This is an exercise that I want to implement in real code
I send a signal to my app (x86-64 linux). My app then executes code that walks the stack and prints out instruction pointers. I'm not sure if I want only the last few or everything to main. Anyway, I'm releasing an optimized binary without debug information. I strip symbols before its distributed.
I was wondering, how do I translate it back? I don't need to translate it in the app. I can use the machine I build to go from rip's to functions. I was thinking maybe I should also distribute one with debug information and maybe have the user be able to see the function+line but I think line will be unlikely if its optimized well
Another problem I have is my code doesn't seem to walk past the signal function. backtrace figures it out but I'm trying to do this without libc. Here's some code
#include <signal.h>
#include <cstdio>
typedef unsigned long long u64;
int mybacktrace();
#include <execinfo.h>
#include <unistd.h>
void print_stacktrace(void) {
size_t size;
enum Constexpr { MAX_SIZE = 1024 };
void *array[MAX_SIZE];
size = backtrace(array, MAX_SIZE);
backtrace_symbols_fd(array, size, STDOUT_FILENO);
}
void mysig(int signo) {
mybacktrace();
_exit(1);
}
int mybacktrace() {
u64*p;
p = (u64*)((u64)&p + 16); //seems to work correctly
for (int i = 0; i < 10 && (u64)p >= 1<<16; i++)
{
printf("%d %p\n", i, p[1]);
p = (u64*)(p[0]);
}
print_stacktrace(); return 0;
return 0;
}
int test()
{
return mybacktrace();
}
int main(int argc, char *argv[])
{
signal(SIGILL, mysig);
test();
__builtin_trap();
return 0;
}
I am trying to add an IPv6 address to an ethernet interface, using libmnl. After constructing a message and sending to the kernel, I saw that it was not added to the interface, even though the return codes for the kernel reply did not contain any error. Kindly can anybody have a look, and help me correct it. Am I supposed to add more attributes to the nlmsghdr or something else?
#include <assert.h>
#include <string.h>
#include <inttypes.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <libmnl/libmnl.h>
#include <linux/rtnetlink.h>
#include <net/if.h>
#include <time.h>
static struct mnl_socket *nl;
static unsigned int nlportid;
int mnl_init(void);
int mnl_init(){
nl = mnl_socket_open(NETLINK_ROUTE);
if(nl == NULL){
printf("Error: mnl_socket_open\n");
return 0;
}
if(mnl_socket_bind(nl, 0, MNL_SOCKET_AUTOPID) < 0){
printf("Error: mnl_socket_bind\n");
return 0;
}
nlportid = mnl_socket_get_portid(nl);
return 1;
}
int add_to_interface(const char* eip){
char buf[MNL_SOCKET_BUFFER_SIZE];
struct nlmsghdr *nlh;
struct ifaddrmsg *ifm;
int ret;
uint8_t seq;
nlh = mnl_nlmsg_put_header(buf);
nlh->nlmsg_type = RTM_NEWADDR;
nlh->nlmsg_flags = NLM_F_REQUEST|NLM_F_CREATE|NLM_F_EXCL;
nlh->nlmsg_seq = seq = time(NULL);
ifm = mnl_nlmsg_put_extra_header(nlh, sizeof(*ifm));
ifm->ifa_family = AF_INET6;
ifm->ifa_prefixlen = 64;
ifm->ifa_flags = IFA_F_PERMANENT;
ifm->ifa_scope = RT_SCOPE_UNIVERSE;
/* TODO get interaface name from user or configuration*/
ifm->ifa_index = if_nametoindex("eth0");
unsigned char eipn[16];
inet_pton(AF_INET6, eip, eipn);
mnl_attr_put(nlh, IFA_ADDRESS, 16,eipn);
mnl_nlmsg_fprintf(stdout, nlh, nlh->nlmsg_len, sizeof(struct ifaddrmsg));
if(mnl_socket_sendto(nl,nlh, nlh->nlmsg_len) < 0){
printf("Error: mnl_socket_sendto");
return 0;
}
ret = mnl_socket_recvfrom(nl,buf, sizeof(buf));
if(ret == -1){
printf("Error: mnl_socket_recvfrom");
return 0;
}
ret = mnl_cb_run(buf, ret, seq, nlportid, NULL, NULL);
return 0;
}
int main(int argc, char *argv[]){
if(mnl_init()){
add_to_interface("2001::20c:29ff:fe5f:13c7/64"); // for testing
}
}
I don't really know anything about libmnl, and the following solution is not perfect. But in case you're still stuck...
