On 3/12/21 8:55 AM, Jarkko Sakkinen wrote: >> ENDBR is a special new instruction for the Indirect Branch Tracking >> (IBT) component of CET. IBT prevents attacks by ensuring that (most) >> indirect branches and function calls may only land at ENDBR >> instructions. Branches that don't follow the rules will result in >> control flow (#CF) exceptions. >> >> ENDBR is a noop when IBT is unsupported or disabled. Most ENDBR >> instructions are inserted automatically by the compiler, but branch >> targets written in assembly must have ENDBR added manually, like this one. > Thank you, this clears the whole thing a lot. > > Doesn't this mean that it could be there just as well unconditionally? It could be there unconditionally. But, I think it's still worth the #ifdef just out of the principle of being as tidy as possible. The #ifdef is basically as low cost and low complexity as you get. It is also somewhat self-documenting: "This instruction is only necessary when your CPU supports IBT".
