Correctly handle MAP_FIXED with a zero address

src/scheme/memory.rs

@@ -90,9 +90,11 @@ impl MemoryScheme {
             return Err(Error::new(EOPNOTSUPP));
         }
 
+        let fixed = map.flags.contains(MapFlags::MAP_FIXED) || map.flags.contains(MapFlags::MAP_FIXED_NOREPLACE);
+
         let page = addr_space.acquire_write().mmap(
             addr_space,
-            (map.address != 0).then_some(span.base),
+            (map.address != 0 || fixed).then_some(span.base),
             page_count,
             map.flags,
             &mut notify_files,

src/scheme/proc.rs

@@ -502,7 +502,9 @@ impl KernelScheme for ProcScheme {
                     PageSpan::validate_nonempty(VirtualAddress::new(map.offset), map.size)
                         .ok_or(Error::new(EINVAL))?;
 
-                let requested_dst_base = (map.address != 0).then_some(requested_dst_page);
+                let fixed = map.flags.contains(MapFlags::MAP_FIXED)
+                    || map.flags.contains(MapFlags::MAP_FIXED_NOREPLACE);
+                let requested_dst_base = (map.address != 0 || fixed).then_some(requested_dst_page);
 
                 let mut src_addr_space = addrspace.acquire_write();
 

src/scheme/user.rs

@@ -1234,7 +1234,10 @@ impl UserInner {
         if map.address % PAGE_SIZE != 0 {
             return Err(Error::new(EINVAL));
         };
-        let dst_base = (map.address != 0)
+
+        let fixed = map.flags.contains(MapFlags::MAP_FIXED)
+            || map.flags.contains(MapFlags::MAP_FIXED_NOREPLACE);
+        let dst_base = (map.address != 0 || fixed)
             .then_some(Page::containing_address(VirtualAddress::new(map.address)));
 
         if map.offset % PAGE_SIZE != 0 {

src/syscall/fs.rs

@@ -744,7 +744,9 @@ pub fn mremap(
     let addr_space = AddrSpace::current()?;
     let src_span = PageSpan::new(old_base, old_size.div_ceil(PAGE_SIZE));
     let new_page_count = new_size.div_ceil(PAGE_SIZE);
-    let requested_dst_base = Some(new_base).filter(|_| new_address != 0);
+    let fixed = map_flags.contains(MapFlags::MAP_FIXED)
+        || map_flags.contains(MapFlags::MAP_FIXED_NOREPLACE);
+    let requested_dst_base = (new_address != 0 || fixed).then_some(new_base);
 
     if mremap_flags.contains(MremapFlags::KEEP_OLD) {
         // TODO: This is a hack! Find a better interface for replacing this, perhaps a capability