Notice that you're dropping several error codes. This one, in particular:
inet_pton(AF_INET6, eip, eipn);
Should be something in the lines of this:
ret = inet_pton(AF_INET6, eip, eipn);
if (ret != 1) {
printf("Bad address.\n");
return -22;
}
And I suppose you can tell where I'm going with this. This:
add_to_interface("2001::20c:29ff:fe5f:13c7/64"); // for testing
Should be this:
add_to_interface("2001::20c:29ff:fe5f:13c7"); // for testing
That fixes it for me. Except it hags at mnl_socket_recvfrom() because the kernel does not answer, apparently.
But do be more careful with those error codes; the ones I mentioned aren't the only ones.
Solution from Pablo Neira,
eipn should be 'struct in6_addr' instead.
mnl_attr_put(nlh, IFA_ADDRESS, 16,eipn);
So this looks like:
mnl_attr_put(nlh, IFA_ADDRESS, sizeof(eipn), &eipn);
Since I just created this crude test using the functions from IETF RFC 4634, I don't know for certain whether I've used them correctly for HMAC-SHA-384-192, so I'll start with that code here:
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include "sha.h"
int main(int argc, char *argv[]) {
HMACContext hmac;
const unsigned char *keyarr = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
int err = hmacReset(&hmac, SHA384, keyarr, 48);
if (err != shaSuccess) {
printf("err 1\n");
exit(1);
}
const uint8_t testarray[65] = {'I',' ','a','m',' ','n','o','t',' ','a',' ','c','r','o','o','k','!'};
const unsigned char *prfkey = "abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789";
memcpy((void *)testarray + 17, (void *)prfkey, 48);
const int testlen = 65;
err = hmacInput(&hmac, testarray, testlen);
if (err != shaSuccess) {
printf("err 2\n");
exit(1);
}
uint8_t Message_Digest[USHAMaxHashSize];
err = hmacResult(&hmac, Message_Digest);
if (err != shaSuccess) {
printf("err 3\n");
exit(1);
}
int i;
for(i = 0; i < 24; i++) printf(" %02X", Message_Digest[i]);
putchar('\n');
}
If I've done everything right (other than selecting good keys) so far, I would ordinarily have a 24-byte (i.e., 192-bit) digest, but if I digitally sign the digest prior to appending it, my experience is that the signature block isn't a predictable length. I'm sure I could come up with any number of ways to identify the end of the message portion, but I don't want to make this a hack. What is the accepted way of doing this? (The signature will use ECDSA.)
I should also mention that this will be a multicast message using UDP inside ESP, since that puts constraints on message economy. (That's also the main reason for the problem--keeping it binary. The other is the practice of appending, rather than prefixing it with a byte count in front of it.)
I write a code to print out strings: "Thread 0" to "Thread 4" using pthread.
Here is my code:
Case 1:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *print_message_function(void* parameter) {
long *i = (long *)parameter;
printf("Thread %ld\n", *i);
pthread_exit(0);
}
int main(int argc, char *argv[]) {
pthread_t threads[5];
long i = 0;
for (i = 0; i < 5; i++) {
pthread_create(&threads[i], 0, print_message_function, (void *)&i);
}
pthread_exit(NULL);
}
But the result is:
Thread 2
Thread 3
Thread 3
Thread 4
Thread 5
or:
Thread 0
Thread 0
Thread 0
Thread 0
Thread 0
It changed when I run it again. So I don't know why the values I passed are (2 to 5) or all (0) or ..... (many cases). I think my arguments I passed is from 0 to 4.
