Re: x86_64: vsyscall vs vdso

[Mikael Pettersson]

Francis Moreau writes:
 > On 9/17/07, Ulrich Drepper <drepper@xxxxxxxxx> wrote:
 > > On 9/17/07, Francis Moreau <francis.moro@xxxxxxxxx> 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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