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Does anyone know of any online source that provides instruction alternatives to a NOP opcode ?
Like 'xchg ax, ax' and the likes. I'm pretty sure that there is also a tool for it, can someone point me to that direction please ?
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A NOP sled (also known as a NOP slide) is a long sequence of instructions preceding shellcode, as shown in Figure 19-3. NOP sleds are not required to be present with shellcode, but they are often included as part of an exploit to increase the likelihood of the exploit succeeding.
Shellcode authors can do this by creating a large NOP sled immediately preceding the shellcode. As long as execution is directed somewhere within the NOP sled, the shellcode will eventually run.
A 128-byte NOP sled is just a group of NOP intructions 128 bytes wide. NOTE #1: NOP (No-OPeration) is an instruction available in most (all?) architectures that does nothing, other than occupying memory and some runtime.
NOTE #2: in architectures with variable length instructions, a NOP instruction is usually just one byte in length, so it can be used as a convenient instruction padding. Unfortunately, that also makes it easy to do a NOP sled. Show activity on this post.
This page has a nice list of NOP alternatives with increasing encoding lengths: http://www.asmpedia.org/index.php?title=NOP
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