Case 2:
When I change to the new code:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *print_message_function(void *parameter);
int main(int argc, char *argv[]) {
pthread_t threads[5];
int i = 0;
for (i = 0; i < 5; i++) {
char *msg = (char*)malloc(sizeof(char));
sprintf(msg, "Thread %d", i);
pthread_create(&threads[i], 0, print_message_function, (void *)msg);
}
}
void *print_message_function(void *parameter) {
printf("%s\n", (char *)parameter);
}
The result is:
Thread 1
Thread 0
Thread 3
Thread 2
Thread 4
Thread 4
It means the loop run 6 times! Why?
Change Case 1 to this:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *print_message_function(void* parameter) {
long i = (long)parameter; // <<<
printf("Thread %ld\n", i); // <<<
pthread_exit(0);
}
int main(int argc, char *argv[]) {
pthread_t threads[5];
long i = 0;
for (i = 0; i < 5; i++) {
pthread_create(&threads[i], 0, print_message_function, (void *)i); // <<<
}
pthread_exit(NULL);
}
The reason that you were seeing inconsistent results before was because you were passing a pointer to each thread where each pointer was pointing at the same local variable, which you were then modifying.
In Case 2 you are mallocing only a single char and then trying to write a string to it. It should be fairly easy to fix.
Your case 2 approach is valid, however you need to fix the malloc part to allocate enough bytes. Change it to
char *msg = (char*)malloc(sizeof(char) * (strlen("Thread ") + 10));
// assuming i will take at most 9-digits (unlikely case)
For your case 1, you are passing address of i. But the value of i is changing thread function will get whatever value is there at that location when it tries to print. Also note that address of i may not be valid by the time thread function executes as its allocated on stack and will go away when main function returns.
So I'm trying to write a kernel module that uses the linux/timer.h file. I got it to work inside just the module, and now I am trying to get it to work from a user program.
Here is my kernel module:
//Necessary Includes For Device Drivers.
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/proc_fs.h>
#include <asm/uaccess.h>
#include <linux/timer.h>
#include <linux/ioctl.h>
#define DEVICE_NAME "mytimer"
#define DEVICE_FILE_NAME "mytimer"
#define MAJOR_NUM 61
#define MINOR_NUM 0
MODULE_LICENSE("Dual BSD/GPL");
static struct timer_list my_timer;
struct file_operations FileOps =
{
//No File Operations for this timer.
};
//Function to perform when timer expires.
void TimerExpire(int data)
{
printk("Timer Data: %d\n", data);
}
//Function to set up timers.
void TimerSetup(void)
{
setup_timer(&my_timer, TimerExpire, 5678);
mod_timer(&my_timer, jiffies + msecs_to_jiffies(5000));
}
//Module Init and Exit Functions.
int init_module(void)
{
int initResult = register_chrdev(MAJOR_NUM, "mytimer", &FileOps);
if (initResult < 0)
{
printk("Cannot obtain major number %d\n", MAJOR_NUM);
return initResult;
}
printk("Loading MyTimer Kernel Module...\n");
return 0;
}
void cleanup_module(void)
{
unregister_chrdev(MAJOR_NUM, "mytimer");
printk("Unloading MyTimer Kernel Module...\n");
}
More specifically, I want my user program to call the TimerSetup() function. I know that I'll need to use ioctl() but I'm not sure how to specify in my MODULE FILE that TimerSetup() should be callable via ioctl().
Also, my second question: I was able to insmod my module and also mknod into /dev/mytimer with the correct major number. But when I tried to open() it so that I can get the file descriptor from it, it kept returning -1, which I'm assuming is wrong. I made sure the permissions were fine (in fact, I made it 777 just to be sure)... It still doesn't work... Is there something I'm missing?
Here is the user program just in case:
#include <stdio.h>
int main(int argc, char* argv[])
{
int fd = open("/dev/mytimer", "r");
printf("fd: %d\n", fd);
return 0;
}
The example code you need can be found in drivers/watchdog/softdog.c (from Linux 2.6.33 at the time this was written), which illustrates proper file operations as well as how to permit userland to fill a structure with ioctl().
It's actually a great, working tutorial for anyone who needs to write trivial character device drivers.
I dissected softdog's ioctl interface when answering my own question, which may be helpful to you.
Here's the gist of it (though far from exhaustive) ...
