Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter_enclave

[Dave Hansen]

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".