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

From: H. Peter Anvin
Date: Thu May 22 2008 - 20:37:56 EST

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!


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