[libc] Add a simple x86_64 linux loader.

This adds a very simple loader. This will be extended to a full loader
in future patches. A utility rule to add unittests has been added to
serve us while we are building out the full loader.

Reviewers: abrachet, phosek

Differential Revision: https://reviews.llvm.org/D76412

4 files changed, 113 insertions, 0 deletions

diff --git a/libc/loader/CMakeLists.txt b/libc/loader/CMakeLists.txt
new file mode 100644
index 0000000..b4bbe81c
--- /dev/null
+++ b/libc/loader/CMakeLists.txt
@@ -0,0 +1,3 @@
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+  add_subdirectory(${LIBC_TARGET_OS})
+endif()
diff --git a/libc/loader/linux/CMakeLists.txt b/libc/loader/linux/CMakeLists.txt
new file mode 100644
index 0000000..4ef4b88
--- /dev/null
+++ b/libc/loader/linux/CMakeLists.txt
@@ -0,0 +1,34 @@
+function(add_loader_object name)
+  cmake_parse_arguments(
+    "ADD_LOADER_OBJECT"
+    ""    # No option arguments
+    "SRC" # Single value arguments
+    "DEPENDS;COMPILE_OPTIONS" # Multi value arguments
+    ${ARGN}
+  )
+  add_object(
+    ${name}_object
+    SRC ${ADD_LOADER_OBJECT_SRC}
+    DEPENDS ${ADD_LOADER_OBJECT_DEPENDS}
+    COMPILE_OPTIONS ${ADD_LOADER_OBJECT_COMPILE_OPTIONS}
+  )
+
+  set(objfile ${LIBC_BUILD_DIR}/lib/${name}.o)
+  add_custom_command(
+    OUTPUT ${objfile}
+    COMMAND cp $<TARGET_OBJECTS:${name}_object> ${objfile}
+    DEPENDS $<TARGET_OBJECTS:${name}_object>
+  )
+  add_custom_target(
+    ${name}
+    DEPENDS ${objfile}
+  )
+  set_target_properties(
+    ${name}
+    PROPERTIES
+      "TARGET_TYPE" "LOADER_OBJECT"
+      "OBJECT_FILE" ${objfile}
+  )
+endfunction()
+
+add_subdirectory(${LIBC_TARGET_MACHINE})
diff --git a/libc/loader/linux/x86_64/CMakeLists.txt b/libc/loader/linux/x86_64/CMakeLists.txt
new file mode 100644
index 0000000..e7d7ee5
--- /dev/null
+++ b/libc/loader/linux/x86_64/CMakeLists.txt
@@ -0,0 +1,11 @@
+add_loader_object(
+  crt1
+  SRC
+    start.cpp
+  DEPENDS
+    linux_syscall_h
+    sys_syscall_h
+  COMPILE_OPTIONS
+    -fno-omit-frame-pointer
+    -ffreestanding # To avoid compiler warnings about calling the main function.
+)
diff --git a/libc/loader/linux/x86_64/start.cpp b/libc/loader/linux/x86_64/start.cpp
new file mode 100644
index 0000000..57af0b1
--- /dev/null
+++ b/libc/loader/linux/x86_64/start.cpp
@@ -0,0 +1,65 @@
+//===------------------ Implementation of crt for x86_64 ------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "config/linux/syscall.h"
+#include "include/sys/syscall.h"
+
+#include <linux/auxvec.h>
+#include <stdint.h>
+
+extern "C" int main(int, char **, char **);
+
+struct Args {
+  // At the language level, argc is an int. But we use uint64_t as the x86_64
+  // ABI specifies it as an 8 byte value.
+  uint64_t argc;
+
+  // At the language level, argv is a char** value. However, we use uint64_t as
+  // the x86_64 ABI specifies the argv vector be an |argc| long array of 8-byte
+  // values. Even though a flexible length array would be more suitable here, we
+  // set the array length to 1 to avoid a compiler warning about it being a C99
+  // extension. Length of 1 is not really wrong as |argc| is guaranteed to be
+  // atleast 1, and there is an 8-byte null entry at the end of the argv array.
+  uint64_t argv[1];
+};
+
+// TODO: Would be nice to use the aux entry structure from elf.h when available.
+struct AuxEntry {
+  uint64_t type;
+  uint64_t value;
+};
+
+extern "C" void _start() {
+  uintptr_t *frame_ptr =
+      reinterpret_cast<uintptr_t *>(__builtin_frame_address(0));
+
+  // This TU is compiled with -fno-omit-frame-pointer. Hence, the previous value
+  // of the base pointer is pushed on to the stack. So, we step over it (the
+  // "+ 1" below) to get to the args.
+  Args *args = reinterpret_cast<Args *>(frame_ptr + 1);
+
+  // After the argv array, is a 8-byte long NULL value before the array of env
+  // values. The end of the env values is marked by another 8-byte long NULL
+  // value. We step over it (the "+ 1" below) to get to the env values.
+  uint64_t *env_ptr = args->argv + args->argc + 1;
+  uint64_t *env_end_marker = env_ptr;
+  while (*env_end_marker)
+    ++env_end_marker;
+
+  // After the env array, is the aux-vector. The end of the aux-vector is
+  // denoted by an AT_NULL entry.
+  for (AuxEntry *aux_entry = reinterpret_cast<AuxEntry *>(env_end_marker + 1);
+       aux_entry->type != AT_NULL; ++aux_entry) {
+    // TODO: Read the aux vector and store necessary information in a libc wide
+    // data structure.
+  }
+
+  __llvm_libc::syscall(SYS_exit,
+                       main(args->argc, reinterpret_cast<char **>(args->argv),
+                            reinterpret_cast<char **>(env_ptr)));
+}