diff --git a/crates/fspy_shm/Cargo.toml b/crates/fspy_shm/Cargo.toml
index a9533cd4..eb339b4f 100644
--- a/crates/fspy_shm/Cargo.toml
+++ b/crates/fspy_shm/Cargo.toml
@@ -7,12 +7,12 @@ license.workspace = true
 publish = false
 rust-version.workspace = true
 
-[dependencies]
-memmap2 = { workspace = true }
-
 [target.'cfg(unix)'.dependencies]
 sigsafe = { workspace = true }
 
+[target.'cfg(windows)'.dependencies]
+memmap2 = { workspace = true }
+
 [target.'cfg(target_os = "windows")'.dependencies]
 windows-sys = { workspace = true, features = [
   "Win32_Foundation",
diff --git a/crates/fspy_shm/README.md b/crates/fspy_shm/README.md
index 3b85ee89..cdcb557e 100644
--- a/crates/fspy_shm/README.md
+++ b/crates/fspy_shm/README.md
@@ -27,14 +27,14 @@ One implementation serves every platform: a sparse file at the full path supplie
 
 Only written pages ever occupy memory or disk. The multi-gigabyte capacity fspy asks for therefore costs about as much as the data a run actually records.
 
-Mapping goes through `memmap2` on every platform. The remaining platform-specific parts are three short passages:
+Unix opens and maps through `sigsafe`; on Linux, its raw-syscall backend works before libc initialization. Windows maps through `memmap2`. The remaining platform-specific parts are short passages:
 
 | Concern           | Unix                                     | Windows                                                                     |
 | ----------------- | ---------------------------------------- | --------------------------------------------------------------------------- |
 | Same-user access  | `mode(0o600)` on the backing file        | the per-user `%TEMP%` ACL                                                   |
 | Sparseness        | file holes, produced by setting a length | `FSCTL_SET_SPARSE` before setting a length, or NTFS allocates every cluster |
 | Keeper cleanup    | unlink the path                          | unlink the path; see the fallback below                                     |
-| Descriptor safety | `O_CLOEXEC`, the Rust standard default   | non-inheritable handles, the Rust standard default                          |
+| Descriptor safety | raw `openat` with `O_CLOEXEC`            | non-inheritable handles, the Rust standard default                          |
 
 `FILE_ATTRIBUTE_TEMPORARY` asks Windows to keep the data in memory when it can. Creation fails on a volume without sparse-file support.
 
diff --git a/crates/fspy_shm/src/unix.rs b/crates/fspy_shm/src/unix.rs
index 166c3074..376a8a6f 100644
--- a/crates/fspy_shm/src/unix.rs
+++ b/crates/fspy_shm/src/unix.rs
@@ -4,12 +4,12 @@ use std::{
     ffi::OsStr,
     fs::{self, File, OpenOptions},
     io,
+    num::NonZeroUsize,
     os::unix::{ffi::OsStrExt as _, fs::OpenOptionsExt as _, io::IntoRawFd as _},
     path::PathBuf,
+    ptr::{self, NonNull},
 };
 
-use memmap2::{MmapOptions, MmapRaw};
-
 /// Keeps the shared memory's backing-file name alive and removes it on drop.
 ///
 /// Removal is cleanup, not a stop signal: later opens fail, but existing
@@ -25,7 +25,7 @@ pub struct ShmKeeper {
 /// view of the same bytes. Drop the handle once the mappings exist.
 pub struct ShmHandle {
     file: sigsafe::OwnedFd,
-    size: usize,
+    size: NonZeroUsize,
 }
 
 /// The mapped shared bytes.
@@ -33,9 +33,16 @@ pub struct ShmHandle {
 /// A `Mapping` keeps the bytes alive until it is dropped and cannot affect the
 /// shared memory's path.
 pub struct Mapping {
-    raw: MmapRaw,
+    ptr: NonNull<u8>,
+    len: NonZeroUsize,
 }
 
+// SAFETY: a mapping owns no thread-affine state; access synchronization is
+// supplied by the fspy channel built on top of it.
+unsafe impl Send for Mapping {}
+// SAFETY: sharing a `Mapping` does not itself access its bytes, and all actual
+// concurrent access is synchronized by the fspy channel.
+unsafe impl Sync for Mapping {}
 /// Creates `size` bytes of zero-initialized shared memory at `path`.
 ///
 /// Returns its [`ShmKeeper`] and an already opened [`ShmHandle`], so the
@@ -48,13 +55,10 @@ pub struct Mapping {
 ///
 /// Returns an error if the shared memory cannot be created or sized.
 pub fn create(path: &OsStr, size: usize) -> io::Result<(ShmKeeper, ShmHandle)> {
-    if size == 0 {
-        return Err(io::Error::new(
-            io::ErrorKind::InvalidInput,
-            "shared-memory size must be nonzero",
-        ));
-    }
-    let size_u64 = u64::try_from(size).map_err(|_| {
+    let size = NonZeroUsize::new(size).ok_or_else(|| {
+        io::Error::new(io::ErrorKind::InvalidInput, "shared-memory size must be nonzero")
+    })?;
+    let size_u64 = u64::try_from(size.get()).map_err(|_| {
         io::Error::new(io::ErrorKind::InvalidInput, "shared-memory size exceeds u64")
     })?;
     let path = PathBuf::from(path);
@@ -89,9 +93,7 @@ pub fn open(path: &OsStr) -> io::Result<ShmHandle> {
     // resize cannot make a mapping access invalid memory.
     let size = usize::try_from(sigsafe::fs::fstat(&file).map_err(errno_to_io)?.st_size)
         .map_err(|_| io::ErrorKind::InvalidData)?;
-    if size == 0 {
-        return Err(io::ErrorKind::InvalidData.into());
-    }
+    let size = NonZeroUsize::new(size).ok_or(io::ErrorKind::InvalidData)?;
     Ok(ShmHandle { file, size })
 }
 
