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// Copyright 2009 The Chromium Authors
// 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.
//
// (This copy has been modified for use in the Mozilla codebase.)
//
// 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.
//
// When the kernel reads the command line arguments for a process, it looks at
// the range of memory 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.
//
// Thus to change the process title, we must move any arguments and 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.
//
// See the following kernel commit for the details of the contract between
// kernel and setproctitle:
//
// 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/set_process_title_linux.h"
#include "mozilla/UniquePtrExtensions.h"
#include <fcntl.h>
#include <stdarg.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <string>
#include <vector>
extern char** environ;
// g_orig_argv0 is the original process name found in argv[0].
// It is set to a copy of argv[0] in setproctitle_init. It is nullptr if
// setproctitle_init was unsuccessful or not called.
static const char* g_orig_argv0 = nullptr;
// Following pointers hold the initial argv/envp memory range.
// They are initialized in setproctitle_init and are used to overwrite the
// argv/envp memory range with a new process title to be read by the kernel.
// They are nullptr if setproctitle_init was unsuccessful or not called.
// Note that g_envp_start is not necessary because it is the same as g_argv_end.
static char* g_argv_start = nullptr;
static char* g_argv_end = nullptr;
static char* g_envp_end = nullptr;
void setproctitle(const char* fmt, ...) {
va_list ap;
// Sanity check before we try and set the process title.
// The BSD version allows a null fmt to restore the original title.
if (!g_orig_argv0 || !fmt) {
return;
}
// The title can be up to the end of envp.
const size_t avail_size = g_envp_end - g_argv_start - 1;
// Linux 4.18--5.2 have a bug where we can never set a process title
// shorter than the initial argv. Check if the bug exists in the current
// kernel on the first call of setproctitle.
static const bool buggy_kernel = [avail_size]() {
// Attempt to set an empty title. This will set cmdline to:
// "" (on Linux --4.17)
// "\0\0\0...\0\0\0.\0" (on Linux 4.18--5.2)
// "\0" (on Linux 5.3--)
memset(g_argv_start, 0, avail_size + 1);
g_argv_end[-1] = '.';
mozilla::UniqueFileHandle fd(
open("/proc/self/cmdline", O_RDONLY | O_CLOEXEC));
if (!fd) {
return false;
}
// We just want to see if there are at least 2 bytes in the file;
// we don't need to read the whole contents. Short reads probably
// aren't possible given how this procfs node is implemented, but
// it's not much more code to handle it anyway.
char buf[2];
ssize_t total_read = 0;
while (total_read < 2) {
ssize_t rd = read(fd.get(), buf, 2);
if (rd <= 0) {
return false;
}
total_read += rd;
}
return true;
}();
memset(g_argv_start, 0, avail_size + 1);
size_t size;
va_start(ap, fmt);
if (fmt[0] == '-') {
size = vsnprintf(g_argv_start, avail_size, &fmt[1], ap);
} else {
size = snprintf(g_argv_start, avail_size, "%s ", g_orig_argv0);
if (size < avail_size) {
size += vsnprintf(&g_argv_start[size], avail_size - size, fmt, ap);
}
}
va_end(ap);
// Kernel looks for a null terminator instead of the initial argv space
// when the end of the space is not terminated with a null.
//
// If the length of the new title is shorter than the original argv space,
// set the last byte of the space to an arbitrary non-null character to tell
// the kernel that setproctitle was called.
//
// On buggy kernels we can never make the process title shorter than the
// initial argv. In that case, just leave the remaining bytes filled with
// null characters.
const size_t argv_size = g_argv_end - g_argv_start - 1;
if (!buggy_kernel && size < argv_size) {
g_argv_end[-1] = '.';
}
}
// 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) {
static bool init_called = false;
if (init_called) {
return;
}
init_called = true;
if (!main_argv) {
return;
}
// Verify that the memory layout matches expectation.
char** const argv = main_argv;
char* argv_start = argv[0];
char* p = argv_start;
for (size_t i = 0; argv[i]; ++i) {
if (p != argv[i]) {
return;
}
p += strlen(p) + 1;
}
char* argv_end = p;
size_t environ_size = 0;
for (size_t i = 0; environ[i]; ++i, ++environ_size) {
if (p != environ[i]) {
return;
}
p += strlen(p) + 1;
}
char* envp_end = p;
// Copy the arg and env strings into the heap. Leak Sanitizer
// doesn't seem to object to these strdup()s; if it ever does, we
// can always ensure the pointers are reachable from globals or add
// a suppresion for this function.
//
// Note that Chromium's version of this code didn't copy the
// arguments; this is probably because they access args via the
// CommandLine class, which copies into a std::vector<std::string>,
// but in general that's not a safe assumption for Gecko.
for (size_t i = 0; argv[i]; ++i) {
argv[i] = strdup(argv[i]);
}
for (size_t i = 0; environ[i]; ++i) {
environ[i] = strdup(environ[i]);
}
if (!argv[0]) {
return;
}
g_orig_argv0 = argv[0];
g_argv_start = argv_start;
g_argv_end = argv_end;
g_envp_end = envp_end;
}