Re: [RFC][PATCH 07/22] x86,extable: Extend extable functionality

[Sean Christopherson <seanjc@google.com>]

On Fri, Nov 05, 2021, Peter Zijlstra wrote:
> On Fri, Nov 05, 2021 at 05:32:14PM +0000, Sean Christopherson wrote:
> 
> > > +#define EX_IMM_MASK	0xFFFF0000
> 
> > > +	imm  = FIELD_GET(EX_IMM_MASK,  e->type);
> > 
> > FIELD_GET casts the result based on the type of the mask, but doesn't explicitly
> > sign extend the masked field, i.e. there's no intermediate cast to tell the compiler
> > that the imm is a 16-bit value that should be sign extended.
> > 
> > Modifying FIELD_GET to sign extended is probably a bad idea as I'm guessing the
> > vast, vast majority of use cases don't want that behavior.  I'm not sure how that
> > would even work with masks that are e.g. 5 bits or so.
> 
> So the way I was reading it was that typeof(_mask) is 'int', e->type is
> also 'int', we mask out the top bits, and since it's all 'int' we do an
> arith shift right (ie. preserves sign).
> 
> Where did that reading go wrong?

Hmm, C99 standard says that right shift with a negative value is implementation
specific:

  E1 has a signed type and a negative value, the resulting value is implementation-defined.

gcc-10 generates a bare "shr", i.e. doesn't special case negative values, and "shr"
is explicitly defined as an unsigned divide.