linux-kernel - Re: vdso feature requests from the Go people

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Message-ID: <539A8EA8.9080901@zytor.com>
Date:	Thu, 12 Jun 2014 22:39:52 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Russ Cox <rsc@...ang.org>,
	Ian Taylor <iant@...ang.org>
Subject: Re: vdso feature requests from the Go people

On 06/12/2014 09:36 PM, Andy Lutomirski wrote:
> 
> 1. Parsing the vDSO is a PITA.  What if we bundled the reference
> parser inside the vdso?  Concretely, we could have AT_VDSO_FINDENTRY
> point to a function like:
> 
> void *vdso_find_entry(const char *name, const char *version)
> 
> Then things like Go and maybe even musl (and klibc?) could just call
> that function.  And we'd never have to worry about maintaining
> compatibility with more and more weird vdso parsers.
> 
> Implementing this could be as simple as shoving parse_vdso.c into the
> vdso, although vdso2c could help and allow a really simple in-vdso
> implementation.
> 

I'm not really sure how much of a win that is... you have to parse
*something*, and for the vast majority of all implementations there will
be a dynamic linker just sitting there, and that is what it *does*.

> 2. Go uses a segmented stack, and the vdso is quite unfriendly for
> segmented stack.  If we can get compiler support, is there a
> reasonable way that we could advertise the maximum stack usage of each
> vdso entry point?

I suspect an easier way to do that would just be to define a maximum
stack usage for *any* vdso entry point, and then enable the gcc stack
depth warning (perhaps even with Werror)... we can do this now.

	-hpa

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