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

From: Suresh Siddha
Date: Tue May 20 2008 - 13:53:35 EST

On Tue, May 20, 2008 at 05:20:43PM +0200, Mikael Pettersson wrote:
> H. Peter Anvin writes:
> > Mikael Pettersson wrote:
> > > So the question now is what if anything has the Linux kernel written
> > > to those reserved fields. (Looking..) Hmm, signal delivery on x86-64
> > > seems to do fxsave directly to the fxsave area in the user's sigframe,
> > > which would imply that the reserved fields have unpredictable values.
> > >
> >
> > OK, so that's not a usable path unless we can find some area in the
> > existing data set to put a flag. Groan.
> An ugly workaround could be to start clearing one of these fields,
> and say that the data there is only valid for kernels >= 2.6.26.
> (I said it was ugly...)
> Or we go back to stashing a flag in uc_flags (which is kosher),
> and try to figure out how to mark non-rt sigframes.

This issue of not-zeroing, is present in only 64bit kernels and for 64bit apps,

64bit app signal handling uses only rt_frame, so we can add an uc_flag for
them and for 32bit apps, kernel was always zero'ing the reserved bits
at the end of _fpstate.

In short, for non-rt frames, they can check the reserved bits at the end
of fpstate frame and for rt-frames (perhaps even for 32bit rt frame handling)
apps can check for uc_flag aswell, for extended state presence. Is this
good enough?

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