From 3941378c4f463b3c27bc3fa5ed5b96cc0a0976e3 Mon Sep 17 00:00:00 2001
From: relyt871 <104436315+relyt871@users.noreply.github.com>
Date: Fri, 16 Jun 2023 22:13:10 +0800
Subject: [PATCH] notes

---
 W12D2.md | 38 ++++++++++++++++++++++++++++++++++++++
 W4D2.md  | 38 ++++++++++++++++++++++++++++++++++++++
 W8D2.md  | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 129 insertions(+)
 create mode 100644 W12D2.md
 create mode 100644 W4D2.md
 create mode 100644 W8D2.md

diff --git a/W12D2.md b/W12D2.md
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+# W12D2
+
+## Distributed Shared Memory
+
+Q: Addressing
+
+- Message
+  - packet
+  - document
+  - file (page)
+- Shared Memory (OS does it)
+  - directory (translation)
+
+
+
+#### DHT(Chord)
+
+Background: distributed searching
+
+Basic: any content $K_i = Hash(...)$
+
+- machine $i$ has file A.mpg: $Hash(A.mpg) = 1$, then let machine 1 hold the information that A.mpg is owned by $i$
+- query A.mpg: Hash = 1 $\to$ ask machine 1 for the owner $\to$ go to owner $i$
+
+Challenge: some machine may be offline
+
+- $succ(i)$: $i$'s next online machine
+- finger table: for each online machine, maintain $succ$ of $s+2^k,k=0,1,2,...$ on machine $s$
+- every time a machine becomes online: broadcast, other machines update finger table
+
+
+
+#### Distributed Systems
+
+- Chip: SoC (System on Chip)  
+- Board: SMP (Symmetric Pultiprocessing)  recall: reentrant functions
+- LAN: Cluster
+- Internet: Grid $\to$ Cloud
\ No newline at end of file
diff --git a/W4D2.md b/W4D2.md
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+# W4D2
+
+## Memory Management
+
+#### Concepts
+
+- No Memory Abstraction
+  - reallocation (static/dynamic)
+  - swapping
+- Virutal Memory
+  - need to run programs that are too large to fit in memory
+  - no explicit swapping -> OS does it
+- Page Replacement Algorithm
+  - Optimal algorithm (based on the future)
+  - NRU (not recently used) algorithm
+    - Referenced bit and Modified bit
+    - class0: not R not M
+    - class 1: not R, M
+    - class 2: R, not M
+    - class 3: R and M
+  - FIFO algorithm
+  - Second-chance algorithm
+  - Clock algorithm
+    - optimize the queue: only the clock ptr needs to move
+  - LRU (least recently used) algorithm
+    - complex for hardware
+    - simulating LRU in software: aging algorithm
+  - Working set algorithm
+    - each process focus on a set of pages
+  - WSClock algorithm
+
+##### Physical Memory Management
+
+- Buddy System (UNIX)
+  - allocate $2^m$ continual physical pages(frames)
+  - free_areas[] (list head)
+  - buddy: $2k*2^n$ and $(2k+1)*2^n$
+  - used in Linux since 1970s.   why efficient?
\ No newline at end of file
diff --git a/W8D2.md b/W8D2.md
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+# W8D2
+
+#### Buffer Overflow
+
+- Architecture level mechanism: data area not executable (page table: DEP)
+
+- Stack Canary
+
+  - compiler adds canary in stack and check whether canary is modified
+
+  - eg: 
+
+    ```c
+    foo() {
+    	//compiler: add code to insert canary
+    	char s[128];
+    	get(s);
+    	//compiler: add code to check canary
+    }
+    ```
+
+
+
+#### Code Reuse Attacks (Return to libc)
+
+```c
+system("/bin/sh");
+=
+fork();
+execve("/bin/sh");
+```
+
+- use instructions in libc to build useful functions (including stack overflow)
+
+
+
+#### Format String Attack
+
+format string:
+
+```c
+printf("Hello, %s", name);
+printf("%x add %x equals %x", a, b, c);
+```
+
+- if not enough args provided: 
+  - compiler will reserve space for these args
+  - can be used to either leak information or modify registers
+
+
+
+#### Back Doors
+