In softdog_ioctl() you see a simple initialization of struct watchdog_info that advertises functionality, version and device information:
static const struct watchdog_info ident = {
.options = WDIOF_SETTIMEOUT |
WDIOF_KEEPALIVEPING |
WDIOF_MAGICCLOSE,
.firmware_version = 0,
.identity = "Software Watchdog",
};
We then look at a simple case where the user just wants to obtain these capabilities:
switch (cmd) {
case WDIOC_GETSUPPORT:
return copy_to_user(argp, &ident, sizeof(ident)) ? -EFAULT : 0;
... which of course, will fill the corresponding userspace watchdog_info with the initialized values above. If copy_to_user() fails, -EFAULT is returned which causes the corresponding userspace ioctl() call to return -1 with a meaningful errno being set.
Note, the magic requests are actually defined in linux/watchdog.h , so that the kernel and userspace share them:
#define WDIOC_GETSUPPORT _IOR(WATCHDOG_IOCTL_BASE, 0, struct watchdog_info)
#define WDIOC_GETSTATUS _IOR(WATCHDOG_IOCTL_BASE, 1, int)
#define WDIOC_GETBOOTSTATUS _IOR(WATCHDOG_IOCTL_BASE, 2, int)
#define WDIOC_GETTEMP _IOR(WATCHDOG_IOCTL_BASE, 3, int)
#define WDIOC_SETOPTIONS _IOR(WATCHDOG_IOCTL_BASE, 4, int)
#define WDIOC_KEEPALIVE _IOR(WATCHDOG_IOCTL_BASE, 5, int)
#define WDIOC_SETTIMEOUT _IOWR(WATCHDOG_IOCTL_BASE, 6, int)
#define WDIOC_GETTIMEOUT _IOR(WATCHDOG_IOCTL_BASE, 7, int)
#define WDIOC_SETPRETIMEOUT _IOWR(WATCHDOG_IOCTL_BASE, 8, int)
#define WDIOC_GETPRETIMEOUT _IOR(WATCHDOG_IOCTL_BASE, 9, int)
#define WDIOC_GETTIMELEFT _IOR(WATCHDOG_IOCTL_BASE, 10, int)
WDIOC obviously signifying "Watchdog ioctl"
You can easily take that a step further, having your driver do something and place the result of that something in the structure and copy it to userspace. For instance, if struct watchdog_info also had a member __u32 result_code. Note, __u32 is just the kernel's version of uint32_t.
With ioctl(), the user passes the address of an object, be it a structure, integer, whatever to the kernel expecting the kernel to write its reply in an identical object and copy the results to the address that was provided.
The second thing you are going to need to do is make sure your device knows what to do when someone opens, reads from it, writes to it, or uses a hook like ioctl(), which you can easily see by studying softdog.
Of interest is:
static const struct file_operations softdog_fops = {
.owner = THIS_MODULE,
.llseek = no_llseek,
.write = softdog_write,
.unlocked_ioctl = softdog_ioctl,
.open = softdog_open,
.release = softdog_release,
};
Where you see the unlocked_ioctl handler going to ... you guessed it, softdog_ioctl().
I think you might be juxtaposing a layer of complexity that really doesn't exist when dealing with ioctl(), it really is that simple. For that same reason, most kernel developers frown on new ioctl interfaces being added unless they are absolutely necessary. Its just too easy to lose track of the type that ioctl() is going to fill vs the magic you use to do it, which is the primary reason that copy_to_user() fails often resulting in the kernel rotting with hordes of userspace processes stuck in disk sleep.
For a timer, I agree, ioctl() is the shortest path to sanity.
You are missing a .open function pointer in your file_operations structure to specify the function to be called when a process attempts to open the device file. You will need to specify a .ioctl function pointer for your ioctl function as well.
Try reading through The Linux Kernel Module Programming Guide, specifically chapters 4 (Character Device Files) and 7 (Talking to Device Files).
Chapter 4 introduces the file_operations structure, which holds pointers to functions defined by the module/driver that perform various operations such as open or ioctl.
Chapter 7 provides information on communicating with a module/drive via ioctls.
Linux Device Drivers, Third Edition is another good resource.
