Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the fast path

From: H. Peter Anvin
Date: Mon Apr 18 2016 - 02:19:49 EST

On 04/17/16 23:14, Andy Lutomirski wrote:
>> It's not "weird", it is the ABI as defined. We have to do this for all
>> the system call arguments, too; you just don't notice it because the
>> compiler does it for us. Some other architectures, e.g. s390, has the
>> opposite convention where the caller is responsible for normalizing the
>> result; in that case we have to do it *again* in the kernel, which is
>> one of the major reasons for the SYSCALL_*() macros.
> What ABI?

The C ABI for int. I hadn't seen the below, because I think syscall(3)
is just braindamaged, but the odds are that if we'd ever use the upper
32 bits for anything we'd be in a world of hurt, so that would be highly
theoretical IMO. Bit 31 might be possible, but I wouldn't really want
to brave it unless we really have no choice.

> Also, the behavior in which fail the syscall if any high bits are set
> is faster -- it's one fewer instruction. Admittedly, the CPU can
> probably do that instruction for free, but still...

Yes, it can; at least on any remotely modern hardware.