obstack.h __PTR_ALIGN vs. ubsan

obstack.h __PTR_ALIGN vs. ubsan

[Alan Modra]

Current ubsan complains on every use of __PTR_ALIGN (when ptrdiff_t is
as large as a pointer), due to making calculations relative to a NULL
pointer.  This patch avoids the problem by extracting out and
simplifying __BPTR_ALIGN for the usual case.  I've continued to use
ptrdiff_t here, where it might be better to throw away __BPTR_ALIGN
entirely and just assume uintptr_t exists.

OK to apply for gcc?

	* obstack.h (__PTR_ALIGN): Expand and simplify __BPTR_ALIGN
	rather than calculating relative to a NULL pointer.

diff --git a/include/obstack.h b/include/obstack.h
index a6eb6c95587..0d8746f835b 100644
--- a/include/obstack.h
+++ b/include/obstack.h
@@ -137,9 +137,9 @@
    relative to B.  Otherwise, use the faster strategy of computing the
    alignment relative to 0.  */
 
-#define __PTR_ALIGN(B, P, A)						      \
-  __BPTR_ALIGN (sizeof (ptrdiff_t) < sizeof (void *) ? (B) : (char *) 0,      \
-                P, A)
+#define __PTR_ALIGN(B, P, A)						\
+  (sizeof (ptrdiff_t) < sizeof (void *) ? __BPTR_ALIGN (B, P, A)	\
+   : (char *) (((ptrdiff_t) (P) + (A)) & ~(A)))
 
 #ifndef __attribute_pure__
 # if defined __GNUC_MINOR__ && __GNUC__ * 1000 + __GNUC_MINOR__ >= 2096

-- 
Alan Modra
Australia Development Lab, IBM

Re: obstack.h __PTR_ALIGN vs. ubsan

[Jeff Law]

On 9/1/2021 10:03 PM, Alan Modra via Binutils wrote:
> Current ubsan complains on every use of __PTR_ALIGN (when ptrdiff_t is
> as large as a pointer), due to making calculations relative to a NULL
> pointer.  This patch avoids the problem by extracting out and
> simplifying __BPTR_ALIGN for the usual case.  I've continued to use
> ptrdiff_t here, where it might be better to throw away __BPTR_ALIGN
> entirely and just assume uintptr_t exists.
>
> OK to apply for gcc?
>
> 	* obstack.h (__PTR_ALIGN): Expand and simplify __BPTR_ALIGN
> 	rather than calculating relative to a NULL pointer.
OK
jeff