@@ -137,8 +139,36 @@ impl ShmHandle {
     ///
     /// Returns an error if the mapping cannot be established.
     pub fn map(&self) -> io::Result<Mapping> {
-        let file = sigsafe::AsRawFd::as_raw_fd(&self.file);
-        Ok(Mapping { raw: MmapOptions::new().len(self.size).map_raw(file)? })
+        // SAFETY: the address is only a hint, the nonzero length is the
+        // validated backing-file size, the descriptor remains borrowed,
+        // and the resulting shared mapping is owned by `Mapping`.
+        let mapped = unsafe {
+            sigsafe::mm::mmap(
+                ptr::null_mut(),
+                self.size.get(),
+                sigsafe::mm::ProtFlags::READ | sigsafe::mm::ProtFlags::WRITE,
+                sigsafe::mm::MapFlags::SHARED,
+                &self.file,
+                0,
+            )
+        }
+        .map_err(errno_to_io)?;
+        let Some(ptr) = NonNull::new(mapped.cast()) else {
+            // A non-fixed mapping should not be placed at address zero,
+            // which Rust cannot represent as a non-null allocation.
+            // SAFETY: release the successful mapping before rejecting it.
+            let _ = unsafe { sigsafe::mm::munmap(mapped, self.size.get()) };
+            return Err(io::ErrorKind::Other.into());
+        };
+        Ok(Mapping { ptr, len: self.size })
+    }
+}
+
+impl Drop for Mapping {
+    fn drop(&mut self) {
+        // SAFETY: this is the complete mapping owned by `self`, and dropping
+        // it proves that no safe borrow through `self` remains.
+        let _ = unsafe { sigsafe::mm::munmap(self.ptr.as_ptr().cast(), self.len.get()) };
     }
 }
 
@@ -146,14 +176,14 @@ impl ShmHandle {
 impl Mapping {
     /// Returns the mapped length in bytes.
     #[must_use]
-    pub fn len(&self) -> usize {
-        self.raw.len()
+    pub const fn len(&self) -> usize {
+        self.len.get()
     }
 
     /// Returns a raw pointer to the first mapped byte.
     #[must_use]
-    pub fn as_ptr(&self) -> *mut u8 {
-        self.raw.as_mut_ptr()
+    pub const fn as_ptr(&self) -> *mut u8 {
+        self.ptr.as_ptr()
     }
 
     /// Returns the mapped bytes as a shared slice.
@@ -163,7 +193,7 @@ impl Mapping {
     /// The caller must ensure that no process or thread mutates the mapping for
     /// the lifetime of the returned slice.
     #[must_use]
-    pub unsafe fn as_slice(&self) -> &[u8] {
+    pub const unsafe fn as_slice(&self) -> &[u8] {
         // SAFETY: The mapping is valid for its full length, and the caller
         // guarantees that it is not mutated while the slice is borrowed.
         unsafe { std::slice::from_raw_parts(self.as_ptr().cast_const(), self.len()) }
diff --git a/crates/sigsafe/README.md b/crates/sigsafe/README.md
index e708d0da..8216c9f7 100644
--- a/crates/sigsafe/README.md
+++ b/crates/sigsafe/README.md
@@ -28,9 +28,9 @@ rustix can be built with a libc backend instead of raw syscalls, and anything in
 
 Functions whose rustix implementation already meets the rules are re-exposed as-is; being listed in a module here is what marks a call as allowed, and the backend check above is what keeps that true.
 
-- `mm` — anonymous memory mappings: `mmap_anonymous`, `munmap`.
+- `mm` — anonymous and file-backed memory mappings, protection changes, and unmapping.
 - `env` — allocation-free iteration over process arguments and environment entries.
-- `fs` — caller-buffer filesystem operations: `getcwd`, plus macOS `fcntl_getpath`.
+- `fs` — caller-buffer and descriptor operations such as `openat`, `fstat`, `getcwd`, and `readlinkat`.
 - `param` — `page_size`.
 
 Allocation without malloc lives in [`sigsafe_alloc`](../sigsafe_alloc).
diff --git a/crates/sigsafe/src/mm.rs b/crates/sigsafe/src/mm.rs
index c7cdecd0..7de3763f 100644
--- a/crates/sigsafe/src/mm.rs
+++ b/crates/sigsafe/src/mm.rs
@@ -8,4 +8,4 @@
 //! pre-libc startup) and the crate-level backend check, which guarantees
 //! they cannot silently turn into libc calls on Linux.
 
-pub use rustix::mm::{MapFlags, MprotectFlags, ProtFlags, mmap_anonymous, mprotect, munmap};
+pub use rustix::mm::{MapFlags, MprotectFlags, ProtFlags, mmap, mmap_anonymous, mprotect, munmap};