linux-kernel - Re: [PATCH] selftest/mm: fix pointer comparison in mremap

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Message-Id: <20251107160855.58891ac6df6854a3b608185f@linux-foundation.org>
Date: Fri, 7 Nov 2025 16:08:55 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Ankit Khushwaha <ankitkhushwaha.linux@...il.com>, Lorenzo Stoakes
 <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Suren
 Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, Shuah
 Khan <shuah@...nel.org>, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftest/mm: fix pointer comparison in mremap_test

On Fri, 7 Nov 2025 10:27:27 +0100 "David Hildenbrand (Red Hat)" <david@...nel.org> wrote:

> On 06.11.25 13:02, Ankit Khushwaha wrote:
> > On Thu, Nov 06, 2025 at 12:18:57PM +0100, David Hildenbrand (Red Hat) wrote:
> >> On 06.11.25 11:49, Ankit Khushwaha wrote:
> >>> Pointer arthemitic with 'void * addr' and 'unsigned long long dest_alignment'
> >>> triggers following warning:
> >>>
> >>> mremap_test.c:1035:31: warning: pointer comparison always evaluates to
> >>> false [-Wtautological-compare]
> >>>    1035 |                 if (addr + c.dest_alignment < addr) {
> >>>         |                                             ^
> >>>
> >>> typecasting 'addr' to 'unsigned long long' to fix pointer comparison.
> >>
> >> With which compiler are you seeing this?
> > 
> > Hi David,
> > 
> > clang version 20.1.8 (Fedora 20.1.8-4.fc42) raised this warning.
> > 
> > To reproduce:
> > 	make -C tools/testing/selftests/mm CC=clang
> 
> Thanks, and thanks to Lorenzo for the details.
> 
> Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>

I must say, applying this would be an unhappy life event.

	if (void* + ulong < void*)

makes perfect sense in a world which permits void* arithmetic (ie,
ours).  So what the heck is clang doing??

If we do

	void *addr2 = addr + c.dest_alignment;
	if (addr2 < addr)
		...

then which statement warns, and why?