Race condition in os

Prerequisite – Concurrency is the execution of the multiple instruction sequences at the same time. It happens in the operating system when there are several process threads running in parallel. The running process threads always communicate with each other through shared memory or message passing. Concurrency results in sharing of resources result in problems like deadlocks and resources starvation. It helps in techniques like coordinating execution of processes, memory allocation and execution scheduling for maximizing throughput. There are several motivations for allowing concurrent execution: • Physical resource sharing : Multiuser environment since hardware resources are limited. • Logical resource sharing: Shared file(same piece of information). • Computation speedup: Parallel execution • Modularity: Divide system functions into separation processes. Relationship Between Processes • The result of execution depends is only on the input state. • The result of the execution will always be the same for the same input. • The termination of the independent process will not terminate any other. Cooperating System: Its state is shared along other processes. • The result of the execution depends on relative execution sequence and cannot be predicted in advanced(Non-deterministic). • The result of the execution will not always be the same for the same input. • The termination of the cooperating process may affect other process. Operation on a Process Most systems support at le...

In this article Another area of concern is the potential for security holes exploited by race conditions. There are several ways in which this might happen. The subtopics that follow outline some of the major pitfalls that the developer must avoid. Race Conditions in the Dispose Method If a class's Dispose method (for more information, see Dispose can be run more than once, as shown in the following example. Sub Dispose() If Not (myObj Is Nothing) Then Cleanup(myObj) myObj = Nothing End If End Sub void Dispose() If there are other paths to DoOtherWork that can be called from another thread with the same object, an untrusted caller can slip past a demand. If your code caches security information, make sure that you review it for this vulnerability. Race Conditions in Finalizers Race conditions can also occur in an object that references a static or unmanaged resource that it then frees in its finalizer. If multiple objects share a resource that is manipulated in a class's finalizer, the objects must synchronize all access to that resource. See also • •

What Is a Race Condition? In any computing system, there are some tasks that need to be completed in a specific order. For example, before allowing someone to log in, a security system first receives their username and password and then checks it against a database before allowing access. Attackers can exploit this fact by interfering with processes to access secure areas and content in what's known as a race condition attack. What Is a Race Condition Vulnerability? Race condition attacks (also called Time of Check to Time of Use, or TOCTTOU attacks) take advantage of the need that computing systems must execute some tasks in a specific sequence. In any such sequence, there is a small period of time when the system has carried out the first task but not started on the second. If this period is long enough or the attacker is lucky and knowledgeable, a race condition vulnerability exists where an attacker can trick the system into carrying out unauthorized actions in addition to its normal processes. There are two main ways this attack is carried out: • Interference by an untrusted process - The attacker inserts a piece of code in between the steps of a secure process. • Interference by a trusted process - The attacker exploits two different processes that share some state in common. If you have a situation you think might be a race condition vulnerability, why not Our analysis shows that most race condition flaws in software are race conditions within the same thread. Race ...

When two or more processes are reading or writing some shared data and the final result depends on who runs precisely when, are called race conditions. Example: a print spooler. When a process wants to print a file, it enters the file name in a special spooler directory. Another process, the printer daemon, periodically checks to see if there are any files to be printed, and if there are, it prints them and then removes their names from the directory. Consider our spooler directory has a very large number of slots, numbered 0, 1, 2 each one capable of holding a file name. Also imagine that there are two shared variables, out, which points to the next file to be printed, and in, which points to the next free slot in the directory. These two variables might well be kept on a two-word file available to all processes. At a certain instant, slots 0 to 3 are empty and slots 4 to 6 are full. More or less simultaneously, processes A and B decide they want to queue a file for printing. Process A reads in and stores the value, 7, in a local variable called next-free slot. Just then a clock interrupt occurs and the CPU decides that process A has run long enough, so it switches to process B. Process B also reads in, and also gets a 7. It too stores it in its local variable next-free slot. At this instant both processes think that the next available slot is 7. Process B now continues to run. It stores the name of its file in slot 7 and updates in to be an 8. Then it goes off and does o...

A race condition occurs when two or more threads can access shared data and they try to change it at the same time. Because the thread scheduling algorithm can swap between threads at any time, you don't know the order in which the threads will attempt to access the shared data. Therefore, the result of the change in data is dependent on the thread scheduling algorithm, i.e. both threads are "racing" to access/change the data. Problems often occur when one thread does a "check-then-act" (e.g. "check" if the value is X, then "act" to do something that depends on the value being X) and another thread does something to the value in between the "check" and the "act". E.g: if (x == 5) // The "Check" // release lock for x @Ian In a multithreaded system there will always be times when resources need to be shared. To say that one approach is bad without giving an alternative just isn't productive. I'm always looking for ways to improve and if there is an alternative I will gladly research it and weigh the pro's and cons. @Despertar ...also, its not necessarily the case that resources will always need to be shared in a milti-threaded system. For example you might have an array where each element needs processing. You could possibly partition the array and have a thread for each partition and the threads can do their work completely independently of one another. A "race condition" exists when multithreaded (or otherwise parallel) code that would access a shared resource could do so...

