linux-kernel - Re: [RFC 0/2] __vdso

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Message-ID: <9a4fb9c9-4b1c-4593-a99e-a2ef2325d55d@email.android.com>
Date:	Sun, 15 Jun 2014 12:31:17 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ian Lance Taylor <iant@...ang.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Rich Felker <dalias@...c.org>,
	Mikael Pettersson <mikpelinux@...il.com>,
	Russ Cox <rsc@...ang.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>
Subject: Re: [RFC 0/2] __vdso_findsym

The weak symbols are well-known names.  The __vdso symbols are strong.

On June 15, 2014 12:22:17 PM PDT, Ian Lance Taylor <iant@...ang.org> wrote:
>On Sun, Jun 15, 2014 at 12:14 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> If it doesn't, then you incur an additional indirection penalty.  The
>strong __vdso symbol allows the libc wrapper to fall back to the vdso
>implementation, the weak symbol allows three to be no wrapper at all. 
>This is good.
>>
>> The reason for changing ABI would be shifting types.  This is very
>much how glibc manages transitions.
>
>The purpose of symbol versioning is so that symbols with well known
>names, like stat, can continue to use those same names while changing
>types.  Both old and new programs can continue to use the name stat
>and continue to work even though they use different types.
>
>I don't see how this applies to the kernel VDSO.  Those symbols do not
>use well-known names; they use names like __vdso_time.  If you change
>the types used by those symbols, you can change the name as well.
>What is the downside?
>
>Ian

-- 
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