Kernel: Expose a process's filesystem root as a /proc/PID/root symlink

In order to preserve the absolute path of the process root, we save the
custody used by chroot() before stripping it to become the new "/".
There's probably a better way to do this.

Kernel/FileSystem/ProcFS.cpp

@@ -80,6 +80,7 @@ enum ProcFileType {
     FI_PID_fds,
     FI_PID_exe, // symlink
     FI_PID_cwd, // symlink
+    FI_PID_root, // symlink
     FI_PID_fd,  // directory
     __FI_PID_End,
 
@@ -571,6 +572,14 @@ Optional<KBuffer> procfs$pid_cwd(InodeIdentifier identifier)
     return handle->process().current_directory().absolute_path().to_byte_buffer();
 }
 
+Optional<KBuffer> procfs$pid_root(InodeIdentifier identifier)
+{
+    auto handle = ProcessInspectionHandle::from_pid(to_pid(identifier));
+    if (!handle)
+        return {};
+    return handle->process().root_directory_for_procfs().absolute_path().to_byte_buffer();
+}
+
 Optional<KBuffer> procfs$self(InodeIdentifier)
 {
     char buffer[16];
@@ -1030,6 +1039,7 @@ InodeMetadata ProcFSInode::metadata() const
         break;
     case FI_PID_cwd:
     case FI_PID_exe:
+    case FI_PID_root:
         metadata.mode = 0120400;
         break;
     case FI_Root:
@@ -1408,6 +1418,7 @@ ProcFS::ProcFS()
     m_entries[FI_PID_fds] = { "fds", FI_PID_fds, false, procfs$pid_fds };
     m_entries[FI_PID_exe] = { "exe", FI_PID_exe, false, procfs$pid_exe };
     m_entries[FI_PID_cwd] = { "cwd", FI_PID_cwd, false, procfs$pid_cwd };
+    m_entries[FI_PID_root] = { "root", FI_PID_root, false, procfs$pid_root };
     m_entries[FI_PID_fd] = { "fd", FI_PID_fd, false };
 }
 

Kernel/Process.cpp

@@ -4146,6 +4146,7 @@ int Process::sys$chroot(const char* user_path, size_t path_length)
     auto directory_or_error = VFS::the().open_directory(path.value(), current_directory());
     if (directory_or_error.is_error())
         return directory_or_error.error();
+    m_root_directory_for_procfs = directory_or_error.value();
     set_root_directory(Custody::create(nullptr, "", directory_or_error.value()->inode()));
     return 0;
 }
@@ -4157,6 +4158,13 @@ Custody& Process::root_directory()
     return *m_root_directory;
 }
 
+Custody& Process::root_directory_for_procfs()
+{
+    if (!m_root_directory_for_procfs)
+        m_root_directory_for_procfs = root_directory();
+    return *m_root_directory_for_procfs;
+}
+
 void Process::set_root_directory(const Custody& root)
 {
     m_root_directory = root;

Kernel/Process.h

@@ -311,6 +311,7 @@ public:
     u32 priority_boost() const { return m_priority_boost; }
 
     Custody& root_directory();
+    Custody& root_directory_for_procfs();
     void set_root_directory(const Custody&);
 
 private:
@@ -374,6 +375,7 @@ private:
     RefPtr<Custody> m_executable;
     RefPtr<Custody> m_cwd;
     RefPtr<Custody> m_root_directory;
+    RefPtr<Custody> m_root_directory_for_procfs;
 
     RefPtr<TTY> m_tty;