linux-kernel - Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT

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Date:	Mon, 21 Mar 2016 14:49:44 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2

On Mon, Mar 21, 2016 at 12:16 PM, Andi Kleen <andi@...stfloor.org> wrote:
> From: Andi Kleen <ak@...ux.intel.com>
>
> The kernel needs to explicitely enable RD/WRFSBASE to handle context
> switch correctly. So the application needs to know if it can safely use
> these instruction. Just looking at the CPUID bit is not enough because it
> may be running in a kernel that does not enable the instructions.
>
> One way for the application would be to just try and catch the SIGILL.
> But that is difficult to do in libraries which may not want
> to overwrite the signal handlers of the main application.
>
> So we need to provide a way for the application to discover the kernel
> capability.
>
> I used AT_HWCAP2 in the ELF aux vector which is already used by
> PPC for similar things. We define a new Linux defined bitmap
> returned in AT_HWCAP.  Currently it has only one bit set,
> for kernel is FSGSBASE capable.
>
> The application can then access it manually or using
> the getauxval() function in newer glibc.

How about adding a VDSO function instead?  The VDSO can use
alternatives, so it can use the new instructions if supported, or else
use the old syscall.

--
Brian Gerst