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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3EC7BC3DA7D for ; Thu, 5 Jan 2023 16:32:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233378AbjAEQcl (ORCPT ); Thu, 5 Jan 2023 11:32:41 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34910 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231889AbjAEQcj (ORCPT ); Thu, 5 Jan 2023 11:32:39 -0500 Received: from mail-pg1-x52f.google.com (mail-pg1-x52f.google.com [IPv6:2607:f8b0:4864:20::52f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id ACB1A5D423 for ; Thu, 5 Jan 2023 08:32:38 -0800 (PST) Received: by mail-pg1-x52f.google.com with SMTP id d10so24831579pgm.13 for ; Thu, 05 Jan 2023 08:32:38 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=googlemail.com; s=20210112; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=Ow2r2TZdrVhfIknj/l1EkyeUhZdKgzWR4bhV6QJcLZM=; b=RNH1RNWDmugvTkQ9quJjXS8UMMS4yhPMAQRQBWh3M0TIU/APglmLfVUWsL6REJwkEL 8VxHIJJyQj/1Z1Zj51W5g73OX32SIU7+asrHG3rH35LdvX8vrRqmLhLiUruXrjetDkR4 oWcMAf0U9mkfV6dDO7KvUB3TIUjrf2F7t0auzUkhirxlw0IB96yuxPdQnn10rAl3WMvd HhO74sTBTqlviH5WSwP1OEXhRYepVovwhLcX7t2zmC0GyVV8BHVRUs5D4lZgFRTV4C3P qeh9PI85Wi54dxuWvekAuXW37w5VkIKVQ2e6kY9YfnPIsgUb3Kz+2MxmXGuX/d88dLsS lPXQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=Ow2r2TZdrVhfIknj/l1EkyeUhZdKgzWR4bhV6QJcLZM=; b=A1lYUfHbcPeZ98AmUTi8GVFFf5Dmtbnin4K9a5N8PEsvJgVc0JF26b9JFrE073YYsZ fOhdCtd2phBgnzjxlKs5AJ2xwr33xtiqcOpFH8IPqdR8zdF5jhV/zrSLKKEG0mT1l/Bi /UzW4rs5qgELLEAdPw0q1RWKNDpqTTbjeB0JcLn6vfBiCDiOdT/cTyfhQ+hf1XC7cMme I9Vwh1u+kHl0XDyzRgmTUDp17raUtzC22dup52EPT5Lz27zMGQrnmTjjeoIrXhlGK91v 6jqyKNnm14/ARsT6ssQrrDKJzmY3UdFqhO7kMFtil/rLmy1cCv+wSXsFHYMFk+jPi9Gp hx0w== X-Gm-Message-State: AFqh2kp12tZn/yb4lW03rTjoCzyL60f5x1Cd68H6OXo5TkjPVZt8P+xH VJnY7Nk7L48OOOQNnTAOQLxHnHxY4UC89A== X-Google-Smtp-Source: AMrXdXtNvg7212lkqlfpekfb99VROoN4KnstcQvEkXn+PPPfGkjVc6UW8ejjLUsgNiINCVmesT628w== X-Received: by 2002:a62:14cb:0:b0:583:3a9c:1df8 with SMTP id 194-20020a6214cb000000b005833a9c1df8mr1059898pfu.23.1672936357872; Thu, 05 Jan 2023 08:32:37 -0800 (PST) Received: from mariner-vm.. (c-71-197-160-159.hsd1.wa.comcast.net. [71.197.160.159]) by smtp.gmail.com with ESMTPSA id k26-20020aa79d1a000000b0058130f1eca1sm19224375pfp.182.2023.01.05.08.32.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Jan 2023 08:32:37 -0800 (PST) From: dthaler1968@googlemail.com To: bpf@vger.kernel.org Cc: Dave Thaler Subject: [PATCH] bpf, docs: Fix modulo zero, division by zero, overflow, and underflow Date: Thu, 5 Jan 2023 16:32:23 +0000 Message-Id: <20230105163223.3472-1-dthaler1968@googlemail.com> X-Mailer: git-send-email 2.33.4 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org From: Dave Thaler Fix modulo zero, division by zero, overflow, and underflow. Also clarify how a negative immediate value is used in unsigned division Signed-off-by: Dave Thaler --- Documentation/bpf/instruction-set.rst | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index e672d5ec6cc..2ba7c618f33 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -99,19 +99,26 @@ code value description BPF_ADD 0x00 dst += src BPF_SUB 0x10 dst -= src BPF_MUL 0x20 dst \*= src -BPF_DIV 0x30 dst /= src +BPF_DIV 0x30 dst = (src != 0) ? (dst / src) : 0 BPF_OR 0x40 dst \|= src BPF_AND 0x50 dst &= src BPF_LSH 0x60 dst <<= src BPF_RSH 0x70 dst >>= src BPF_NEG 0x80 dst = ~src -BPF_MOD 0x90 dst %= src +BPF_MOD 0x90 dst = (src != 0) ? (dst % src) : dst BPF_XOR 0xa0 dst ^= src BPF_MOV 0xb0 dst = src BPF_ARSH 0xc0 sign extending shift right BPF_END 0xd0 byte swap operations (see `Byte swap instructions`_ below) ======== ===== ========================================================== +Underflow and overflow are allowed during arithmetic operations, +meaning the 64-bit or 32-bit value will wrap. If +eBPF program execution would result in division by zero, +the destination register is instead set to zero. +If execution would result in modulo by zero, +the destination register is instead left unchanged. + ``BPF_ADD | BPF_X | BPF_ALU`` means:: dst_reg = (u32) dst_reg + (u32) src_reg; @@ -128,6 +135,10 @@ BPF_END 0xd0 byte swap operations (see `Byte swap instructions`_ below) dst_reg = dst_reg ^ imm32 +Also note that the division and modulo operations are unsigned, +where 'imm' is first sign extended to 64 bits and then converted +to an unsigned 64-bit value. There are no instructions for +signed division or modulo. Byte swap instructions ~~~~~~~~~~~~~~~~~~~~~~ -- 2.33.4