From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EB8E5C43381 for ; Mon, 4 Jan 2021 15:59:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B5E882253D for ; Mon, 4 Jan 2021 15:59:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727825AbhADP6u (ORCPT ); Mon, 4 Jan 2021 10:58:50 -0500 Received: from mail.kernel.org ([198.145.29.99]:36188 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727745AbhADP6t (ORCPT ); Mon, 4 Jan 2021 10:58:49 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 2A36D2245C; Mon, 4 Jan 2021 15:58:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1609775888; bh=Td1I9RpMziDCkwfnLuS6v//9XgaDsMl180JA+UBQCWo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ZeIdnvHHPYukkM2h344J1wLKzvFx+QxSfXnenINyHYShstFC4ouZuL7AdeKHaa4Mb UK/p0zlULAV5iGBAfiBW5LXdjuvGYkWc03Xmd+KQNu/h3IrF6rCqZ4VYTYuGagmJrp DuTh8Lj0sFgOgKgwRavXbh4fjAOTb8fh+U0c409o= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Christophe Leroy , Segher Boessenkool , Michael Ellerman , Sasha Levin Subject: [PATCH 4.19 11/35] powerpc/bitops: Fix possible undefined behaviour with fls() and fls64() Date: Mon, 4 Jan 2021 16:57:14 +0100 Message-Id: <20210104155703.950803319@linuxfoundation.org> X-Mailer: git-send-email 2.30.0 In-Reply-To: <20210104155703.375788488@linuxfoundation.org> References: <20210104155703.375788488@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Christophe Leroy [ Upstream commit 1891ef21d92c4801ea082ee8ed478e304ddc6749 ] fls() and fls64() are using __builtin_ctz() and _builtin_ctzll(). On powerpc, those builtins trivially use ctlzw and ctlzd power instructions. Allthough those instructions provide the expected result with input argument 0, __builtin_ctz() and __builtin_ctzll() are documented as undefined for value 0. The easiest fix would be to use fls() and fls64() functions defined in include/asm-generic/bitops/builtin-fls.h and include/asm-generic/bitops/fls64.h, but GCC output is not optimal: 00000388 : 388: 2c 03 00 00 cmpwi r3,0 38c: 41 82 00 10 beq 39c 390: 7c 63 00 34 cntlzw r3,r3 394: 20 63 00 20 subfic r3,r3,32 398: 4e 80 00 20 blr 39c: 38 60 00 00 li r3,0 3a0: 4e 80 00 20 blr 000003b0 : 3b0: 2c 03 00 00 cmpwi r3,0 3b4: 40 82 00 1c bne 3d0 3b8: 2f 84 00 00 cmpwi cr7,r4,0 3bc: 38 60 00 00 li r3,0 3c0: 4d 9e 00 20 beqlr cr7 3c4: 7c 83 00 34 cntlzw r3,r4 3c8: 20 63 00 20 subfic r3,r3,32 3cc: 4e 80 00 20 blr 3d0: 7c 63 00 34 cntlzw r3,r3 3d4: 20 63 00 40 subfic r3,r3,64 3d8: 4e 80 00 20 blr When the input of fls(x) is a constant, just check x for nullity and return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction directly. For fls64() on PPC64, do the same but with __builtin_clzll() and cntlzd instruction. On PPC32, lets take the generic fls64() which will use our fls(). The result is as expected: 00000388 : 388: 7c 63 00 34 cntlzw r3,r3 38c: 20 63 00 20 subfic r3,r3,32 390: 4e 80 00 20 blr 000003a0 : 3a0: 2c 03 00 00 cmpwi r3,0 3a4: 40 82 00 10 bne 3b4 3a8: 7c 83 00 34 cntlzw r3,r4 3ac: 20 63 00 20 subfic r3,r3,32 3b0: 4e 80 00 20 blr 3b4: 7c 63 00 34 cntlzw r3,r3 3b8: 20 63 00 40 subfic r3,r3,64 3bc: 4e 80 00 20 blr Fixes: 2fcff790dcb4 ("powerpc: Use builtin functions for fls()/__fls()/fls64()") Cc: stable@vger.kernel.org Signed-off-by: Christophe Leroy Acked-by: Segher Boessenkool Signed-off-by: Michael Ellerman Link: https://lore.kernel.org/r/348c2d3f19ffcff8abe50d52513f989c4581d000.1603375524.git.christophe.leroy@csgroup.eu Signed-off-by: Sasha Levin --- arch/powerpc/include/asm/bitops.h | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) diff --git a/arch/powerpc/include/asm/bitops.h b/arch/powerpc/include/asm/bitops.h index ff71566dadee5..76db1c5000bd6 100644 --- a/arch/powerpc/include/asm/bitops.h +++ b/arch/powerpc/include/asm/bitops.h @@ -221,15 +221,34 @@ static __inline__ void __clear_bit_unlock(int nr, volatile unsigned long *addr) */ static __inline__ int fls(unsigned int x) { - return 32 - __builtin_clz(x); + int lz; + + if (__builtin_constant_p(x)) + return x ? 32 - __builtin_clz(x) : 0; + asm("cntlzw %0,%1" : "=r" (lz) : "r" (x)); + return 32 - lz; } #include +/* + * 64-bit can do this using one cntlzd (count leading zeroes doubleword) + * instruction; for 32-bit we use the generic version, which does two + * 32-bit fls calls. + */ +#ifdef CONFIG_PPC64 static __inline__ int fls64(__u64 x) { - return 64 - __builtin_clzll(x); + int lz; + + if (__builtin_constant_p(x)) + return x ? 64 - __builtin_clzll(x) : 0; + asm("cntlzd %0,%1" : "=r" (lz) : "r" (x)); + return 64 - lz; } +#else +#include +#endif #ifdef CONFIG_PPC64 unsigned int __arch_hweight8(unsigned int w); -- 2.27.0