Date: Thu, 7 Apr 2016 13:19:38 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, linux-api <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>, Andrew Hunter <ahh@google.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Andi Kleen <andi@firstfloor.org>, Dave Watson <davejwatson@fb.com>,
        Chris Lameter <cl@linux.com>, Ben Maurer <bmaurer@fb.com>,
        rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU
 number of running thread
Message-ID: <20160407111938.GR3430@twins.programming.kicks-ass.net>
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On Thu, Apr 07, 2016 at 12:39:21PM +0200, Florian Weimer wrote:
> On 04/07/2016 12:31 PM, Peter Zijlstra wrote:
> > On Thu, Apr 07, 2016 at 11:01:25AM +0200, Florian Weimer wrote:
> >>> Because ideally this structure would be part of the initial (glibc) TCB
> >>> with fixed offset etc.
> >>
> >> This is not possible because we have layering violations and code
> >> assumes it knows the precise of the glibc TCB.  I think Address
> >> Sanitizer is in this category.  This means we cannot adjust the TCB size
> >> based on the kernel headers used to compile glibc, and there will have
> >> to be some indirection.
> > 
> > So with the proposed fixed sized object it would work, right?
> 
> I didn't see a proposal for a fixed size buffer, in the sense that the
> size of struct sockaddr_in is fixed.

This thing proposed a single 64byte structure (with the possibility of
eventually adding more 64byte structures). Basically:

struct tlabi {
	union {
		__u8[64] __foo;
		struct {
			/* fields go here */
		};
	};
} __aligned__(64);

People objected against the fixed size scheme, but it being possible to
get a fixed TCB offset and reduce indirections is a big win IMO.