Re: [PATCH 02/19] x86, fpu: wrap get_xsave_addr() to make it safer

Thomas Gleixner
2015-05-18T19:42:21Z

On Mon, 18 May 2015, Thomas Gleixner wrote:

> On Fri, 8 May 2015, Dave Hansen wrote:
> > The MPX code appears to be saving off the FPU in an unsafe
> > way. It does not disable preemption or ensure that the
> > FPU state has been allocated.
> >
> > This patch introduces a new helper which will do both of
> > those things internally.
>
> This changelog does not really match the implementation. Unless I'm
> missing something I can't find anything preemption related.

Gah. Hit send before finishing the mail.

It's unlazy_fpu (which I agree is a horrible name) which does the
right thing.

> > +
> > +/*
> > + * This wraps up the common operations that need to occur when retrieving
> > + * data from xsave state. It first ensures that the current task was
> > + * using the FPU and retrieves the data in to a buffer. It then calculates
> > + * the offset of the requested field in the buffer.
> > + *
> > + * This function is safe to call whether the FPU is in use or not.
> > + *
> > + * Note that this only works on the current task.
> > + *
> > + * Inputs:
> > + * @xsave_field: state which is defined in xsave.h (e.g. XSTATE_FP,
> > + * XSTATE_SSE, etc...)
> > + * Output:
> > + * address of the state in the xsave area.
>
> or NULL in case of .....
>
> > + */
> > +void *get_xsave_field(int xsave_field)
> > +{
> > + union thread_xstate *xstate;
> > +
> > + if (!tsk_used_math(current))
> > + return NULL;
> > + /*
> > + * unlazy_fpu() is poorly named and will actually
> > + * save the xstate off in to the memory buffer.
> > + */
> > + unlazy_fpu(current);
> > + xstate = current->thread.fpu.state;
> > +
> > + return get_xsave_addr(&xstate->xsave, xsave_field);
> > +}
>
> Thanks,
>
> tglx
>