Kernel+LibC: Add MAP_RANDOMIZED flag for sys$mmap()

This can be used to request random VM placement instead of the highly
predictable regular mmap(nullptr, ...) VM allocation strategy.

It will soon be used to implement ASLR in the dynamic loader. :^)

1 files changed, 15 insertions, 7 deletions

diff --git a/Kernel/Syscalls/mmap.cpp b/Kernel/Syscalls/mmap.cpp
index aa555b71f3..be1ebdf818 100644
--- a/Kernel/Syscalls/mmap.cpp
+++ b/Kernel/Syscalls/mmap.cpp
@@ -119,6 +119,7 @@ void* Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> user_params)
     bool map_stack = flags & MAP_STACK;
     bool map_fixed = flags & MAP_FIXED;
     bool map_noreserve = flags & MAP_NORESERVE;
+    bool map_randomized = flags & MAP_RANDOMIZED;
 
     if (map_shared && map_private)
         return (void*)-EINVAL;
@@ -133,16 +134,23 @@ void* Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> user_params)
         return (void*)-EINVAL;
 
     Region* region = nullptr;
-    auto range = allocate_range(VirtualAddress(addr), size, alignment);
-    if (!range.has_value()) {
-        if (addr && !map_fixed) {
-            // If there's an address but MAP_FIXED wasn't specified, the address is just a hint.
-            range = allocate_range({}, size, alignment);
+    Optional<Range> range;
+
+    if (map_randomized) {
+        range = page_directory().range_allocator().allocate_randomized(size, alignment);
+    } else {
+        range = allocate_range(VirtualAddress(addr), size, alignment);
+        if (!range.has_value()) {
+            if (addr && !map_fixed) {
+                // If there's an address but MAP_FIXED wasn't specified, the address is just a hint.
+                range = allocate_range({}, size, alignment);
+            }
         }
-        if (!range.has_value())
-            return (void*)-ENOMEM;
     }
 
+    if (!range.has_value())
+        return (void*)-ENOMEM;
+
     if (map_anonymous) {
         auto strategy = map_noreserve ? AllocationStrategy::None : AllocationStrategy::Reserve;
         auto region_or_error = allocate_region(range.value(), !name.is_null() ? name : "mmap", prot, strategy);