OpenBSD_Notes_re_DEP: Added an md file in docs/ explaining how to disable W^X protections on the filesystem you're using to execute Unicorn-using programmes, so that the OS will allow those programmes to run.

docs/OPENBSD-NOTES.md

@@ -0,0 +1,69 @@
+## Circumventing OpenBSD 6.0's W^X Protections
+
+OpenBSD 6.0 and above enforces data-execution prevention (DEP or
+W^X) by default, preventing memory from being mapped as 
+simultaneously writeable and executable (i.e., W|X). This causes
+problems for Unicorn, if left in place.  If you're seeing
+errors like the following:
+```
+/home/git/unicorn >> ./sample_arm
+Emulate ARM code
+zsh: abort (core dumped)  ./sample_arm
+```
+then W^X is likely the culprit. If we run it again with ktrace
+and look at the output with kdump, we see that this is indeed
+the issue:
+``` 
+ 82192 sample_arm CALL  mmap(0,0x800000,0x7<PROT_READ|PROT_WRITE|PROT_EXEC>,0x1002<MAP_PRIVATE|MAP_ANON>,-1,0)
+ 82192 sample_arm PSIG  SIGABRT SIG_DFL
+ 82192 sample_arm NAMI  "sample_arm.core"
+```
+Right now, we're in the /home filesystem. Let's look at its mount
+options in /etc/fstab:
+```
+1234abcdcafef00d.g /home ffs rw,nodev,nosuid 1 2
+```
+If we edit the options to include ```wxallowed```, appending
+this after nosuid, for example, then we're golden:
+```
+1234abcdcafef00d.g /home ffs rw,nodev,nosuid,wxallowed 1 2
+```
+
+Note that this *does* diminish the security of your filesystem 
+somewhat, and so if you're particularly particular about such
+things, we recommend setting up a dedicated filesystem for 
+any activities that require ```(W|X)```, such as unicorn
+development and testing. 
+
+In order for these changes to take effect, you will need to
+reboot. 
+
+_Time passes..._
+
+Let's try this again. There's no need to recompile unicorn or 
+the samples, as (W^X) is strictly a runtime issue. 
+
+First, we double check to see if /home has been mounted with
+wxallowed:
+```
+/home >> mount | grep home
+/dev/sd3g on /home type ffs (local, nodev, nosuid, wxallowed)
+```
+Okay, now let's try running that sample again...
+```
+/home/git/unicorn/samples >> ./sample_arm
+Emulate ARM code
+>>> Tracing basic block at 0x10000, block size = 0x8
+>>> Tracing instruction at 0x10000, instruction size = 0x4
+>>> Emulation done. Below is the CPU context
+>>> R0 = 0x37
+>>> R1 = 0x3456
+==========================
+Emulate THUMB code
+>>> Tracing basic block at 0x10000, block size = 0x2
+>>> Tracing instruction at 0x10000, instruction size = 0x2
+>>> Emulation done. Below is the CPU context
+>>> SP = 0x1228
+```
+works fine. 
+