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

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Message-ID: <57147837.2030706@zytor.com>
Date:	Sun, 17 Apr 2016 23:01:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ben Hutchings <ben@...adent.org.uk>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the
 fast path

On 04/17/16 22:48, Andy Lutomirski wrote:
> 
> I think I prefer the "reject weird input" behavior over the "accept
> and normalize weird input" if we can get away with it, and I'm fairly
> confident that we can get away with "reject weird input" given that
> distro kernels do exactly that already.
> 

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.

So I'm not sure this is a valid consideration.  The reason it generally
works is because the natural way for the user space code to work is to
load a value into %eax which will naturally zero-extend to %rax, but it
isn't inherently required to work that way.

	-hpa