Kernel: Add a basic implementation of unveil()

This syscall is a complement to pledge() and adds the same sort of
incremental relinquishing of capabilities for filesystem access.

The first call to unveil() will "drop a veil" on the process, and from
now on, only unveiled parts of the filesystem are visible to it.

Each call to unveil() specifies a path to either a directory or a file
along with permissions for that path. The permissions are a combination
of the following:

- r: Read access (like the "rpath" promise)
- w: Write access (like the "wpath" promise)
- x: Execute access
- c: Create/remove access (like the "cpath" promise)

Attempts to open a path that has not been unveiled with fail with
ENOENT. If the unveiled path lacks sufficient permissions, it will fail
with EACCES.

Like pledge(), subsequent calls to unveil() with the same path can only
remove permissions, not add them.

Once you call unveil(nullptr, nullptr), the veil is locked, and it's no
longer possible to unveil any more paths for the process, ever.

This concept comes from OpenBSD, and their implementation does various
things differently, I'm sure. This is just a first implementation for
SerenityOS, and we'll keep improving on it as we go. :^)

Kernel/Process.h

@@ -82,6 +82,24 @@ enum class Pledge : u32 {
 #undef __ENUMERATE_PLEDGE_PROMISE
 };
 
+enum class UnveilState {
+    None,
+    VeilDropped,
+    VeilLocked,
+};
+
+struct UnveiledPath {
+    enum Access {
+        Read = 1,
+        Write = 2,
+        Execute = 4,
+        CreateOrRemove = 8,
+    };
+
+    String path;
+    unsigned permissions { 0 };
+};
+
 class Process : public InlineLinkedListNode<Process>
     , public Weakable<Process> {
     friend class InlineLinkedListNode<Process>;
@@ -282,6 +300,7 @@ public:
     int sys$set_process_boost(pid_t, int amount);
     int sys$chroot(const char* path, size_t path_length, int mount_flags);
     int sys$pledge(const Syscall::SC_pledge_params*);
+    int sys$unveil(const Syscall::SC_unveil_params*);
 
     static void initialize();
 
@@ -380,6 +399,9 @@ public:
     bool has_promises() const { return m_promises; }
     bool has_promised(Pledge pledge) const { return m_promises & (1u << (u32)pledge); }
 
+    UnveilState unveil_state() const { return m_unveil_state; }
+    const Vector<UnveiledPath>& unveiled_paths() const { return m_unveiled_paths; }
+
 private:
     friend class MemoryManager;
     friend class Scheduler;
@@ -481,6 +503,9 @@ private:
     u32 m_promises { 0 };
     u32 m_execpromises { 0 };
 
+    UnveilState m_unveil_state { UnveilState::None };
+    Vector<UnveiledPath> m_unveiled_paths;
+
     WaitQueue& futex_queue(i32*);
     HashMap<u32, OwnPtr<WaitQueue>> m_futex_queues;
 };