feat: pass file mapping info via PEB with label support

Add FileMappingInfo struct (guest_addr, size, label) and file_mappings
field to HyperlightPEB so file mapping metadata is communicated to the
guest through the PEB. Space for MAX_FILE_MAPPINGS (32) entries is
statically reserved after the PEB struct to avoid dynamic layout changes.

- Add label parameter to map_file_cow (optional, defaults to filename)
- Add shared memory overlap validation (full mapped range)
- Add inter-mapping overlap detection
- Add write_file_mapping_entry on SandboxMemoryManager
- Add MAX_FILE_MAPPINGS limit enforcement at registration time
- Update PEB region sizing in get_memory_regions and layout tests

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

Author: simongdavies

src/hyperlight_common/src/mem.rs

@@ -28,11 +28,54 @@ pub struct GuestMemoryRegion {
     pub ptr: u64,
 }
 
+/// Maximum length of a file mapping label (excluding null terminator).
+pub const FILE_MAPPING_LABEL_MAX_LEN: usize = 63;
+
+/// Maximum number of file mappings that can be registered in the PEB.
+///
+/// Space for this many [`FileMappingInfo`] entries is statically
+/// reserved immediately after the [`HyperlightPEB`] struct within the
+/// same memory region. The reservation happens at layout time
+/// (see `SandboxMemoryLayout::new`) so the guest heap never overlaps
+/// the array, regardless of how many entries are actually used.
+pub const MAX_FILE_MAPPINGS: usize = 32;
+
+/// Describes a single file mapping in the guest address space.
+///
+/// Stored in the PEB's file mappings array so the guest can discover
+/// which files have been mapped, at what address, and with what label.
+#[derive(Debug, Clone, Copy)]
+#[repr(C)]
+pub struct FileMappingInfo {
+    /// The guest address where the file is mapped.
+    pub guest_addr: u64,
+    /// The page-aligned size of the mapping in bytes.
+    pub size: u64,
+    /// Null-terminated C-style label (max 63 chars + null).
+    pub label: [u8; FILE_MAPPING_LABEL_MAX_LEN + 1],
+}
+
+impl Default for FileMappingInfo {
+    fn default() -> Self {
+        Self {
+            guest_addr: 0,
+            size: 0,
+            label: [0u8; FILE_MAPPING_LABEL_MAX_LEN + 1],
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
 #[repr(C)]
 pub struct HyperlightPEB {
     pub input_stack: GuestMemoryRegion,
     pub output_stack: GuestMemoryRegion,
     pub init_data: GuestMemoryRegion,
     pub guest_heap: GuestMemoryRegion,
+    /// File mappings array descriptor.
+    /// **Note:** `size` holds the **entry count** (number of valid
+    /// [`FileMappingInfo`] entries), NOT a byte size. `ptr` holds the
+    /// guest address of the preallocated array (immediately after the
+    /// PEB struct).
+    pub file_mappings: GuestMemoryRegion,
 }

src/hyperlight_host/examples/crashdump/main.rs

@@ -152,7 +152,7 @@ fn guest_crash_auto_dump(guest_path: &str) -> hyperlight_host::Result<()> {
     // Map a file as read-only into the guest at a known address.
     let mapping_file = create_mapping_file();
     let guest_base: u64 = 0x200000000;
-    let len = sandbox.map_file_cow(mapping_file.as_path(), guest_base)?;
+    let len = sandbox.map_file_cow(mapping_file.as_path(), guest_base, None)?;
     println!("Mapped {len} bytes at guest address {guest_base:#x} (read-only).");
 
     // Call WriteMappedBuffer — the guest maps the address in its page tables
@@ -259,7 +259,7 @@ fn guest_crash_with_dump_disabled(guest_path: &str) -> hyperlight_host::Result<(
 
     let mapping_file = create_mapping_file();
     let guest_base: u64 = 0x200000000;
-    let len = sandbox.map_file_cow(mapping_file.as_path(), guest_base)?;
+    let len = sandbox.map_file_cow(mapping_file.as_path(), guest_base, None)?;
 
     println!("Calling guest function 'WriteMappedBuffer' on read-only region...");
     let result = sandbox.call::<bool>("WriteMappedBuffer", (guest_base, len));
@@ -401,7 +401,7 @@ mod tests {
         // automatically. This mapping lets us verify that GDB can read
         // a specific sentinel string from a known address.
         let len = sbox
-            .map_file_cow(&data_file, MAP_GUEST_BASE)
+            .map_file_cow(&data_file, MAP_GUEST_BASE, None)
             .expect("map_file_cow");
 
         // Read the mapped region back through the guest and verify it

src/hyperlight_host/src/mem/layout.rs

@@ -63,7 +63,7 @@ limitations under the License.
 use std::fmt::Debug;
 use std::mem::{offset_of, size_of};
 