Minimal runnable example
Tested in a fully reproducible QEMU + Buildroot environment, so might help others get their ioctl working. GitHub upstream:
kernel module |
shared header |
userland.
The most annoying part was understanding that some low ids are hijacked: ioctl is not called if cmd = 2 , you have to use _IOx macros.
Kernel module:
#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
#include <linux/debugfs.h>
#include <linux/module.h>
#include <linux/printk.h> /* printk */
#include "ioctl.h"
MODULE_LICENSE("GPL");
static struct dentry *dir;
static long unlocked_ioctl(struct file *filp, unsigned int cmd, unsigned long argp)
{
void __user *arg_user;
union {
int i;
lkmc_ioctl_struct s;
} arg_kernel;
arg_user = (void __user *)argp;
pr_info("cmd = %x\n", cmd);
switch (cmd) {
case LKMC_IOCTL_INC:
if (copy_from_user(&arg_kernel.i, arg_user, sizeof(arg_kernel.i))) {
return -EFAULT;
}
pr_info("0 arg = %d\n", arg_kernel.i);
arg_kernel.i += 1;
if (copy_to_user(arg_user, &arg_kernel.i, sizeof(arg_kernel.i))) {
return -EFAULT;
}
break;
case LKMC_IOCTL_INC_DEC:
if (copy_from_user(&arg_kernel.s, arg_user, sizeof(arg_kernel.s))) {
return -EFAULT;
}
pr_info("1 arg = %d %d\n", arg_kernel.s.i, arg_kernel.s.j);
arg_kernel.s.i += 1;
arg_kernel.s.j -= 1;
if (copy_to_user(arg_user, &arg_kernel.s, sizeof(arg_kernel.s))) {
return -EFAULT;
}
break;
default:
return -EINVAL;
break;
}
return 0;
}
static const struct file_operations fops = {
.owner = THIS_MODULE,
.unlocked_ioctl = unlocked_ioctl
};
static int myinit(void)
{
dir = debugfs_create_dir("lkmc_ioctl", 0);
/* ioctl permissions are not automatically restricted by rwx as for read / write,
* but we could of course implement that ourselves:
* https://stackoverflow.com/questions/29891803/user-permission-check-on-ioctl-command */
debugfs_create_file("f", 0, dir, NULL, &fops);
return 0;
}
static void myexit(void)
{
debugfs_remove_recursive(dir);
}
module_init(myinit)
module_exit(myexit)
Shared header between the kernel module and userland:
ioctl.h
#ifndef IOCTL_H
#define IOCTL_H
#include <linux/ioctl.h>
typedef struct {
int i;
int j;
} lkmc_ioctl_struct;
#define LKMC_IOCTL_MAGIC 0x33
#define LKMC_IOCTL_INC _IOWR(LKMC_IOCTL_MAGIC, 0, int)
#define LKMC_IOCTL_INC_DEC _IOWR(LKMC_IOCTL_MAGIC, 1, lkmc_ioctl_struct)
#endif
Userland:
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include "../ioctl.h"
int main(int argc, char **argv)
{
int fd, arg_int, ret;
lkmc_ioctl_struct arg_struct;
if (argc < 2) {
puts("Usage: ./prog <ioctl-file>");
return EXIT_FAILURE;
}
fd = open(argv[1], O_RDONLY);
if (fd == -1) {
perror("open");
return EXIT_FAILURE;
}
/* 0 */
{
arg_int = 1;
ret = ioctl(fd, LKMC_IOCTL_INC, &arg_int);
if (ret == -1) {
perror("ioctl");
return EXIT_FAILURE;
}
printf("arg = %d\n", arg_int);
printf("ret = %d\n", ret);
printf("errno = %d\n", errno);
}
puts("");
/* 1 */
{
arg_struct.i = 1;
arg_struct.j = 1;
ret = ioctl(fd, LKMC_IOCTL_INC_DEC, &arg_struct);
if (ret == -1) {
perror("ioctl");
return EXIT_FAILURE;
}
printf("arg = %d %d\n", arg_struct.i, arg_struct.j);
printf("ret = %d\n", ret);
printf("errno = %d\n", errno);
}
close(fd);
return EXIT_SUCCESS;
}