linux-kernel - Re: x86_64: vsyscall vs vdso

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Message-ID: <18159.36076.350387.910660@alkaid.it.uu.se>
Date:	Tue, 18 Sep 2007 10:31:40 +0200
From:	Mikael Pettersson <mikpe@...uu.se>
To:	"Francis Moreau" <francis.moro@...il.com>
Cc:	"Ulrich Drepper" <drepper@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: x86_64: vsyscall vs vdso

Francis Moreau writes:
 > On 9/17/07, Ulrich Drepper <drepper@...il.com> wrote:
 > > On 9/17/07, Francis Moreau <francis.moro@...il.com> wrote:
 > > > I think signal trampolines will still need them too. So making
 > > > vsyscalls configurable doesn't seem to work, does it ?
 > >
 > > vsyscalls aren't used for that.  We have a restorer in libc and could
 > > easily use one in the vdso.  That's what is done on x86.
 > >
 > 
 > Sorry for my ignorance but what' is 'a restorer' ?

When the kernel sets up the context for a user-space signal
handler, it needs to supply a return address (in a register
or on the stack). That's the restorer. The restorer points
to a stub that performs sys_{rt_,}sigreturn(). Depending on
architecture and kernel version, the restorer stub can be
defined by libc, or be provided automatically by the kernel.
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