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An example of sharing global variables between different cores |
Sharing memory (variables) between different cores is one of the challenges in the operating systems, and it is tricky to handle. HyperDbg has the same limitations in handling shared resources between different cores.
Imagine we want to count the number of times that nt!NtCreateFile system-call is called in our system.
We know that the syscall number for nt!NtCreateFile is 0x55. In the !syscall command, both the $context and RAX register contain the syscall number.
One of the solutions to avoid simultaneous read/write from the shared objects (global variables) is using spinlocks.
Spinlocks are implemented through functions in the script engine.
First of all, we should define two global variables. One is for the counter, and the other will be used as the lock.
This can be achieved by using the following commands.
? .my_lock = 0;
? .my_counter = 0;
{% hint style="warning" %} Keep in mind that the lock variable should be a global variable, not a local variable. {% endhint %}
Now, we protect the critical section of our counter by using the spinlock_lock function. After that, we can safely read our global variable .my_counter and change its value.
spinlock_lock(&.my_lock);
.my_counter = .my_counter + 1;
printf("NtCreateFile syscall (0x0055) is called %llx times\n", .my_counter);
spinlock_unlock(&.my_lock);
Using spinlocks, we are sure that two (or more) cores won't execute our critical section. Only one core runs the critical section simultaneously, and other cores will wait until the current core releases the lock using the spinlock_unlock function.
{% hint style="danger" %} If you don't use spinlocks and access to global variables without using a lock then your result might not be accurate as the debuggee might (and will) run the script at the same time in two cores, and your code has the classic problem of concurrent reading of shared memory in multi-core environment. {% endhint %}
To wrap up, we'll use the following script to accomplish the above task.
? .my_lock = 0;
? .my_counter = 0;
!syscall script {
if ($context == 0x55) {
spinlock_lock(&.my_lock);
.my_counter = .my_counter + 1;
printf("NtCreateFile syscall (0x0055) is called %llx times\n", .my_counter);
spinlock_unlock(&.my_lock);
}
}
You can see the result of the above script:
