Kernel: Make Credentials the authority on process SID

The SID was duplicated between the process credentials and protected
data. And to make matters worse, the credentials SID was not updated in
sys$setsid.

This patch fixes this by removing the SID from protected data and
updating the credentials SID everywhere.

Kernel/Process.h

@@ -114,7 +114,6 @@ class Process final
     public:
         ProcessID pid { 0 };
         ProcessID ppid { 0 };
-        SessionID sid { 0 };
         // FIXME: This should be a NonnullRefPtr
         RefPtr<Credentials> credentials;
         RefPtr<ProcessGroup> process_group;
@@ -233,10 +232,7 @@ public:
     {
         return with_protected_data([](auto& protected_data) { return protected_data.pid; });
     }
-    SessionID sid() const
+    SessionID sid() const { return credentials()->sid(); }
-    {
-        return with_protected_data([](auto& protected_data) { return protected_data.sid; });
-    }
     bool is_session_leader() const { return sid().value() == pid().value(); }
     ProcessGroupID pgid() const
     {

Kernel/Syscalls/fork.cpp

@@ -103,7 +103,6 @@ ErrorOr<FlatPtr> Process::sys$fork(RegisterState& regs)
             child_protected_data.execpromises = my_protected_data.execpromises.load();
             child_protected_data.has_promises = my_protected_data.has_promises.load();
             child_protected_data.has_execpromises = my_protected_data.has_execpromises.load();
-            child_protected_data.sid = my_protected_data.sid;
             child_protected_data.credentials = my_protected_data.credentials;
             child_protected_data.umask = my_protected_data.umask;
             child_protected_data.signal_trampoline = my_protected_data.signal_trampoline;

Kernel/Syscalls/setpgid.cpp

@@ -32,12 +32,26 @@ ErrorOr<FlatPtr> Process::sys$setsid()
     // NOTE: ProcessGroup::create_if_unused_pgid() will fail with EPERM
     //       if a process group with the same PGID already exists.
     auto process_group = TRY(ProcessGroup::create_if_unused_pgid(ProcessGroupID(pid().value())));
-    return with_mutable_protected_data([&](auto& protected_data) -> ErrorOr<FlatPtr> {
+
+    auto new_sid = SessionID(pid().value());
+    auto credentials = this->credentials();
+    auto new_credentials = TRY(Credentials::create(
+        credentials->uid(),
+        credentials->gid(),
+        credentials->euid(),
+        credentials->egid(),
+        credentials->suid(),
+        credentials->sgid(),
+        credentials->extra_gids(),
+        new_sid,
+        credentials->pgid()));
+
+    with_mutable_protected_data([&](auto& protected_data) {
         protected_data.tty = nullptr;
         protected_data.process_group = move(process_group);
-        protected_data.sid = pid().value();
+        protected_data.credentials = move(new_credentials);
-        return protected_data.sid.value();
     });
+    return new_sid.value();
 }
 
 ErrorOr<FlatPtr> Process::sys$getpgid(pid_t pid)
@@ -114,7 +128,7 @@ ErrorOr<FlatPtr> Process::sys$setpgid(pid_t specified_pid, pid_t specified_pgid)
     }
     // FIXME: There are more EPERM conditions to check for here..
     auto process_group = TRY(ProcessGroup::find_or_create(new_pgid));
-    return process->with_mutable_protected_data([&process, &process_group, new_sid, new_pgid](auto& protected_data) -> ErrorOr<FlatPtr> {
+    return process->with_mutable_protected_data([&process, &process_group, new_pgid](auto& protected_data) -> ErrorOr<FlatPtr> {
         auto credentials = process->credentials();
 
         auto new_credentials = TRY(Credentials::create(
@@ -125,7 +139,7 @@ ErrorOr<FlatPtr> Process::sys$setpgid(pid_t specified_pid, pid_t specified_pgid)
             credentials->suid(),
             credentials->sgid(),
             credentials->extra_gids(),
-            new_sid,
+            credentials->sid(),
             new_pgid));
 
         protected_data.credentials = move(new_credentials);