Jens Bendisposto bendisposto

Tools! Eine Toolsammlung aus der EntwickelBar 04/2025

macOS

Windows

Algorithm

A secret byte you want to read is stored at inaccessible memory location priv_mem.
The sender triggers an access exception by attempting to read priv_mem.
Due to CPU optimization (out-of-order execution), the load of secret from priv_mem and the use of its value in (4) and (5) below may execute before the exception is triggered.
Calculate an offset into a known array probe by multiplying secret by the width of a cache line (or whatever block size the CPU typically fetches, like a 4096-byte page). This guarantees each of those 256 possible offsets will cache separately.
Load probe[offset], which causes the CPU to cache exactly one chunk of of our array, populating one cache line.
The exception finally triggers, clearing the modified registers...but cached data is not excised.
Iterate over all 256 offsets into probe to find out which one loads fast. You've determined the value of secret.

Embedding an SSH-accessible REPL in a Clojure process

N.B. This is now a library, thanks to the efforts of the wonderful @mtnygard. And the README does a good job of making clear just how terrible an idea it is to actually do this. :)

As any Clojurist knows, the REPL is an incredibly handy development tool. It can also be useful as a tool for debugging running programs. Of course, this raises the question of how to limit access to the REPL to authorized parties. With the Apache SSHD library, you can embed an SSH server in any JVM process. It takes only a little code to hook this up to a REPL, and to limit access either by public key or

Latency numbers every programmer should know

L1 cache reference ......................... 0.5 ns
Branch mispredict ............................ 5 ns
L2 cache reference ........................... 7 ns
Mutex lock/unlock ........................... 25 ns
Main memory reference ...................... 100 ns             
Compress 1K bytes with Zippy ............. 3,000 ns  =   3 µs
Send 2K bytes over 1 Gbps network ....... 20,000 ns  =  20 µs
SSD random read ........................ 150,000 ns  = 150 µs

Read 1 MB sequentially from memory ..... 250,000 ns = 250 µs

	apply plugin: 'java'

	sourceSets.main.java.srcDir('.')
	sourceSets.main.output.classesDir('bin')

	repositories { mavenCentral(); }

	def java(String x) {
	def arguments = x.split(" ")
	def t = arguments.tail() as List

	(ns turel.core
	(:refer-clojure :exclude [==])
	(:use clojure.core.logic))

	(defn not-membero
	[x l]
	(conde [(emptyo l)]
	[(fresh [head tail]
	(conso head tail l)
	(!= head x)

	# use ImageMagick convert
	# the order is important. the density argument applies to input.pdf and resize and rotate to output.pdf
	convert -density 90 input.pdf -rotate 0.5 -attenuate 0.2 +noise Multiplicative -colorspace Gray output.pdf