content/common/set_process_title_linux.cc

   1 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 // This file implements BSD-style setproctitle() for Linux.
   6 // It is written such that it can easily be compiled outside Chromium.
   7 //
   8 // The Linux kernel sets up two locations in memory to pass arguments and
   9 // environment variables to processes. First, there are two char* arrays stored
  10 // one after another: argv and environ. A pointer to argv is passed to main(),
  11 // while glibc sets the global variable |environ| to point at the latter. Both
  12 // of these arrays are terminated by a NULL pointer; the environment array is
  13 // also followed by some empty space to allow additional variables to be added.
  14 //
  15 // These arrays contain pointers to a second location in memory, where the
  16 // strings themselves are stored one after another: first all the arguments,
  17 // then the environment variables. The kernel will allocate a single page of
  18 // memory for this purpose, so the end of the page containing argv[0] is the
  19 // end of the storage potentially available to store the process title.
  20 //
  21 // When the kernel reads the command line arguments for a process, it looks at
  22 // the range of memory within this page that it initially used for the argument
  23 // list. If the terminating '\0' character is still where it expects, nothing
  24 // further is done. If it has been overwritten, the kernel will scan up to the
  25 // size of a page looking for another. (Note, however, that in general not that
  26 // much space is actually mapped, since argv[0] is rarely page-aligned and only
  27 // one page is mapped.)
  28 //
  29 // Thus to change the process title, we must move any environment variables out
  30 // of the way to make room for a potentially longer title, and then overwrite
  31 // the memory pointed to by argv[0] with a single replacement string, making
  32 // sure its size does not exceed the available space.
  33 //
  34 // It is perhaps worth noting that patches to add a system call to Linux for
  35 // this, like in BSD, have never made it in: this is the "official" way to do
  36 // this on Linux. Presumably it is not in glibc due to some disagreement over
  37 // this position within the glibc project, leaving applications caught in the
  38 // middle. (Also, only a very few applications need or want this anyway.)
  39
  40 #include "content/common/set_process_title_linux.h"
  41
  42 #include <stdarg.h>
  43 #include <stdint.h>
  44 #include <stdio.h>
  45 #include <string.h>
  46 #include <unistd.h>
  47
  48 extern char** environ;
  49
  50 static char** g_main_argv = NULL;
  51 static char* g_orig_argv0 = NULL;
  52
  53 void setproctitle(const char* fmt, ...) {
  54   va_list ap;
  55   size_t i, avail_size;
  56   uintptr_t page_size, page, page_end;
  57   // Sanity check before we try and set the process title.
  58   // The BSD version allows fmt == NULL to restore the original title.
  59   if (!g_main_argv || !environ || !fmt)
  60     return;
  61   if (!g_orig_argv0) {
  62     // Save the original argv[0].
  63     g_orig_argv0 = strdup(g_main_argv[0]);
  64     if (!g_orig_argv0)
  65       return;
  66   }
  67   page_size = sysconf(_SC_PAGESIZE);
  68   // Get the page on which the argument list and environment live.
  69   page = (uintptr_t) g_main_argv[0];
  70   page -= page % page_size;
  71   page_end = page + page_size;
  72   // Move the environment out of the way. Note that we are moving the values,
  73   // not the environment array itself (which may not be on the page we need
  74   // to overwrite anyway).
  75   for (i = 0; environ[i]; ++i) {
  76     uintptr_t env_i = (uintptr_t) environ[i];
  77     // Only move the value if it's actually in the way. This avoids
  78     // leaking copies of the values if this function is called again.
  79     if (page <= env_i && env_i < page_end) {
  80       char* copy = strdup(environ[i]);
  81       // Be paranoid. Check for allocation failure and bail out.
  82       if (!copy)
  83         return;
  84       environ[i] = copy;
  85     }
  86   }
  87   // Put the title in argv[0]. We have to zero out the space first since the
  88   // kernel doesn't actually look for a null terminator unless we make the
  89   // argument list longer than it started.
  90   avail_size = page_end - (uintptr_t) g_main_argv[0];
  91   memset(g_main_argv[0], 0, avail_size);
  92   va_start(ap, fmt);
  93   if (fmt[0] == '-') {
  94     vsnprintf(g_main_argv[0], avail_size, &fmt[1], ap);
  95   } else {
  96     size_t size = snprintf(g_main_argv[0], avail_size, "%s ", g_orig_argv0);
  97     if (size < avail_size)
  98       vsnprintf(g_main_argv[0] + size, avail_size - size, fmt, ap);
  99   }
 100   va_end(ap);
 101   g_main_argv[1] = NULL;
 102 }
 103
 104 // A version of this built into glibc would not need this function, since
 105 // it could stash the argv pointer in __libc_start_main(). But we need it.
 106 void setproctitle_init(const char** main_argv) {
 107   if (g_main_argv)
 108     return;
 109
 110   uintptr_t page_size = sysconf(_SC_PAGESIZE);
 111   // Check that the argv array is in fact on the same page of memory
 112   // as the environment array just as an added measure of protection.
 113   if (((uintptr_t) environ) / page_size == ((uintptr_t) main_argv) / page_size)
 114     g_main_argv = const_cast<char**>(main_argv);
 115 }