diff --git a/src/shims/unix/foreign_items.rs b/src/shims/unix/foreign_items.rs
index 35036ce078..87cfe2907b 100644
--- a/src/shims/unix/foreign_items.rs
+++ b/src/shims/unix/foreign_items.rs
@@ -262,6 +262,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
 
             "mmap" => {
                 let [addr, length, prot, flags, fd, offset] = this.check_shim(abi, Abi::C {unwind: false}, link_name, args)?;
+                let offset = this.read_scalar(offset)?.to_int(this.libc_ty_layout("off_t").size)?;
                 let ptr = this.mmap(addr, length, prot, flags, fd, offset)?;
                 this.write_scalar(ptr, dest)?;
             }
diff --git a/src/shims/unix/linux/foreign_items.rs b/src/shims/unix/linux/foreign_items.rs
index 6937e0f089..a05a6470a5 100644
--- a/src/shims/unix/linux/foreign_items.rs
+++ b/src/shims/unix/linux/foreign_items.rs
@@ -9,6 +9,7 @@ use shims::unix::fs::EvalContextExt as _;
 use shims::unix::linux::fd::EvalContextExt as _;
 use shims::unix::linux::mem::EvalContextExt as _;
 use shims::unix::linux::sync::futex;
+use shims::unix::mem::EvalContextExt as _;
 use shims::unix::sync::EvalContextExt as _;
 use shims::unix::thread::EvalContextExt as _;
 
@@ -43,6 +44,14 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let result = this.linux_readdir64(dirp)?;
                 this.write_scalar(result, dest)?;
             }
+            "mmap64" => {
+                let [addr, length, prot, flags, fd, offset] =
+                    this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
+                let offset = this.read_scalar(offset)?.to_i64()?;
+                let ptr = this.mmap(addr, length, prot, flags, fd, offset.into())?;
+                this.write_scalar(ptr, dest)?;
+            }
+
             // Linux-only
             "sync_file_range" => {
                 let [fd, offset, nbytes, flags] =
diff --git a/src/shims/unix/mem.rs b/src/shims/unix/mem.rs
index d7dc17fa89..d3470893db 100644
--- a/src/shims/unix/mem.rs
+++ b/src/shims/unix/mem.rs
@@ -26,7 +26,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         prot: &OpTy<'tcx, Provenance>,
         flags: &OpTy<'tcx, Provenance>,
         fd: &OpTy<'tcx, Provenance>,
-        offset: &OpTy<'tcx, Provenance>,
+        offset: i128,
     ) -> InterpResult<'tcx, Scalar<Provenance>> {
         let this = self.eval_context_mut();
 
@@ -36,7 +36,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         let prot = this.read_scalar(prot)?.to_i32()?;
         let flags = this.read_scalar(flags)?.to_i32()?;
         let fd = this.read_scalar(fd)?.to_i32()?;
-        let offset = this.read_target_usize(offset)?;
 
         let map_private = this.eval_libc_i32("MAP_PRIVATE");
         let map_anonymous = this.eval_libc_i32("MAP_ANONYMOUS");
diff --git a/tests/pass-dep/shims/mmap.rs b/tests/pass-dep/shims/mmap.rs
index e19f54d068..7bbb9dd53c 100644
--- a/tests/pass-dep/shims/mmap.rs
+++ b/tests/pass-dep/shims/mmap.rs
@@ -5,16 +5,25 @@
 use std::io::Error;
 use std::{ptr, slice};
 
-fn test_mmap() {
+fn test_mmap<Offset: Default>(
+    mmap: unsafe extern "C" fn(
+        *mut libc::c_void,
+        libc::size_t,
+        libc::c_int,
+        libc::c_int,
+        libc::c_int,
+        Offset,
+    ) -> *mut libc::c_void,
+) {
     let page_size = page_size::get();
     let ptr = unsafe {
-        libc::mmap(
+        mmap(
             ptr::null_mut(),
             page_size,
             libc::PROT_READ | libc::PROT_WRITE,
             libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
             -1,
-            0,
+            Default::default(),
         )
     };
     assert!(!ptr.is_null());
@@ -35,40 +44,40 @@ fn test_mmap() {
 
     // Test all of our error conditions
     let ptr = unsafe {
-        libc::mmap(
+        mmap(
             ptr::null_mut(),
             page_size,
             libc::PROT_READ | libc::PROT_WRITE,
             libc::MAP_PRIVATE | libc::MAP_SHARED, // Can't be both private and shared
             -1,
-            0,
+            Default::default(),
         )
     };
     assert_eq!(ptr, libc::MAP_FAILED);
     assert_eq!(Error::last_os_error().raw_os_error().unwrap(), libc::EINVAL);
 
     let ptr = unsafe {
-        libc::mmap(
+        mmap(
             ptr::null_mut(),
             0, // Can't map no memory
             libc::PROT_READ | libc::PROT_WRITE,
             libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
             -1,
-            0,
+            Default::default(),
         )
     };
     assert_eq!(ptr, libc::MAP_FAILED);
     assert_eq!(Error::last_os_error().raw_os_error().unwrap(), libc::EINVAL);
 
     let ptr = unsafe {
-        libc::mmap(
+        mmap(
             ptr::invalid_mut(page_size * 64),
             page_size,
             libc::PROT_READ | libc::PROT_WRITE,
             // We don't support MAP_FIXED
             libc::MAP_PRIVATE | libc::MAP_ANONYMOUS | libc::MAP_FIXED,
             -1,
-            0,
+            Default::default(),
         )
     };
     assert_eq!(ptr, libc::MAP_FAILED);
@@ -77,13 +86,13 @@ fn test_mmap() {
     // We don't support protections other than read+write
     for prot in [libc::PROT_NONE, libc::PROT_EXEC, libc::PROT_READ, libc::PROT_WRITE] {
         let ptr = unsafe {
-            libc::mmap(
+            mmap(
                 ptr::null_mut(),
                 page_size,
                 prot,
                 libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
                 -1,
-                0,
+                Default::default(),
             )
         };
         assert_eq!(ptr, libc::MAP_FAILED);
@@ -93,13 +102,13 @@ fn test_mmap() {
     // We report an error for mappings whose length cannot be rounded up to a multiple of
     // the page size.
     let ptr = unsafe {
-        libc::mmap(
+        mmap(
             ptr::null_mut(),
             usize::MAX - 1,
             libc::PROT_READ | libc::PROT_WRITE,
             libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
             -1,
-            0,
+            Default::default(),
         )
     };
     assert_eq!(ptr, libc::MAP_FAILED);
@@ -163,7 +172,9 @@ fn test_mremap() {
 }
 
 fn main() {
-    test_mmap();
+    test_mmap(libc::mmap);
+    #[cfg(target_os = "linux")]
+    test_mmap(libc::mmap64);
     #[cfg(target_os = "linux")]
     test_mremap();
 }