseHandle(mapping_handle);
+            }
+            log_then_return!(
+                "MapViewOfFile failed: {:?}",
+                std::io::Error::last_os_error()
+            );
+        }
+
+        Ok(PreparedFileMapping {
+            guest_base,
+            size,
+            host_resources: Some(HostFileResources::Windows {
+                mapping_handle: HandleWrapper::from(mapping_handle),
+                view_base: view.Value,
+            }),
+        })
+    }
+    #[cfg(unix)]
+    {
+        use std::os::fd::AsRawFd;
+
+        let file = std::fs::File::options().read(true).open(file_path)?;
+        let file_size = file.metadata()?.len();
+        if file_size == 0 {
+            log_then_return!("map_file_cow: cannot map an empty file: {:?}", file_path);
+        }
+        let size = usize::try_from(file_size).map_err(|_| {
+            crate::HyperlightError::Error(format!(
+                "File size {file_size} exceeds addressable range on this platform"
+            ))
+        })?;
+        let size = size.div_ceil(page_size) * page_size;
+        let base = unsafe {
+            libc::mmap(
+                std::ptr::null_mut(),
+                size,
+                libc::PROT_READ | libc::PROT_WRITE | libc::PROT_EXEC,
+                libc::MAP_PRIVATE,
+                file.as_raw_fd(),
+                0,
+            )
+        };
+        if base == libc::MAP_FAILED {
+            log_then_return!("mmap error: {:?}", std::io::Error::last_os_error());
+        }
+
+        Ok(PreparedFileMapping {
+            guest_base,
+            size,
+            host_resources: Some(HostFileResources::Unix {
+                mmap_base: base,
+                mmap_size: size,
+            }),
+        })
+    }
+}