[kernel] Track capability reference counts

First pass at reference-counting capabilities.

src/kernel/capabilities.cpp

@@ -20,6 +20,8 @@ cap_table::create(obj::kobject *target, j6_cap_t caps)
 {
     j6_handle_t new_handle = make_handle(m_count);
 
+    util::scoped_lock lock {m_lock};
+
     m_caps.insert({
         new_handle,
         j6_handle_invalid,
@@ -29,12 +31,17 @@ cap_table::create(obj::kobject *target, j6_cap_t caps)
         target,
         });
 
+    if (target)
+        target->handle_retain();
+
     return new_handle;
 }
 
 j6_handle_t
 cap_table::derive(j6_handle_t base, j6_cap_t caps)
 {
+    util::scoped_lock lock {m_lock};
+
     capability *existing = m_caps.find(base);
     if (!existing)
         return j6_handle_invalid;
@@ -52,11 +59,43 @@ cap_table::derive(j6_handle_t base, j6_cap_t caps)
         existing->object,
         });
 
+    if (existing->object)
+        existing->object->handle_retain();
+
     return new_handle;
 }
 
 capability *
-cap_table::find(j6_handle_t id)
+cap_table::find_without_retain(j6_handle_t id)
 {
     return m_caps.find(id);
 }
+
+capability *
+cap_table::retain(j6_handle_t id)
+{
+    util::scoped_lock lock {m_lock};
+
+    capability *cap = m_caps.find(id);
+    if (!cap)
+        return nullptr;
+
+    ++cap->holders;
+    return cap;
+}
+
+void
+cap_table::release(j6_handle_t id)
+{
+    util::scoped_lock lock {m_lock};
+
+    capability *cap = m_caps.find(id);
+    if (!cap)
+        return;
+
+    if (--cap->holders == 0) {
+        if (cap->object)
+            cap->object->handle_release();
+        m_caps.erase(id);
+    }
+}

src/kernel/capabilities.h

@@ -33,8 +33,16 @@ public:
     /// Create a new capability from an existing one.
     j6_handle_t derive(j6_handle_t id, j6_cap_t caps);
 
-    /// Get the capability referenced by a handle
+    /// Get the capability referenced by a handle.
-    capability * find(j6_handle_t id);
+    capability * retain(j6_handle_t id);
+
+    /// Release a retained capability when it is no longer being used.
+    void release(j6_handle_t id);
+
+    /// Get the capability referenced by a handle, without increasing
+    /// its refcount. WARNING: You probably want retain(), using a capability
+    /// (or the object it points to) without retaining it is dangerous.
+    capability * find_without_retain(j6_handle_t id);
 
 private:
     using map_type = util::inplace_map<j6_handle_t, capability, j6_handle_invalid>;

src/kernel/objects/process.cpp

@@ -1,3 +1,5 @@
+#include <new>
+
 #include <util/no_construct.h>
 
 #include "assert.h"
@@ -119,28 +121,44 @@ process::thread_exited(thread *th)
 void
 process::add_handle(j6_handle_t handle)
 {
+    capability *c = g_cap_table.retain(handle);
+    kassert(c, "Trying to add a non-existant handle to a process!");
+
+    if (c) {
+        util::scoped_lock lock {m_handles_lock};
         m_handles.add(handle);
     }
-
-bool
-process::remove_handle(j6_handle_t id)
-{
-    return m_handles.remove(id);
 }
 
 bool
-process::has_handle(j6_handle_t id)
+process::remove_handle(j6_handle_t handle)
 {
-    return m_handles.contains(id);
+    util::scoped_lock lock {m_handles_lock};
+    bool removed = m_handles.remove(handle);
+    lock.release();
+
+    if (removed)
+        g_cap_table.release(handle);
+
+    return removed;
+}
+
+bool
+process::has_handle(j6_handle_t handle)
+{
+    util::scoped_lock lock {m_handles_lock};
+    return m_handles.contains(handle);
 }
 
 size_t
 process::list_handles(j6_handle_descriptor *handles, size_t len)
 {
+    util::scoped_lock lock {m_handles_lock};
+
     size_t count = 0;
     for (j6_handle_t handle : m_handles) {
 
-        capability *cap = g_cap_table.find(handle);
+        capability *cap = g_cap_table.find_without_retain(handle);
         kassert(cap, "Found process handle that wasn't in the cap table");
         if (!cap) continue;
 

src/kernel/objects/process.h

@@ -3,7 +3,6 @@
 /// Definition of process kobject types
 
 #include <j6/cap_flags.h>
-#include <util/map.h>
 #include <util/node_map.h>
 #include <util/vector.h>
 
@@ -114,6 +113,7 @@ private:
     util::vector<thread*> m_threads;
 
     util::node_set<j6_handle_t, j6_handle_invalid, heap_allocated> m_handles;
+    util::spinlock m_handles_lock;
 
     enum class state : uint8_t { running, exited };
     state m_state;

src/kernel/syscall_verify.cpp.cog

@@ -184,10 +184,22 @@ for id, scope, method in syscalls.methods:
         cog.outl()
 
     if "noreturn" in method.options:
+        if handles:
+            cog.outl(f"""   static_assert(false, "Cannot define a `noreturn` syscall that holds handles.");""")
         cog.outl(f"""    {name}({", ".join(args)});""")
+
     else:
+        cog.outl("    j6_status_t _retval;")
+        cog.outl("    {")
         cog.outl(f"""        profiler<syscall_profiles, {id}> profile {{"{name}"}};""")
-        cog.outl(f"""    return {name}({", ".join(args)});""")
+        cog.outl(f"""        _retval = {name}({", ".join(args)});""")
+        cog.outl("    }\n")
+
+        for type, arg, caps in handles:
+            cog.outl(f"    release_handle({arg});")
+
+        cog.outl(f"""    return _retval;""")
+
     cog.outl("}")
     cog.outl("\n")
 ]]]*/

src/kernel/syscalls/helpers.h

@@ -30,7 +30,7 @@ j6_status_t get_handle(j6_handle_t id, j6_cap_t caps, T *&object)
         return j6_status_ok;
     }
 
-    capability *capdata = g_cap_table.find(id);
+    capability *capdata = g_cap_table.retain(id);
     if (!capdata || capdata->type != T::type)
         return j6_err_invalid_arg;
 
@@ -56,7 +56,7 @@ inline j6_status_t get_handle<obj::kobject>(j6_handle_t id, j6_cap_t caps, obj::
         return j6_status_ok;
     }
 
-    capability *capdata = g_cap_table.find(id);
+    capability *capdata = g_cap_table.retain(id);
     if (!capdata)
         return j6_err_invalid_arg;
 
@@ -68,4 +68,7 @@ inline j6_status_t get_handle<obj::kobject>(j6_handle_t id, j6_cap_t caps, obj::
     return j6_status_ok;
 }
 
+inline void release_handle(j6_handle_t id) { g_cap_table.release(id); }
+inline void release_handle(j6_handle_t *id) { g_cap_table.release(*id); }
+
 } // namespace syscalls