From nobody Sat Oct 24 00:33:00 2026 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass(p=quarantine dis=none) header.from=sloservers.com ARC-Seal: i=1; a=rsa-sha256; t=1790938383; cv=none; d=zohomail.com; s=zohoarc; b=Q9XrHlXWnfhutuRyh9d/eF5NhUWLtsgPkFsy+mZ24fpEIedTtRSKqa42gmb/rapJr/qzLtbfvQWD9ajhfalxg8K3SZtPlLNkf1MGzHtT4QvEqC5jYQBq9X3L5739yM7R9d7T/OnkJ4OT9Wxj8h/AH2XKOM51C91GLiq/mBnRp44= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; s=zohoarc; t=1790938383; h=Content-Type:Content-Transfer-Encoding:Cc:Cc:Date:Date:From:From:List-Subscribe:List-Post:List-Id:List-Archive:List-Help:List-Unsubscribe:MIME-Version:Message-ID:Sender:Subject:Subject:To:To:Message-Id:Reply-To; bh=I+U74psbi8tCVpzeQ6OUMDODRtaL1+AvB1PgukSJWCc=; b=MhM8IOZ0PG0bFjyNu+bP20KgIjKRS3YREXgw+k8WXQwDklgCGcgrws38SHArURYuT+r3nLABXk39L4CnPUE1aTiJOUkqZSakeUnMotdfiqFFVfob88hwuYxp1EinkkA0RfoSIuGvrnNrK9yUBw9wd1zQhsWZVNTBe+LWbjp4lpI= ARC-Authentication-Results: i=1; mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass header.from= (p=quarantine dis=none) Return-Path: Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1790938383711197.4581280152396; Fri, 2 Oct 2026 03:53:03 -0700 (PDT) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1xCasJ-0006z8-Fs; Fri, 02 Oct 2026 06:52:23 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xCasF-0006yp-J6 for qemu-devel@nongnu.org; Fri, 02 Oct 2026 06:52:19 -0400 Received: from vps.sloservers.com ([204.109.61.218]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xCasD-0007Dd-RR for qemu-devel@nongnu.org; Fri, 02 Oct 2026 06:52:19 -0400 Received: from thinking.duckdns.org (unknown [192.168.11.4]) by vps.sloservers.com (Postfix) with ESMTPS id 7EEE7239C2; Fri, 02 Oct 2026 10:52:14 +0000 (UTC) Received: from [192.168.1.131] (OPNsense.thinking.duckdns.org [192.168.1.5]) by thinking.duckdns.org (Postfix) with ESMTPSA id CF515CF5; Fri, 02 Oct 2026 03:52:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sloservers.com; s=mail; t=1790938334; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=I+U74psbi8tCVpzeQ6OUMDODRtaL1+AvB1PgukSJWCc=; b=fuL6Oalw3AW5+P4FR1PjfCBVtyRkImvTa4/FoCkNGlyGrbiOjgtUXV6pRQQ54BW8UnZt+K UKdHOXGeHqW+RmFssfKVeD0Kp58w+iC8sJvzVoD7lQdcdH5MQq5MvvU3QH33GhCTm5x6XP 8Qf+3SwEoqiZNJTaKCtLTCNINLRo8hoR5eN73JUITE2NZdVW1EgYiTYWI0RugQska+o6zC zOTgVAxWzPrf5qtdT0ScG+EffzKwf1to9BNhTBwT78hW8Tmq39ctkLfO2Gsm4ewr0QappM HcblWXQpniCeTethi/CSOLNB909Lt8mUD7WommtibzwL3FZAvBtiCDiQj4x6Rg== Message-ID: <605bc800-4263-40af-b8d9-18305d25f3cf@sloservers.com> Date: Fri, 2 Oct 2026 03:52:12 -0700 MIME-Version: 1.0 User-Agent: Betterbird (Windows) From: Rick Richard Subject: [PATCH] common-user: Skip the identity map when qemu is loaded low To: qemu-devel@nongnu.org Content-Language: en-US Cc: richard.henderson@linaro.org, laurent@vivier.eu, deller@gmx.de, pierrick.bouvier@oss.qualcomm.com, imp@bsdimp.com, kevans@freebsd.org Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists1p.gnu.org; Received-SPF: pass client-ip=204.109.61.218; envelope-from=rick@sloservers.com; helo=vps.sloservers.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @sloservers.com) X-ZM-MESSAGEID: 1790938387141158500 Content-Type: text/plain; charset="utf-8" A non-PIE qemu is loaded below 4 GiB. That includes every static build on FreeBSD and Linux builds configured with --disable-pie. With the identity map (guest_base 0), a non-PIE guest program can be mapped over qemu. Only use the identity map when qemu is loaded at 4 GiB or higher. Non-PIE programs are normally linked below 4 GiB, and PIE programs are loaded at or above it (ppc64 Linux loads them at exactly 4 GiB). Signed-off-by: Rick Richard --- On FreeBSD a static qemu can't be PIE, so bsd-user is always loaded at 0x200000. pgb_dynamic() picks guest_base 0 because the initial image doesn't overlap qemu, but rtld later maps a non-PIE executable at its link address on top of qemu's own text. This isn't BSD specific, I was able to reproduce on Linux as well by disabling PIE. I tested the fix on Linux with static-pie and --disable-pie builds, and on FreeBSD with aarch64, riscv64 and x86_64 guests. When qemu is loaded high it still uses the identity map. When it's loaded low it picks a non-zero guest_base and the guest runs normally. I couldn't find an existing way in qemu to get its own load address, so I used __executable_start, which both GNU ld and lld provide. If there's a better way I'm happy to change it. common-user/probe-guest-base.c | 9 +++++++-- include/user/probe-guest-base.h | 3 +++ 2 files changed, 10 insertions(+), 2 deletions(-) diff --git a/common-user/probe-guest-base.c b/common-user/probe-guest-base.c index 6f1247cf..226fe7d4 100644 --- a/common-user/probe-guest-base.c +++ b/common-user/probe-guest-base.c @@ -269,8 +269,13 @@ static void pgb_dynamic(const char *image_name, const = PGBRange *image_range, uintptr_t brk, ret; PGBAddrs ga; - /* Try the identity map first. */ - if (pgb_addr_set(&ga, image_range, commpage_range, true)) { + /* + * Try the identity map first, but only if qemu is loaded high. + * A non-PIE qemu loads low, where a guest's non-PIE executable + * could be mapped over it. + */ + if ((uintptr_t)__executable_start >=3D 4 * GiB && + pgb_addr_set(&ga, image_range, commpage_range, true)) { brk =3D (uintptr_t)sbrk(0); if (pgb_try_mmap_set(&ga, 0, brk)) { guest_base =3D 0; diff --git a/include/user/probe-guest-base.h b/include/user/probe-guest-bas= e.h index d7492ef4..fe7f971f 100644 --- a/include/user/probe-guest-base.h +++ b/include/user/probe-guest-base.h @@ -9,6 +9,9 @@ #include "exec/vaddr.h" +/* Defined by the linker: the start address of qemu's own executable image= . */ +extern char __executable_start[]; + typedef struct PGBRange { vaddr lo; vaddr hi;