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

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Message-ID: <8607961d-b42a-4dba-a736-d3a8586ee42d@email.android.com>
Date:	Sun, 15 Jun 2014 12:56:39 -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

You can link against vdso.so if you want to; the kernel build provides an option to install that image.  Doesn't mean that any particular libc uses it.

On June 15, 2014 12:50:36 PM PDT, Ian Lance Taylor <iant@...ang.org> wrote:
>On Sun, Jun 15, 2014 at 12:31 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> The weak symbols are well-known names.  The __vdso symbols are
>strong.
>
>I see.  But I don't understand how this is supposed to work.  When I
>link a program against gettimeofday, I get a reference to gettimeofday
>with version GLIBC_2.2.5.  After all, I only link against libc.so; I
>don't link against the vDSO.  The VDSO provides gettimeofday with
>version LINUX_2.6.  Since those versions don't match, the gettimeofday
>reference in my executable will not be satisfied by the definition in
>the vDSO.  So at dynamic link time my program is always going to be
>linked with the gettimeofday in libc.so, which will in turn call the
>gettimeofday in the vDSO.
>
>Am I missing something that makes the definition of gettimeofday with
>version LINUX_2.6 in the vDSO useful?
>
>Ian
>
>
>
>> 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
>>
>> --
>> Sent from my mobile phone.  Please pardon brevity and lack of
>formatting.

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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