linux-kernel - Re: [PATCH 3.19 3/3] x86, mpx: Change the MPX enable/disable API to arch

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Message-ID: <CALCETrU3oHO5Rjqp_DmJaOkEBOtZpQ+f99xXiY5=84JfJphZCA@mail.gmail.com>
Date:	Mon, 5 Jan 2015 21:59:27 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Dave Hansen <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3.19 3/3] x86, mpx: Change the MPX enable/disable API to arch_prctl

On Mon, Jan 5, 2015 at 8:04 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> We should avoid arch_prctl because glibc won't add a syscall stub that
>> libgcc or whatever would want?  My mind boggles.
>
> Please turn brain on before posting.

Brain was on.

>
> Of course they would add it. But only in the next version. Which means
> everyone using older glibc would be out of luck. So all the users
> would be stuck using syscall(). Anything you may gain in the kernel
> would be totally made up by that.
>

ISTM libmpx shouldn't link against glibc at all -- what if libmpx
users want to use an alternate runtime (musl, Go, etc.)?

But I decided to check whether libmpx links against glibc, and I can't
find sources for it at all.  Do they exist?  Is there any code with
source available that invokes this prctl?

If not, I personally have very little sympathy for the argument that a
binary buried in the depths of the Intel SDE would need to change if
we switched to using arch_prctl.  And I think that it should issue the
syscall itself without using glibc, in which case the syscall wrapper
issue is moot.

--Andy

> BTW I added arch_prctl, but in hindsight it wasn't good idea.
>
> -Andi

-- 
Andy Lutomirski
AMA Capital Management, LLC
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