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.
5 // This file implements BSD-style setproctitle() for Linux.
6 // It is written such that it can easily be compiled outside Chromium.
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.
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.
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.)
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.
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.)
40 #include "base/setproctitle_linux.h"
48 extern char** environ
;
50 static char** g_main_argv
= NULL
;
51 static char* g_orig_argv0
= NULL
;
53 void setproctitle(const char* fmt
, ...) {
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
)
62 // Save the original argv[0].
63 g_orig_argv0
= strdup(g_main_argv
[0]);
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.
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
);
94 vsnprintf(g_main_argv
[0], avail_size
, &fmt
[1], ap
);
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
);
101 g_main_argv
[1] = NULL
;
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
) {
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
= main_argv
;