set_process_title_linux.cc | Explore in Territory

// 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.

#ifdef UNSAFE_BUFFERS_BUILD
// TODO(crbug.com/40284755): Remove this and spanify to fix the errors.
#pragma allow_unsafe_buffers
#endif

// 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.
//
// 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 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:
// https://github.com/torvalds/linux/commit/2954152298c37804dab49d630aa959625b50cf64
//
// 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/process/set_process_title_linux.h"

#include <stdarg.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#include <string>
#include <vector>

#include "base/files/file_util.h"
#include "base/no_destructor.h"
#include "base/numerics/safe_conversions.h"

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 = …;

// 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 = …;
static char* g_argv_end = …;
static char* g_envp_end = …;

void setproctitle(const char* fmt, ...) { … }

// 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(const char** main_argv) { … }
chromium/base/process/set_process_title_linux.cc
