Move some basic early process init into Service Manager

content/common/set_process_title_linux.cc

-// Copyright (c) 2009 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-// This file implements BSD-style setproctitle() for Linux.
-// It is written such that it can easily be compiled outside Chromium.
-//
-// The Linux kernel sets up two locations in memory to pass arguments and
-// environment variables to processes. First, there are two char* arrays stored
-// one after another: argv and environ. A pointer to argv is passed to main(),
-// while glibc sets the global variable |environ| to point at the latter. Both
-// of these arrays are terminated by a NULL pointer; the environment array is
-// also followed by some empty space to allow additional variables to be added.
-//
-// These arrays contain pointers to a second location in memory, where the
-// strings themselves are stored one after another: first all the arguments,
-// then the environment variables. The kernel will allocate a single page of
-// memory for this purpose, so the end of the page containing argv[0] is the
-// end of the storage potentially available to store the process title.
-//
-// When the kernel reads the command line arguments for a process, it looks at
-// the range of memory within this page that it initially used for the argument
-// list. If the terminating '\0' character is still where it expects, nothing
-// further is done. If it has been overwritten, the kernel will scan up to the
-// size of a page looking for another. (Note, however, that in general not that
-// much space is actually mapped, since argv[0] is rarely page-aligned and only
-// one page is mapped.)
-//
-// Thus to change the process title, we must move any environment variables out
-// of the way to make room for a potentially longer title, and then overwrite
-// the memory pointed to by argv[0] with a single replacement string, making
-// sure its size does not exceed the available space.
-//
-// It is perhaps worth noting that patches to add a system call to Linux for
-// this, like in BSD, have never made it in: this is the "official" way to do
-// this on Linux. Presumably it is not in glibc due to some disagreement over
-// this position within the glibc project, leaving applications caught in the
-// middle. (Also, only a very few applications need or want this anyway.)
-
-#include "content/common/set_process_title_linux.h"
-
-#include <stdarg.h>
-#include <stddef.h>
-#include <stdint.h>
-#include <stdio.h>
-#include <string.h>
-#include <unistd.h>
-
-extern char** environ;
-
-static char** g_main_argv = NULL;
-static char* g_orig_argv0 = NULL;
-
-void setproctitle(const char* fmt, ...) {
-  va_list ap;
-  size_t i, avail_size;
-  uintptr_t page_size, page, page_end;
-  // Sanity check before we try and set the process title.
-  // The BSD version allows fmt == NULL to restore the original title.
-  if (!g_main_argv || !environ || !fmt)
-    return;
-  if (!g_orig_argv0) {
-    // Save the original argv[0].
-    g_orig_argv0 = strdup(g_main_argv[0]);
-    if (!g_orig_argv0)
-      return;
-  }
-  page_size = sysconf(_SC_PAGESIZE);
-  // Get the page on which the argument list and environment live.
-  page = (uintptr_t) g_main_argv[0];
-  page -= page % page_size;
-  page_end = page + page_size;
-  // Move the environment out of the way. Note that we are moving the values,
-  // not the environment array itself (which may not be on the page we need
-  // to overwrite anyway).
-  for (i = 0; environ[i]; ++i) {
-    uintptr_t env_i = (uintptr_t) environ[i];
-    // Only move the value if it's actually in the way. This avoids
-    // leaking copies of the values if this function is called again.
-    if (page <= env_i && env_i < page_end) {
-      char* copy = strdup(environ[i]);
-      // Be paranoid. Check for allocation failure and bail out.
-      if (!copy)
-        return;
-      environ[i] = copy;
-    }
-  }
-  // Put the title in argv[0]. We have to zero out the space first since the
-  // kernel doesn't actually look for a null terminator unless we make the
-  // argument list longer than it started.
-  avail_size = page_end - (uintptr_t) g_main_argv[0];
-  memset(g_main_argv[0], 0, avail_size);
-  va_start(ap, fmt);
-  if (fmt[0] == '-') {
-    vsnprintf(g_main_argv[0], avail_size, &fmt[1], ap);
-  } else {
-    size_t size = snprintf(g_main_argv[0], avail_size, "%s ", g_orig_argv0);
-    if (size < avail_size)
-      vsnprintf(g_main_argv[0] + size, avail_size - size, fmt, ap);
-  }
-  va_end(ap);
-  g_main_argv[1] = NULL;
-}
-
-// A version of this built into glibc would not need this function, since
-// it could stash the argv pointer in __libc_start_main(). But we need it.
-void setproctitle_init(const char** main_argv) {
-  if (g_main_argv)
-    return;
-
-  uintptr_t page_size = sysconf(_SC_PAGESIZE);
-  // Check that the argv array is in fact on the same page of memory
-  // as the environment array just as an added measure of protection.
-  if (((uintptr_t) environ) / page_size == ((uintptr_t) main_argv) / page_size)
-    g_main_argv = const_cast<char**>(main_argv);
-}