-use hyperlight_common::mem::{GuestMemoryRegion, HyperlightPEB, PAGE_SIZE_USIZE};
+use hyperlight_common::mem::{HyperlightPEB, PAGE_SIZE_USIZE};
 use tracing::{Span, instrument};
 
 use super::memory_region::MemoryRegionType::{Code, Heap, InitData, Peb};
@@ -92,6 +92,7 @@ pub(crate) struct SandboxMemoryLayout {
     peb_output_data_offset: usize,
     peb_init_data_offset: usize,
     peb_heap_data_offset: usize,
+    peb_file_mappings_offset: usize,
 
     guest_heap_buffer_offset: usize,
     init_data_offset: usize,
@@ -141,6 +142,10 @@ impl Debug for SandboxMemoryLayout {
                 "Guest Heap Offset",
                 &format_args!("{:#x}", self.peb_heap_data_offset),
             )
+            .field(
+                "File Mappings Offset",
+                &format_args!("{:#x}", self.peb_file_mappings_offset),
+            )
             .field(
                 "Guest Heap Buffer Offset",
                 &format_args!("{:#x}", self.guest_heap_buffer_offset),
@@ -211,13 +216,25 @@ impl SandboxMemoryLayout {
         let peb_output_data_offset = peb_offset + offset_of!(HyperlightPEB, output_stack);
         let peb_init_data_offset = peb_offset + offset_of!(HyperlightPEB, init_data);
         let peb_heap_data_offset = peb_offset + offset_of!(HyperlightPEB, guest_heap);
+        let peb_file_mappings_offset = peb_offset + offset_of!(HyperlightPEB, file_mappings);
 
         // The following offsets are the actual values that relate to memory layout,
         // which are written to PEB struct
         let peb_address = Self::BASE_ADDRESS + peb_offset;
-        // make sure heap buffer starts at 4K boundary
-        let guest_heap_buffer_offset = (peb_heap_data_offset + size_of::<GuestMemoryRegion>())
-            .next_multiple_of(PAGE_SIZE_USIZE);
+        // make sure heap buffer starts at 4K boundary.
+        // The FileMappingInfo array is stored immediately after the PEB struct.
+        // We statically reserve space for MAX_FILE_MAPPINGS entries so that
+        // the heap never overlaps the array, even when all slots are used.
+        // The host writes file mapping metadata here via write_file_mapping_entry;
+        // the guest only reads the entries. We don't know at layout time how
+        // many file mappings the host will register, so we reserve space for
+        // the maximum number.
+        // The heap starts at the next page boundary after this reserved area.
+        let file_mappings_array_end = peb_offset
+            + size_of::<HyperlightPEB>()
+            + hyperlight_common::mem::MAX_FILE_MAPPINGS
+                * size_of::<hyperlight_common::mem::FileMappingInfo>();
+        let guest_heap_buffer_offset = file_mappings_array_end.next_multiple_of(PAGE_SIZE_USIZE);
 
         // make sure init data starts at 4K boundary
         let init_data_offset =
@@ -230,6 +247,7 @@ impl SandboxMemoryLayout {
             peb_output_data_offset,
             peb_init_data_offset,
             peb_heap_data_offset,
+            peb_file_mappings_offset,
             sandbox_memory_config: cfg,
             code_size,
             guest_heap_buffer_offset,
@@ -350,6 +368,28 @@ impl SandboxMemoryLayout {
         self.peb_heap_data_offset
     }
 
+    /// Get the offset in guest memory to the file_mappings count field
+    /// (the `size` field of the `GuestMemoryRegion` in the PEB).
+    pub(crate) fn get_file_mappings_size_offset(&self) -> usize {
+        self.peb_file_mappings_offset
+    }
+
+    /// Get the offset in guest memory to the file_mappings pointer field.
+    fn get_file_mappings_pointer_offset(&self) -> usize {
+        self.get_file_mappings_size_offset() + size_of::<u64>()
+    }
+
+    /// Get the offset in snapshot memory where the FileMappingInfo array starts
+    /// (immediately after the PEB struct, within the same page).
+    pub(crate) fn get_file_mappings_array_offset(&self) -> usize {
+        self.peb_offset + size_of::<HyperlightPEB>()
+    }
+
+    /// Get the guest address of the FileMappingInfo array.
+    fn get_file_mappings_array_gva(&self) -> u64 {
+        (Self::BASE_ADDRESS + self.get_file_mappings_array_offset()) as u64
+    }
+
     /// Get the offset of the heap pointer in guest memory,
     #[instrument(skip_all, parent = Span::current(), level= "Trace")]
     fn get_heap_pointer_offset(&self) -> usize {
@@ -446,9 +486,12 @@ impl SandboxMemoryLayout {
             ));
         }
 
-        // PEB
+        // PEB + preallocated FileMappingInfo array
+        let peb_and_array_size = size_of::<HyperlightPEB>()
+            + hyperlight_common::mem::MAX_FILE_MAPPINGS
+                * size_of::<hyperlight_common::mem::FileMappingInfo>();
         let heap_offset = builder.push_page_aligned(
-            size_of::<HyperlightPEB>(),
+            peb_and_array_size,
             MemoryRegionFlags::READ | MemoryRegionFlags::WRITE,
             Peb,
         );
@@ -588,6 +631,18 @@ impl SandboxMemoryLayout {
         shared_mem.write_u64(self.get_heap_size_offset(), self.heap_size.try_into()?)?;
         shared_mem.write_u64(self.get_heap_pointer_offset(), addr)?;
 
