From nobody Tue Sep 16 02:20:21 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C9B9CC54EBD for ; Mon, 9 Jan 2023 07:56:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236419AbjAIH4H (ORCPT ); Mon, 9 Jan 2023 02:56:07 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53904 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236320AbjAIHzh (ORCPT ); Mon, 9 Jan 2023 02:55:37 -0500 Received: from 1wt.eu (wtarreau.pck.nerim.net [62.212.114.60]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 6BF3F13D55 for ; Sun, 8 Jan 2023 23:55:36 -0800 (PST) Received: (from willy@localhost) by pcw.home.local (8.15.2/8.15.2/Submit) id 3097soBf026019; Mon, 9 Jan 2023 08:54:50 +0100 From: Willy Tarreau To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Willy Tarreau Subject: [PATCH 5/6] tools/nolibc: prevent gcc from making memset() loop over itself Date: Mon, 9 Jan 2023 08:54:41 +0100 Message-Id: <20230109075442.25963-6-w@1wt.eu> X-Mailer: git-send-email 2.17.5 In-Reply-To: <20230109075442.25963-1-w@1wt.eu> References: <20230109075442.25963-1-w@1wt.eu> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" When building on ARM in thumb mode with gcc-11.3 at -O2 or -O3, nolibc-test segfaults during the select() tests. It turns out that at this level, gcc recognizes an opportunity for using memset() to zero the fd_set, but it miscompiles it because it also recognizes a memset pattern as well, and decides to call memset() from the memset() code: 000122bc : 122bc: b510 push {r4, lr} 122be: 0004 movs r4, r0 122c0: 2a00 cmp r2, #0 122c2: d003 beq.n 122cc 122c4: 23ff movs r3, #255 ; 0xff 122c6: 4019 ands r1, r3 122c8: f7ff fff8 bl 122bc 122cc: 0020 movs r0, r4 122ce: bd10 pop {r4, pc} Simply placing an empty asm() statement inside the loop suffices to avoid this. Signed-off-by: Willy Tarreau --- tools/include/nolibc/string.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h index 0932db3817d2..fffdaf6ff467 100644 --- a/tools/include/nolibc/string.h +++ b/tools/include/nolibc/string.h @@ -88,8 +88,11 @@ void *memset(void *dst, int b, size_t len) { char *p =3D dst; =20 - while (len--) + while (len--) { + /* prevent gcc from recognizing memset() here */ + asm volatile(""); *(p++) =3D b; + } return dst; } =20 --=20 2.35.3