invalid opcode

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Ali.Z

Joined: 08 Jan 2018
Posts: 772

Ali.Z 23 Dec 2018, 11:52

just wanted to say, an interesting fact about eflags:

i was messing around and generating invalid opcodes, when ever i attempt to execute that opcode resume flags is set (RF)

and im not sure why lol, anyone knows anything about the relation of RF and invalid opcode?

edit:
found something, but i did not understand it.
now how would explain it, and it feels there are more hidden things behind it.

Quote:

The RF bit in EFLAGS controls the recognition of debug faults. This permits
debug faults to be raised for a given instruction at most once, no matter
how many times the instruction is restarted.

edit edit:
now whats going on?

Quote:

Resume flag (RF) of flags register
Allows an instruction to be restarted after a debug exception without
immediately causing another debug exception due to the same condition.

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Asm For Wise Humans

23 Dec 2018, 11:52

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20518
Location: In your JS exploiting you and your system

revolution 23 Dec 2018, 12:46

If you are trying to restart an invalid instruction; well you can't, It will always be invalid.

If you are trying to restart a valid instruction in a protected OS, then the OS can decide to create whatever error it wants to and pass that down to the app, or simply give up and tell the user the app did something invalid.

23 Dec 2018, 12:46

Furs

Joined: 04 Mar 2016
Posts: 2595

Furs 23 Dec 2018, 13:34

revolution wrote:

If you are trying to restart an invalid instruction; well you can't, It will always be invalid.

I heard that if you restart it 42 times while having your monitor facing a mirror without you obstructing them, an image of Cthulhu will appear in the mirror and then... stuff happens.

23 Dec 2018, 13:34

Mikl___

Joined: 30 Dec 2014
Posts: 143
Location: Russian Federation, Irkutsk

Mikl___ 23 Dec 2018, 13:38

23 Dec 2018, 13:38

Ali.Z

Joined: 08 Jan 2018
Posts: 772

Ali.Z 23 Dec 2018, 22:14

thanks Mikl, that figure explains it well.

_________________
Asm For Wise Humans

23 Dec 2018, 22:14

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