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

From	"H. Peter Anvin" <hpa@zytor.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the fast path
Date	2016-04-18 08:20 +0200
Message-ID	<rpaHo-Nu-5@gated-at.bofh.it> (permalink)
References	(5 earlier) <rpa4G-f9-3@gated-at.bofh.it> <rpaem-jl-9@gated-at.bofh.it> <rpaem-jl-7@gated-at.bofh.it> <rpaxH-Jy-5@gated-at.bofh.it> <rpaHo-Nu-7@gated-at.bofh.it>
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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.

	-hpa

Thread

Re: System call number masking Ben Hutchings <ben@decadent.org.uk> - 2016-04-18 02:50 +0200
  [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Ben Hutchings <ben@decadent.org.uk> - 2016-04-18 02:50 +0200
    Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 07:00 +0200
      Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Andy Lutomirski <luto@amacapital.net> - 2016-04-18 07:20 +0200
        Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 07:30 +0200
        Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 07:30 +0200
          Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Andy Lutomirski <luto@amacapital.net> - 2016-04-18 07:40 +0200
            Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 07:50 +0200
              Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Andy Lutomirski <luto@amacapital.net> - 2016-04-18 07:50 +0200
                Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 08:10 +0200
                Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 08:20 +0200
                Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Andy Lutomirski <luto@amacapital.net> - 2016-04-18 08:20 +0200

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