Fix integer overflow during file length calculation on 32bit targets.

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
+### Fixed
+- Integer overflow during file length calculation on 32bit targets.
 
 ## [0.3.0] - 2021-06-10
 ### Changed

src/lib.rs

@@ -188,13 +188,25 @@ impl MmapOptions {
         self.len.map(Ok).unwrap_or_else(|| {
             let desc = file.as_raw_desc();
             let file_len = file_len(desc.0)?;
-            let len = file_len as u64 - self.offset;
-            if len > (usize::MAX as u64) {
+
+            if file_len < self.offset {
                 return Err(Error::new(
                     ErrorKind::InvalidData,
-                    "memory map length overflows usize",
+                    "memory map offset is larger than length",
                 ));
             }
+            let len = file_len - self.offset;
+
+            #[cfg(not(target_pointer_width = "64"))]
+            {
+                if len > (usize::MAX as u64) {
+                    return Err(Error::new(
+                        ErrorKind::InvalidData,
+                        "memory map length overflows usize",
+                    ));
+                }
+            }
+
             Ok(len as usize)
         })
     }

src/stub.rs

@@ -75,6 +75,6 @@ impl MmapInner {
     }
 }
 
-pub fn file_len(file: &File) -> io::Result<usize> {
-    Ok(file.metadata()?.len() as usize)
+pub fn file_len(file: &File) -> io::Result<u64> {
+    Ok(file.metadata()?.len())
 }

src/unix.rs

@@ -233,13 +233,13 @@ fn page_size() -> usize {
     unsafe { libc::sysconf(libc::_SC_PAGESIZE) as usize }
 }
 
-pub fn file_len(file: RawFd) -> io::Result<usize> {
+pub fn file_len(file: RawFd) -> io::Result<u64> {
     unsafe {
         let mut stat: libc::stat = std::mem::zeroed();
 
         let result = libc::fstat(file, &mut stat);
         if result == 0 {
-            Ok(stat.st_size as usize)
+            Ok(stat.st_size as u64)
         } else {
             Err(io::Error::last_os_error())
         }

src/windows.rs

@@ -435,6 +435,6 @@ fn allocation_granularity() -> usize {
     }
 }
 
-pub fn file_len(file: &File) -> io::Result<usize> {
-    Ok(file.metadata()?.len() as usize)
+pub fn file_len(file: &File) -> io::Result<u64> {
+    Ok(file.metadata()?.len())
 }