Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU number of running thread

[Peter Zijlstra]

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.