+        // Set up the file_mappings descriptor in the PEB.
+        // - The `size` field holds the number of valid FileMappingInfo
+        //   entries currently written (initially 0 — entries are added
+        //   later by map_file_cow / evolve).
+        // - The `ptr` field holds the guest address of the preallocated
+        //   FileMappingInfo array
+        shared_mem.write_u64(self.get_file_mappings_size_offset(), 0)?;
+        shared_mem.write_u64(
+            self.get_file_mappings_pointer_offset(),
+            self.get_file_mappings_array_gva(),
+        )?;
+
         // End of setting up the PEB
 
         // The input and output data regions do not have their layout
@@ -611,7 +666,11 @@ mod tests {
         // in order of layout
         expected_size += layout.code_size;
 
-        expected_size += size_of::<HyperlightPEB>().next_multiple_of(PAGE_SIZE_USIZE);
+        // PEB + preallocated FileMappingInfo array
+        let peb_and_array = size_of::<HyperlightPEB>()
+            + hyperlight_common::mem::MAX_FILE_MAPPINGS
+                * size_of::<hyperlight_common::mem::FileMappingInfo>();
+        expected_size += peb_and_array.next_multiple_of(PAGE_SIZE_USIZE);
 
         expected_size += layout.heap_size.next_multiple_of(PAGE_SIZE_USIZE);
 

src/hyperlight_host/src/mem/mgr.rs

@@ -13,6 +13,8 @@ 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.
 */
+use std::mem::offset_of;
+
 use flatbuffers::FlatBufferBuilder;
 use hyperlight_common::flatbuffer_wrappers::function_call::{
     FunctionCall, validate_guest_function_call_buffer,
@@ -351,6 +353,66 @@ impl SandboxMemoryManager<ExclusiveSharedMemory> {
 }
 
 impl SandboxMemoryManager<HostSharedMemory> {
+    /// Write a [`FileMappingInfo`] entry into the PEB's preallocated array.
+    ///
+    /// Reads the current entry count from the PEB, validates that the
+    /// array isn't full ([`MAX_FILE_MAPPINGS`]), writes the entry at the
+    /// next available slot, and increments the count.
+    ///
+    /// This is the **only** place that writes to the PEB file mappings
+    /// array — both `MultiUseSandbox::map_file_cow` and the evolve loop
+    /// call through here so the logic is not duplicated.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if [`MAX_FILE_MAPPINGS`] has been reached.
+    ///
+    /// [`FileMappingInfo`]: hyperlight_common::mem::FileMappingInfo
+    /// [`MAX_FILE_MAPPINGS`]: hyperlight_common::mem::MAX_FILE_MAPPINGS
+    pub(crate) fn write_file_mapping_entry(
+        &mut self,
+        guest_addr: u64,
+        size: u64,
+        label: &[u8; hyperlight_common::mem::FILE_MAPPING_LABEL_MAX_LEN + 1],
+    ) -> Result<()> {
+        use hyperlight_common::mem::{FileMappingInfo, MAX_FILE_MAPPINGS};
+
+        // Read the current entry count from the PEB. This is the source
+        // of truth — it survives snapshot/restore because the PEB is
+        // part of shared memory that gets snapshotted.
+        let current_count =
+            self.shared_mem
+                .read::<u64>(self.layout.get_file_mappings_size_offset())? as usize;
+
+        if current_count >= MAX_FILE_MAPPINGS {
+            return Err(crate::new_error!(
+                "file mapping limit reached ({} of {})",
+                current_count,
+                MAX_FILE_MAPPINGS,
+            ));
+        }
+
+        // Write the entry into the next available slot.
+        let entry_offset = self.layout.get_file_mappings_array_offset()
+            + current_count * std::mem::size_of::<FileMappingInfo>();
+        let guest_addr_offset = offset_of!(FileMappingInfo, guest_addr);
+        let size_offset = offset_of!(FileMappingInfo, size);
+        let label_offset = offset_of!(FileMappingInfo, label);
+        self.shared_mem
+            .write::<u64>(entry_offset + guest_addr_offset, guest_addr)?;
+        self.shared_mem
+            .write::<u64>(entry_offset + size_offset, size)?;
+        self.shared_mem
+            .copy_from_slice(label, entry_offset + label_offset)?;
+
+        // Increment the entry count.
+        let new_count = (current_count + 1) as u64;
+        self.shared_mem
+            .write::<u64>(self.layout.get_file_mappings_size_offset(), new_count)?;
+
+        Ok(())
+    }
+
     /// Reads a host function call from memory
     #[instrument(err(Debug), skip_all, parent = Span::current(), level= "Trace")]
     pub(crate) fn get_host_function_call(&mut self) -> Result<FunctionCall> {