From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (bilbo.ozlabs.org [203.11.71.1]) (using TLSv1.2 with cipher ADH-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 41H1VN3hD4zF1PM for ; Fri, 29 Jun 2018 12:55:40 +1000 (AEST) From: Michael Ellerman To: Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , malat@debian.org, aneesh.kumar@linux.vnet.ibm.com Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH] powerpc/mm: fix always true/false warning in slice.c In-Reply-To: <63b696ab7be8b941fa1e1589f28260320d12a32a.1529589640.git.christophe.leroy@c-s.fr> References: <63b696ab7be8b941fa1e1589f28260320d12a32a.1529589640.git.christophe.leroy@c-s.fr> Date: Fri, 29 Jun 2018 12:55:39 +1000 Message-ID: <871scqwbbo.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Christophe Leroy writes: > This patch fixes the following warnings (obtained with make W=1). > > arch/powerpc/mm/slice.c: In function 'slice_range_to_mask': > arch/powerpc/mm/slice.c:73:12: error: comparison is always true due to limited range of data type [-Werror=type-limits] > if (start < SLICE_LOW_TOP) { Presumably only on 32-bit ? > diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c > index 9530c6db406a..17c57760e06c 100644 > --- a/arch/powerpc/mm/slice.c > +++ b/arch/powerpc/mm/slice.c > @@ -79,7 +86,7 @@ static void slice_range_to_mask(unsigned long start, unsigned long len, > - (1u << GET_LOW_SLICE_INDEX(start)); > } > > - if ((start + len) > SLICE_LOW_TOP) { > + if (!slice_addr_is_low(end)) { > unsigned long start_index = GET_HIGH_SLICE_INDEX(start); > unsigned long align_end = ALIGN(end, (1UL << SLICE_HIGH_SHIFT)); > unsigned long count = GET_HIGH_SLICE_INDEX(align_end) - start_index; This worries me. By casting before the comparison in the helper you squash the compiler warning, but the code is still broken if (start + len) overflows. Presumably that "never happens", but it just seems fishy. The other similar check in that file does: if (SLICE_NUM_HIGH && ((start + len) > SLICE_LOW_TOP)) { Where SLICE_NUM_HIGH == 0 on 32-bit. Could we fix the less than comparisons with SLICE_LOW_TOP with something similar, eg: if (!SLICE_NUM_HIGH || start < SLICE_LOW_TOP) { ie. limit them to the 64-bit code? cheers