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// 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[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(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 = main_argv;
}
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