Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

["H. Peter Anvin" <hpa@zytor.com>]

Mikael Pettersson wrote:
> 
>  > > While restoring from the user, kernel also need to find out what layout
>  > > the user is passing. So it's bi-directional. I prefer the same mechanism
>  > > (using cookies/magic numbers etc inaddition to uc_flags or cpuid checks) to
>  > > interpret the fpstate for both user/kernel.
>  > 
>  > No, it really doesn't: the kernel only needs to be able to read the same 
>  > format as it itself wrote.
> 
> The kernel needs to accept one(*) of the formats it can produce, which
> is not necessarily what it last produced. It's not inconceivable that
> user-space will construct sigframes on the fly (to emulate setcontext),
> or that it will mangle sigframes (e.g. to map non-rt to rt before sigreturn).
> 
> (*) The format is determined by which version of sys_sigreturn the
> user invokes.
> 

No.  You CANNOT restore from a frame that doesn't have the full state - 
you don't have enough information to do so!

	-hpa