[ld.so] Add a minimal dynamic linker

This commit includes a number of changes to enable loading of PIE
executables:

- The loader in srv.init checks for a `PT_INTERP` segment in the program
  its loading, and if it exists, loads the specified interpreter and
  passes control to it instead of the program itself.
- Added ld.so the dynamic linker executable and set it as the
  interpreter for all user-target programs.
- Program initial stack changed again to now contain a number of
  possible tagged structures, including a new one for ld.so's arguments,
  and for passing handles tagged with protocol ids.
- Added a stub for a new VFS protocol. Unused so far, but srv.init will
  need to serve VFS requests from ld.so once I transition libraries to
  shared libs for user-target programs. (Right now all executables are
  PIE but statically linked, so they only need internal relocations.)
- Added 16 and 8 bit variants of `util::bitset`. This ended up not being
  used, but could be useful.

src/libraries/j6/include/j6/init.h

@@ -2,17 +2,74 @@
 /// \file init.h
 /// Process initialization utility functions
 
+#include <stdint.h>
 #include <j6/types.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+#ifdef __cplusplus
+#define add_header(name) \
+    static constexpr j6_arg_type type_id = j6_arg_type_ ## name; \
+    j6_arg_header header;
+#else
+#define add_header(name) \
+    j6_arg_header header;
+#endif
+
+enum j6_arg_type {
+    j6_arg_type_none,
+    j6_arg_type_sysv_init,
+    j6_arg_type_loader,
+    j6_arg_type_driver,
+    j6_arg_type_handles,
+};
+
+struct j6_arg_header
+{
+    uint32_t size;
+    uint16_t type;
+};
+
+struct j6_arg_loader
+{
+    add_header(loader);
+    uintptr_t loader_base;
+    uintptr_t image_base;
+    uintptr_t phdr;
+    size_t phdr_size;
+    size_t phdr_count;
+    uintptr_t entrypoint;
+};
+
+struct j6_arg_driver
+{
+    add_header(driver);
+    uint64_t device;
+    uint8_t data [0];
+};
+
+struct j6_arg_handle_entry
+{
+    uint64_t proto;
+    j6_handle_t handle;
+};
+
+struct j6_arg_handles
+{
+    add_header(handles);
+    size_t nhandles;
+    j6_arg_handle_entry handles[0];
+};
+
 struct j6_init_args
 {
     uint64_t args[2];
 };
 
+
+
 /// Find the first handle of the given type held by this process
 j6_handle_t j6_find_first_handle(j6_object_type obj_type);
 
@@ -22,3 +79,5 @@ int driver_main(unsigned, const char **, const char **, const j6_init_args *);
 #ifdef __cplusplus
 } // extern "C"
 #endif
+
+#undef add_header

src/libraries/j6/include/j6/protocols/vfs.h

@@ -0,0 +1,11 @@
+#pragma once
+/// \file j6/protocols/vfs.h
+/// Definitions for the virtual file system protocol
+
+#include <j6/protocols.h>
+
+enum j6_proto_vfs_tag
+{
+    j6_proto_vfs_load = j6_proto_base_first_proto_id,
+    j6_proto_vfs_file,
+};

src/libraries/j6/include/j6/protocols/vfs.hh

@@ -0,0 +1,22 @@
+#include <j6/protocols/vfs.h>
+#include <j6/types.h>
+
+namespace j6::proto::vfs {
+
+class client
+{
+public:
+    /// Constructor.
+    /// \arg vfs_mb  Handle to the VFS service's mailbox
+    client(j6_handle_t vfs_mb);
+
+    /// Load a file into a VMA
+    /// \arg path  Path of the file to load
+    /// \arg vma   [out] Handle to the loaded VMA, or invalid if not found
+    j6_status_t load_file(char *path, j6_handle_t &vma);
+
+private:
+    j6_handle_t m_service;
+};
+
+} // namespace j6::proto::vfs

src/libraries/j6/j6.module

@@ -12,6 +12,7 @@ j6 = module("j6",
         "mutex.cpp",
         "protocol_ids.cpp",
         "protocols/service_locator.cpp",
+        "protocols/vfs.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",
         "syslog.cpp",

src/libraries/j6/protocols/vfs.cpp

@@ -0,0 +1,57 @@
+#include <j6/errors.h>
+#include <j6/protocols/vfs.hh>
+#include <j6/syscalls.h>
+#include <j6/syslog.hh>
+
+#ifndef __j6kernel
+
+namespace j6::proto::vfs {
+
+client::client(j6_handle_t vfs_mb) :
+    m_service {vfs_mb}
+{
+}
+
+inline size_t simple_strlen(const char *s) { size_t n = 0; while (s && *s) n++; return n; }
+
+j6_status_t
+client::load_file(char *path, j6_handle_t &vma)
+{
+    if (!path)
+        return j6_err_invalid_arg;
+
+    uint64_t tag = j6_proto_vfs_load;
+    size_t handle_count = 1;
+    vma = j6_handle_invalid;
+
+    // Always need to send a big enough buffer for a status code
+    size_t path_len = simple_strlen(path);
+    size_t data_len = path_len;
+    char *data = path;
+    j6_status_t alternate = 0;
+    if (path_len < sizeof(alternate)) {
+        data = reinterpret_cast<char*>(alternate);
+        for (unsigned i = 0; i < path_len; ++i)
+            data[i] = path[i];
+        data_len = sizeof(alternate);
+    }
+
+    j6_status_t s = j6_mailbox_call(m_service, &tag,
+        data, &data_len, path_len,
+        &vma, &handle_count);
+
+    if (s != j6_status_ok)
+        return s;
+
+    if (tag == j6_proto_vfs_file)
+        return j6_status_ok; // handle is already in `vma`
+
+    else if (tag == j6_proto_base_status)
+        return *reinterpret_cast<j6_status_t*>(data); // contains a status
+
+    return j6_err_unexpected;
+}
+
+
+} // namespace j6::proto::vfs
+#endif // __j6kernel