Bug#822236: vDSO is not in effect in static linked executable on jessie
control: retitle -1 vsyscall is not in effect in static linked executable on jessie
thanks
On 2016-04-24 21:01, jian wang wrote:
> On Sun, Apr 24, 2016 at 6:31 PM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> > control: tag -1 + moreinfo
> >
> > On 2016-04-22 11:11, Mattia Rizzolo wrote:
> >> control: reassign -1 libc6 2.19-18+deb8u4
> >>
> >> On Fri, Apr 22, 2016 at 06:27:34PM +0800, jian wang wrote:
> >> > Package: lib6
> >>
> >> lost a 'c' :)
> >> reassigning.
> >>
> >> > Version: 2.19-18+deb8u4
> >> > Severity: important
> >> >
> >> > vDSO is not in effect in static linked executable on jessie. This is
> >> > not the case with wheezy.
> >> >
> >> > We build binary to run on both wheezy and jessie, so -static. When
> >> > building platform is wheezy, vdso works fine, but jessie the opposite.
> >> >
> >> > This leads to performance hit. Unfortunately, we use g++ 4.9 for some
> >> > application, so wheezy can't be used as build platform.
> >
> > There is not of information about your environment, so given what you
> > describe, it seems you are using an i386 installation.
> >
>
> The context is for lots of servers. We use amd64 all over datacenters.
>
> > The vDSO support requires at least a i686 CPU, while Debian i386 targets
> > i586. When using dynamic linking, one can install libc6-i686 which
> > provides an i686 optimized version of the libc, therefore with vDSO
> > support. It is automatically loaded if the CPU supports it.
> >
> > In your case, given you use static linking, the i586 version of the libc
> > is used, and it therefore doesn't support vDSO . That is true on wheezy,
> > jessie, stretch, sid. I don't believe this is a regression, and in
> > addition I can't reproduce it.
> >
>
> No.
>
> We have looked into the code now.
>
> Glibc 2.13 (wheezy) uses an asm version of gettimeofday.S for x86_64
> which uses vDSO.
To be more precise, glibc 2.13 uses vDSO for shared builds and vsyscall
for static ones.
> On the way up to glibc 2.19 (jessie), vDSO was rewritten into
> gettimeofday.c. And as we dig out, Ulrich Drepper decided to not
> optimize static linking, or even to dwarf static linking
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=12813
vsyscalls are consider deprecated due to limitation and security
consideration. I guess that the reason the vsyscall part of the code has
been removed.
Aurelien
--
Aurelien Jarno GPG: 4096R/1DDD8C9B
aurelien@aurel32.net http://www.aurel32.net
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