Race condition in software is an undesirable event that can happen when multiple entities access or modify shared resources in a system. The system behaves correctly when these entities use the shared resources as expected. But sometimes due to uncontrollable delays, the sequence of operations may change due to relative timing of events. When this happens, the system may enter a state not designed for and hence fail. The "race" happens because this type of failure is dependent on which entity gains access to the shared resource first. One definition of race condition describes it as "anomalous behavior due to unexpected critical dependence on the relative timing of events." Discussion • Can you give one example of race condition? Illustrating software race condition. Source: Devopedia 2020. Suppose a process calls a function that's supposed to increment a value (v) stored in a database or some shared memory. This operation is not atomic, meaning that it can be broken down into smaller steps. The function will read the current value from database (A), store it in memory as a function variable (B), increment it (C) and finally write the new value to database (D). This function's execution is therefore a sequence of steps A-D. Race condition will happen if process P1 calls this function and proceeds to step C. Meanwhile, it gets preempted by the operating system and another process P2 gets its chance to run. Race condition would not have happened if P1 had completed all steps...

I'm having trouble analyzing a problem on a past OS exam. It is as follows: Describe the output of the following program. Does a race condition exist? int count=0; int main(void) I'm not that good yet with C and race conditions, so my answer is mostly a guess. But if I saw this on an exam, I would say "The program splits a process into a parent and child process; the child process prints 'Output1' and the parent prints 'Output2', one character at a time. Then the total number of letters is printed at the end of the program; however, this variable 'count' may be inaccurate, as a race condition exists between the child and parent. Either can access and update count at any time, which can lead to inaccuracies." From my understanding, race conditions arise when two or more threads or processes try to access or set the same shared variable, and the only incident of that I can see in this program is 'count'. Am I anywhere near correct, and if not, what could be added to this program to create a race condition (hypothetically, of course; I know that's not what we would want to do)? Aren't these two processes the moment you skip past that fork(). At that time isn't the only race between who gets to dump a putc() in the IO queue to the console before the other? There is no threading issue that I see. And regarding Neil's comment, it may interlace between processes, but I would be surprised if the child weren't outright finished before the parent even resumed (or vice-versa).

Race condition in software is an undesirable event that can happen when multiple entities access or modify shared resources in a system. The system behaves correctly when these entities use the shared resources as expected. But sometimes due to uncontrollable delays, the sequence of operations may change due to relative timing of events. When this happens, the system may enter a state not designed for and hence fail. The "race" happens because this type of failure is dependent on which entity gains access to the shared resource first. One definition of race condition describes it as "anomalous behavior due to unexpected critical dependence on the relative timing of events." Discussion • Can you give one example of race condition? Illustrating software race condition. Source: Devopedia 2020. Suppose a process calls a function that's supposed to increment a value (v) stored in a database or some shared memory. This operation is not atomic, meaning that it can be broken down into smaller steps. The function will read the current value from database (A), store it in memory as a function variable (B), increment it (C) and finally write the new value to database (D). This function's execution is therefore a sequence of steps A-D. Race condition will happen if process P1 calls this function and proceeds to step C. Meanwhile, it gets preempted by the operating system and another process P2 gets its chance to run. Race condition would not have happened if P1 had completed all steps...

Prerequisite – Concurrency is the execution of the multiple instruction sequences at the same time. It happens in the operating system when there are several process threads running in parallel. The running process threads always communicate with each other through shared memory or message passing. Concurrency results in sharing of resources result in problems like deadlocks and resources starvation. It helps in techniques like coordinating execution of processes, memory allocation and execution scheduling for maximizing throughput. There are several motivations for allowing concurrent execution: • Physical resource sharing : Multiuser environment since hardware resources are limited. • Logical resource sharing: Shared file(same piece of information). • Computation speedup: Parallel execution • Modularity: Divide system functions into separation processes. Relationship Between Processes • The result of execution depends is only on the input state. • The result of the execution will always be the same for the same input. • The termination of the independent process will not terminate any other. Cooperating System: Its state is shared along other processes. • The result of the execution depends on relative execution sequence and cannot be predicted in advanced(Non-deterministic). • The result of the execution will not always be the same for the same input. • The termination of the cooperating process may affect other process. Operation on a Process Most systems support at le...

In this article Another area of concern is the potential for security holes exploited by race conditions. There are several ways in which this might happen. The subtopics that follow outline some of the major pitfalls that the developer must avoid. Race Conditions in the Dispose Method If a class's Dispose method (for more information, see Dispose can be run more than once, as shown in the following example. Sub Dispose() If Not (myObj Is Nothing) Then Cleanup(myObj) myObj = Nothing End If End Sub void Dispose() If there are other paths to DoOtherWork that can be called from another thread with the same object, an untrusted caller can slip past a demand. If your code caches security information, make sure that you review it for this vulnerability. Race Conditions in Finalizers Race conditions can also occur in an object that references a static or unmanaged resource that it then frees in its finalizer. If multiple objects share a resource that is manipulated in a class's finalizer, the objects must synchronize all access to that resource. See also • •