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Side by Side Diff: base/setproctitle_linux.c

Issue 196009: Linux: set the process title (that shows in "ps" etc.) of renderers correctly when using the zygote. (Closed) Base URL: svn://chrome-svn/chrome/trunk/src/
Patch Set: '' Created 11 years, 3 months ago
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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 "base/setproctitle_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, 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(char** main_argv) {
107 uintptr_t page_size = sysconf(_SC_PAGESIZE);
108 // Check that the argv array is in fact on the same page of memory
109 // as the environment array just as an added measure of protection.
110 if (((uintptr_t) environ) / page_size == ((uintptr_t) main_argv) / page_size)
111 g_main_argv = main_argv;
112 }
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