c++: Further fix for -fsanitize=vptr [PR94325]

Hi!

For -fsanitize=vptr, we insert a NULL store into the vptr instead of just
adding a CLOBBER of this.  build_clobber_this makes the CLOBBER conditional
on in_charge (implicit) parameter whenever CLASSTYPE_VBASECLASSES, but when
adding this conditionalization to the -fsanitize=vptr code in PR87095,
I wanted it to catch some more cases when the class has CLASSTYPE_VBASECLASSES,
but the vptr is still not shared with something else, otherwise the
sanitization would be less effective.
The following testcase shows that the chosen test that CLASSTYPE_PRIMARY_BINFO
is non-NULL and has BINFO_VIRTUAL_P set wasn't sufficient,
the D class has still sizeof(D) == sizeof(void*) and thus contains just
a single vptr, but while in B::~B() this results in the vptr not being
cleared, in C::~C() this condition isn't true, as CLASSTYPE_PRIMARY_BINFO
in that case is B and is not BINFO_VIRTUAL_P, so it clears the vptr, but the
D::~D() dtor after invoking C::~C() invokes A::~A() with an already cleared
vptr, which is then reported.
The following patch is just a shot in the dark, keep looking through
CLASSTYPE_PRIMARY_BINFO until we find BINFO_VIRTUAL_P, but it works on the
existing testcase as well as this new one.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Or do we want some other test?

2020-04-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94325
	* decl.c (begin_destructor_body): For CLASSTYPE_VBASECLASSES class
	dtors, if CLASSTYPE_PRIMARY_BINFO is non-NULL, but not BINFO_VIRTUAL_P,
	look at CLASSTYPE_PRIMARY_BINFO of its BINFO_TYPE if it is not
	BINFO_VIRTUAL_P, and so on.

	* g++.dg/ubsan/vptr-15.C: New test.


	Jakub

On 4/6/20 6:43 PM, Jakub Jelinek wrote:
> Hi!
> 
> For -fsanitize=vptr, we insert a NULL store into the vptr instead of just
> adding a CLOBBER of this.  build_clobber_this makes the CLOBBER conditional
> on in_charge (implicit) parameter whenever CLASSTYPE_VBASECLASSES, but when
> adding this conditionalization to the -fsanitize=vptr code in PR87095,
> I wanted it to catch some more cases when the class has CLASSTYPE_VBASECLASSES,
> but the vptr is still not shared with something else, otherwise the
> sanitization would be less effective.
> The following testcase shows that the chosen test that CLASSTYPE_PRIMARY_BINFO
> is non-NULL and has BINFO_VIRTUAL_P set wasn't sufficient,
> the D class has still sizeof(D) == sizeof(void*) and thus contains just
> a single vptr, but while in B::~B() this results in the vptr not being
> cleared, in C::~C() this condition isn't true, as CLASSTYPE_PRIMARY_BINFO
> in that case is B and is not BINFO_VIRTUAL_P, so it clears the vptr, but the
> D::~D() dtor after invoking C::~C() invokes A::~A() with an already cleared
> vptr, which is then reported.
> The following patch is just a shot in the dark, keep looking through
> CLASSTYPE_PRIMARY_BINFO until we find BINFO_VIRTUAL_P, but it works on the
> existing testcase as well as this new one.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> Or do we want some other test?

OK.

> 2020-04-06  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/94325
> 	* decl.c (begin_destructor_body): For CLASSTYPE_VBASECLASSES class
> 	dtors, if CLASSTYPE_PRIMARY_BINFO is non-NULL, but not BINFO_VIRTUAL_P,
> 	look at CLASSTYPE_PRIMARY_BINFO of its BINFO_TYPE if it is not
> 	BINFO_VIRTUAL_P, and so on.
> 
> 	* g++.dg/ubsan/vptr-15.C: New test.
> 
> --- gcc/cp/decl.c.jj	2020-03-27 09:59:26.407083563 +0100
> +++ gcc/cp/decl.c	2020-04-06 13:25:03.321511554 +0200
> @@ -16662,14 +16662,20 @@ begin_destructor_body (void)
>   	    /* If the vptr is shared with some virtual nearly empty base,
>   	       don't clear it if not in charge, the dtor of the virtual
>   	       nearly empty base will do that later.  */
> -	    if (CLASSTYPE_VBASECLASSES (current_class_type)
> -		&& CLASSTYPE_PRIMARY_BINFO (current_class_type)
> -		&& BINFO_VIRTUAL_P
> -			  (CLASSTYPE_PRIMARY_BINFO (current_class_type)))
> +	    if (CLASSTYPE_VBASECLASSES (current_class_type))
>   	      {
> -		stmt = convert_to_void (stmt, ICV_STATEMENT,
> -					tf_warning_or_error);
> -		stmt = build_if_in_charge (stmt);
> +		tree c = current_class_type;
> +		while (CLASSTYPE_PRIMARY_BINFO (c))
> +		  {
> +		    if (BINFO_VIRTUAL_P (CLASSTYPE_PRIMARY_BINFO (c)))
> +		      {
> +			stmt = convert_to_void (stmt, ICV_STATEMENT,
> +						tf_warning_or_error);
> +			stmt = build_if_in_charge (stmt);
> +			break;
> +		      }
> +		    c = BINFO_TYPE (CLASSTYPE_PRIMARY_BINFO (c));
> +		  }
>   	      }
>   	    finish_decl_cleanup (NULL_TREE, stmt);
>   	  }
> --- gcc/testsuite/g++.dg/ubsan/vptr-15.C.jj	2020-04-06 13:32:43.501627756 +0200
> +++ gcc/testsuite/g++.dg/ubsan/vptr-15.C	2020-04-06 13:37:52.642001353 +0200
> @@ -0,0 +1,14 @@
> +// PR c++/94325
> +// { dg-do run { target c++11 } }
> +// { dg-options "-fsanitize=vptr -fno-sanitize-recover=vptr" }
> +
> +struct A { virtual ~A () = default; };
> +struct B : public virtual A {};
> +struct C : public B {};
> +struct D : public C {};
> +
> +int
> +main ()
> +{
> +  D a;
> +}
> 
> 	Jakub
>