Linux: set the process title (that shows in "ps" etc.) of renderers correctly when using the zygote.

base/setproctitle_linux.c

+// 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 "base/setproctitle_linux.h"
+
+#include <stdarg.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, 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(char** main_argv) {
+  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 = main_argv;
+}