mach: ignore fileoff only when filesize is empty

Fixes #172

src/mach/segment.rs

@@ -388,6 +388,16 @@ impl<'a> ctx::IntoCtx<container::Ctx> for Segment<'a> {
     }
 }
 
+/// Read data that belongs to a segment if the offset is within the boundaries of bytes.
+fn segment_data(bytes: &[u8], fileoff :u64, filesize :u64) -> Result<&[u8], error::Error> {
+    let data :&[u8] = if filesize != 0 {
+        bytes.pread_with(fileoff as usize, filesize as usize)?
+    } else {
+        &[]
+    };
+    Ok(data)
+}
+
 impl<'a> Segment<'a> {
     /// Create a new, blank segment, with cmd either `LC_SEGMENT_64`, or `LC_SEGMENT`, depending on `ctx`.
     /// **NB** You are responsible for providing a correctly marshalled byte array as the sections. You should not use this for anything other than writing.
@@ -424,7 +434,6 @@ impl<'a> Segment<'a> {
     }
     /// Convert the raw C 32-bit segment command to a generalized version
     pub fn from_32(bytes: &'a[u8], segment: &SegmentCommand32, offset: usize, ctx: container::Ctx) -> Result<Self, error::Error> {
-        let data = bytes.pread_with(segment.fileoff as usize, segment.filesize as usize)?;
         Ok(Segment {
             cmd:      segment.cmd,
             cmdsize:  segment.cmdsize,
@@ -437,15 +446,14 @@ impl<'a> Segment<'a> {
             initprot: segment.initprot,
             nsects:   segment.nsects,
             flags:    segment.flags,
-            data,
+            data: segment_data(bytes, segment.fileoff as u64, segment.filesize as u64)?,
             offset,
             raw_data: bytes,
             ctx,
         })
     }
     /// Convert the raw C 64-bit segment command to a generalized version
     pub fn from_64(bytes: &'a [u8], segment: &SegmentCommand64, offset: usize, ctx: container::Ctx) -> Result<Self, error::Error> {
-        let data = bytes.pread_with(segment.fileoff as usize, segment.filesize as usize)?;
         Ok(Segment {
             cmd:      segment.cmd,
             cmdsize:  segment.cmdsize,
@@ -458,8 +466,8 @@ impl<'a> Segment<'a> {
             initprot: segment.initprot,
             nsects:   segment.nsects,
             flags:    segment.flags,
+            data: segment_data(bytes, segment.fileoff, segment.filesize)?,
             offset,
-            data,
             raw_data: bytes,
             ctx,
         })