Fix "clone grant using fmap"

This does the same as the previous MR, but fixes the issue where the
parent process got the mapping (and at the wrong address) and not the
child process.

src/context/file.rs

@@ -7,7 +7,7 @@ use crate::scheme::{self, SchemeNamespace, SchemeId};
 use crate::syscall::error::{Result, Error, EBADF};
 
 /// A file description
-#[derive(Debug)]
+#[derive(Clone, Copy, Debug)]
 pub struct FileDescription {
     /// The namespace the file was opened from (used for debugging)
     pub namespace: SchemeNamespace,

src/context/memory.rs

@@ -36,12 +36,12 @@ pub fn page_flags(flags: MapFlags) -> PageFlags<RmmA> {
 }
 
 pub struct UnmapResult {
-    pub file_desc: Option<FileDescriptor>,
+    pub file_desc: Option<GrantFileRef>,
 }
 impl Drop for UnmapResult {
     fn drop(&mut self) {
         if let Some(fd) = self.file_desc.take() {
-            let _ = fd.close();
+            let _ = fd.desc.close();
         }
     }
 }
@@ -301,7 +301,15 @@ pub struct Grant {
     mapped: bool,
     owned: bool,
     //TODO: This is probably a very heavy way to keep track of fmap'd files, perhaps move to the context?
-    pub desc_opt: Option<FileDescriptor>,
+    pub desc_opt: Option<GrantFileRef>,
+}
+#[derive(Clone, Debug)]
+pub struct GrantFileRef {
+    pub desc: FileDescriptor,
+    pub offset: usize,
+    // TODO: Can the flags maybe be stored together with the page flags. Should some flags be kept,
+    // and others discarded when re-fmapping on clone?
+    pub flags: MapFlags,
 }
 
 impl Grant {
@@ -374,7 +382,7 @@ impl Grant {
         }
     }
 
-    pub fn map_inactive(src: VirtualAddress, dst: VirtualAddress, size: usize, flags: PageFlags<RmmA>, desc_opt: Option<FileDescriptor>, inactive_table: &mut InactivePageTable) -> Grant {
+    pub fn map_inactive(src: VirtualAddress, dst: VirtualAddress, size: usize, flags: PageFlags<RmmA>, desc_opt: Option<GrantFileRef>, inactive_table: &mut InactivePageTable) -> Grant {
         let active_table = unsafe { ActivePageTable::new(src.kind()) };
         let mut inactive_mapper = inactive_table.mapper();