Re: [RFC 0/2] __vdso_findsym

H. Peter Anvin
2014-06-15T19:57:31Z

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@golang.org> wrote:
>On Sun, Jun 15, 2014 at 12:31 PM, H. Peter Anvin <hpa@zytor.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@golang.org>
>wrote:
>>>On Sun, Jun 15, 2014 at 12:14 PM, H. Peter Anvin <hpa@zytor.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.