linux-kernel - Re: [PATCH] mm: only enable sys_pkey* when ARCH_HAS

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Date:   Wed, 2 Nov 2016 12:15:50 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH] mm: only enable sys_pkey* when ARCH_HAS_PKEYS

On 10/31/2016 05:08 PM, Mark Rutland wrote:
> When an architecture does not select CONFIG_ARCH_HAS_PKEYS, the pkey_alloc
> syscall will return -ENOSPC for all (otherwise well-formed) requests, as the
> generic implementation of mm_pkey_alloc() returns -1. The other pkey syscalls
> perform some work before always failing, in a similar fashion.
> 
> This implies the absence of keys, but otherwise functional pkey support. This
> is odd, since the architecture provides no such support. Instead, it would be
> preferable to indicate that the syscall is not implemented, since this is
> effectively the case.

This makes the behavior of an x86 cpu without pkeys and an arm cpu
without pkeys differ.  Is that what we want?  An application that
_wants_ to use protection keys but can't needs to handle -ENOSPC anyway.

On an architecture that will never support pkeys, it makes sense to do
-ENOSYS, but that's not the case for arm, right?