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

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Message-ID: <CALCETrWnQyCk5vhDj-yAhRdAFOtFKSU6H-E_=LBC+FbA2REhrg@mail.gmail.com>
Date:	Fri, 20 Jun 2014 08:55:01 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Rich Felker <dalias@...c.org>
Cc:	Ian Lance Taylor <iant@...ang.org>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	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

On Mon, Jun 16, 2014 at 8:42 AM, Rich Felker <dalias@...c.org> wrote:
> On Mon, Jun 16, 2014 at 08:31:39AM -0700, Ian Lance Taylor wrote:
>> On Mon, Jun 16, 2014 at 7:38 AM, Andi Kleen <andi@...stfloor.org> wrote:
>> >> I think this issue started when some of the Go developers questioned
>> >> why the kernel needed to provide a very complex interface--parsing an
>> >> ELF shared shared library--for very simple functionality--looking up
>> >> the address of a magic function.  This approach has required special
>> >> support not just in Go, but also in the dynamic linker and gdb, and
>> >> does not work well for statically linked binaries.  The support in gdb
>> >> is perhaps a good idea, but elsewhere it does not make sense.
>> >>
>> >> So why not provide a simple interface?
>> >
>> > What good would it do now that everyone already supports it?
>>
>> Do statically linked binaries use the vDSO calls?
>
> Under glibc, I believe so (not checked). Under musl, yes, and even in
> the dynamic-linked case we use the same code that's used for static
> linking rather than trying to get the dynamic linker to do them
> correctly. I still have some cruft lying around from where (in the
> past) we tried to do it via the dynamic linker, but I'm probably going
> to remove that and make the vdso behave as RTLD_LOCAL so that there's
> no risk of weird symbols it exports interfering with the application
> (applications could still make it global via an explicit dlopen). The
> only reason for keeping it around at all in the dynamic linker is for
> the sake of gdb.
>

What about backtrace_symbols, dl_addr, and the unwinder (e.g.
siglongjmp)?  It would be nice to wean the vdso off of frame pointers
some day.

--Andy
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