Re: [Xen-devel] [PATCH for-4.10 1/2] ubsan: add clang 5.0 support

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Oct 17, 2017 at 06:43:28AM -0600, Jan Beulich wrote:
> >>> On 17.10.17 at 13:36, <roger.pau@xxxxxxxxxx> wrote:
> > clang 5.0 changed the layout of the type_mismatch_data structure and
> > introduced __ubsan_handle_type_mismatch_v1 and
> > __ubsan_handle_pointer_overflow.
> > 
> > This commit adds support for the new structure layout, adds the
> > missing handlers and the new types for type_check_kinds.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> with a remark:
> 
> > +void __ubsan_handle_pointer_overflow(struct pointer_overflow_data *data,
> > +                           unsigned long base, unsigned long result)
> > +{
> > +   unsigned long flags;
> > +
> > +   if (suppress_report(&data->location))
> > +           return;
> > +
> > +   ubsan_prologue(&data->location, &flags);
> > +
> > +   pr_err("pointer overflow:\n");
> > +
> > +   if (((long)base >= 0) == ((long)result >= 0))
> > +           pr_err("%s of unsigned offset to %p overflowed to %p\n",
> > +                   base > result ? "addition" : "subtraction",
> 
> Strictly speaking you also want to make "to" conditional upon this
> being an add; for subtract it ought to be "from". Or perhaps just
> say overflow and underflow?
> 
> And then - is "base > result" really a valid determination of
> add/subtract (or overflow/underflow)? If the pointed to type is
> wider than one byte, an addition may wrap one or more times
> and still yield a value larger than the starting pointer.

That code is mostly adapted from upstream llvm:

https://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/ubsan/ubsan_handlers.cc?revision=313572&view=markup

But I see your point, if the types have different widths that check
won't work properly. In any case, I don't see a better way to deal
with this.

Thanks, Roger.

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