> Luoqi Chen wrote:
>
> > > o restoring %gs is now handled in the proper sigreturn.
> >
> > Restoration of %gs should not be in the kernel because it comes from
> > user application and maybe invalid, if you restore it inside the kernel
> > it could be fatal to the whole system, and on the other hand just a core
> > dump if done in the trampoline code which is still in user mode.
>
> Hmmm... What if the application passes a (possibly handcrafted)
> sigcontext to an explicit call to sigreturn. %gs should be restored in
> that case too, right?
>
> Isn't it therefore better to have %gs in the trapframe?
>
> --
> Marcel Moolenaar mailto:[EMAIL PROTECTED]
> SCC Internetworking & Databases http://www.scc.nl/
> The FreeBSD project mailto:[EMAIL PROTECTED]
>
The only place sigreturn is called explicitly is to enter VM86 mode, and
in that case, %gs is restored inside kernel as part of vm86 trapframe.
In fact, you may choose to restore %gs in the kernel, but you have to be
prepared to take a fault on the load_gs operation and to recover from
the fault properly.
The reason why %gs is not in the trapframe is that trapframe is used at
all entrances to the kernel (syscall/trap/intr), if %gs is included, then
we need to save and restore %gs for each syscall/trap/intr, that's about
40 clock cycles wasted each time.
-lq
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