While hooking code in userland seems to be fairly common for various purposes (such as sandboxing malware by API hooking), hooking system calls is usually not done in userland. As you can get the same information from employing such hooks in kernelland (just after the transition), people usually choose to deploy their hooks there, since they benefit from added security and stability if implemented properly. That being said, there is one application of system call hooking that rightfully belongs into userland: Hooking of 32bit system calls on a native 64bit environment.http://2011.5.16.hooking-32bit-system-c ... g.oxff.net
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