Re: [RFC PATCH] -fsanitize=pointer-overflow support (PR sanitizer/80998)

[Richard Biener <rguenther at suse dot de>]

On Wed, 21 Jun 2017, Jakub Jelinek wrote:

> On Tue, Jun 20, 2017 at 10:18:20AM +0200, Richard Biener wrote:
> > > It would be an attempt to avoid sanitizing int foo (int *p) { return p[10] + p[-5]; }
> > > (when the offset is constant and small and we dereference it).
> > > If there is no page mapped at NULL or at the highest page in the virtual
> > > address space, then the above will crash in case p + 10 or p - 5 wraps
> > > around.
> > 
> > Ah, so merely an optimization to avoid excessive instrumentation then,
> > yes, this might work (maybe make 4096 a --param configurable to be able
> > to disable it).
> 
> Yes.  And I think it can be implemented incrementally.
> 
> > > > > I've bootstrapped/regtested the patch on x86_64-linux and i686-linux
> > > > > and additionally bootstrapped/regtested with bootstrap-ubsan on both too.
> > > > > The latter revealed a couple of issues I'd like to discuss:
> > > > > 
> > > > > 1) libcpp/symtab.c contains a couple of spots reduced into:
> > > > > #define DELETED ((char *) -1)
> > > > > void bar (char *);
> > > > > void
> > > > > foo (char *p)
> > > > > {
> > > > >   if (p && p != DELETED)
> > > > >     bar (p);
> > > > > }
> > > > > where we fold it early into if ((p p+ -1) <= (char *) -3)
> > > > > and as the instrumentation is done during ubsan pass, if p is NULL,
> > > > > we diagnose this as invalid pointer overflow from NULL to 0xffff*f.
> > > > > Shall we change the folder so that during GENERIC folding it
> > > > > actually does the addition and comparison in pointer_sized_int
> > > > > instead (my preference), or shall I move the UBSAN_PTR instrumentation
> > > > > earlier into the FEs (but then I still risk stuff is folded earlier)?
> > > > 
> > > > Aww, so we turn the pointer test into a range test ;)  That it uses
> > > > a pointer type rather than an unsigned integer type is a bug, probably
> > > > caused by pointers being TYPE_UNSIGNED.
> > > > 
> > > > Not sure if the folding itself is worthwhile to keep though, thus an
> > > > option would be to not generate range tests from pointers?
> > > 
> > > I'll have a look.  Maybe only do it during reassoc and not earlier.
> > 
> > It certainly looks somewhat premature in fold-const.c.
> 
> So for this, I have right now 2 variant patches:
> 
> The first one keeps doing what we were except for the
> -fsanitize=pointer-overflow case and has been bootstrap-ubsan
> bootstrapped/regtested on x86_64-linux and i686-linux.
> 
> The second one performs the addition and comparison in pointer sized
> unsigned type instead (not bootstrapped yet).
> 
> I think the second one would be my preference.  Note build_range_check
> is used not just during early folding, but e.g. during ifcombine, reassoc
> etc.
> 
> Martin is contemplating instrumentation of pointer <=/</>=/> comparisons
> and in that case we'd need some further build_range_check changes,
> because while ptr == (void *) 0 || ptr == (void *) 1 || ptr == (void *) 2
> would be without UB, ptr <= (void *) 2 would be UB, so we'd need to perform
> all pointer range checks in integral type except the ones where we just do
> EQ_EXPR/NE_EXPR.

Yes, exactly.

The 2nd patch is ok if it passes bootstrap/test.

Richard.