linux-kernel - Re: [patch 17/26] Xen-paravirt_ops: Add nosegneg capability to the vsyscall page notes

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Date:	Fri, 16 Mar 2007 14:56:24 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Roland McGrath <roland@...hat.com>
CC:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com, Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: [patch 17/26] Xen-paravirt_ops: Add nosegneg capability to the
 vsyscall page notes

Roland McGrath wrote:
> This should be:
>
> NOTE_KERNELCAP_BEGIN(1, 1)
> NOTE_KERNELCAP(0, "nosegneg")
> NOTE_KERNELCAP_END
>
> i.e. 1->0 in the "bit" member.  (Note the ld.so.conf.d file must have the
> matching bit number for ldconfig-based lookups to do the right thing.)
> Or else:
>
> NOTE_KERNELCAP_BEGIN(1, 2)
> NOTE_KERNELCAP(0, "nosegneg")
> NOTE_KERNELCAP_END
>
> i.e. 1->2 in the "mask" member.  (The mask value should be 1<<bit.)
>   

Thanks Roland.  I've never really understood this stuff, and I just
copied this cargo-cultishly.

I'm not quite sure what you're suggesting here though.  Do you mean one of:

NOTE_KERNELCAP_BEGIN(1, 1)
NOTE_KERNELCAP(0, "nosegneg")
NOTE_KERNELCAP_END

or

NOTE_KERNELCAP_BEGIN(1, 2)
NOTE_KERNELCAP(1, "nosegneg")
NOTE_KERNELCAP_END

is the correct thing to use?

> Some pre-release glibc's (before 2.4) had a bug in the code that parses
> this, and would crash parsing the correct note.  Using the wrong bit value
> with nonmatching mask worked around this.  IIRC, no glibc release ever
> included the buggy version of the code.  In nonbuggy glibc, the mismatched
> value causes the "nosegneg" to be omitted from the directory search (under
> LD_LIBRARY_PATH and default directories), though ldconfig-based lookups
> will work (the most common case).
>   

Are you saying that one of the corrected forms might cause old glibcs to
crash, or just ignore nosegneg?

    J

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