From owner-freebsd-current@freebsd.org Wed Jun 10 20:14:53 2020 Return-Path: Delivered-To: freebsd-current@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id C702933E831 for ; Wed, 10 Jun 2020 20:14:53 +0000 (UTC) (envelope-from tuexen@freebsd.org) Received: from drew.franken.de (drew.ipv6.franken.de [IPv6:2001:638:a02:a001:20e:cff:fe4a:feaa]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client CN "*.franken.de", Issuer "COMODO RSA Domain Validation Secure Server CA" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 49hyrs3JWfz4WXs; Wed, 10 Jun 2020 20:14:52 +0000 (UTC) (envelope-from tuexen@freebsd.org) Received: from [IPv6:2a02:8109:1140:c3d:7000:7152:c399:835] (unknown [IPv6:2a02:8109:1140:c3d:7000:7152:c399:835]) (Authenticated sender: macmic) by drew.franken.de (Postfix) with ESMTPSA id B71347220B802; Wed, 10 Jun 2020 22:14:48 +0200 (CEST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 13.4 \(3608.80.23.2.2\)) Subject: Re: gcc versus clang issue for 32-bit binaries From: Michael Tuexen In-Reply-To: <20200610165908.GA81346@raichu> Date: Wed, 10 Jun 2020 22:14:47 +0200 Cc: "freebsd-current@FreeBSD.org" Content-Transfer-Encoding: quoted-printable Message-Id: <49FD752E-9BA0-4F1A-8B6C-47EBFB5911D3@freebsd.org> References: <128AB51F-0950-448F-8463-12C573C1AA38@freebsd.org> <20200610165908.GA81346@raichu> To: Mark Johnston X-Mailer: Apple Mail (2.3608.80.23.2.2) X-Spam-Status: No, score=-2.9 required=5.0 tests=ALL_TRUSTED,BAYES_00 autolearn=disabled version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on mail-n.franken.de X-Rspamd-Queue-Id: 49hyrs3JWfz4WXs X-Spamd-Bar: / Authentication-Results: mx1.freebsd.org; none X-Spamd-Result: default: False [0.00 / 15.00]; local_wl_from(0.00)[freebsd.org]; ASN(0.00)[asn:680, ipnet:2001:638::/32, country:DE] X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.33 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 10 Jun 2020 20:14:53 -0000 > On 10. Jun 2020, at 18:59, Mark Johnston wrote: >=20 > On Wed, Jun 10, 2020 at 06:41:50PM +0200, Michael Tuexen wrote: >> Dear all, >>=20 >> consider the following program test.c: >>=20 >> #include >> #include >>=20 >> int=20 >> main(void) >> { >> void *p; >> =09 >> p =3D mmap((void *)0x20000000, 0x1000000, PROT_READ | PROT_WRITE = | PROT_EXEC, MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0); >> printf("p=3D %p\n", p); >> return (0); >> } >>=20 >> On i386 the following happens: >> * when compiling it with cc and running it, it crashes. >> * when compiling it with gcc it runs fine. >>=20 >> On amd64 the following happens: >> * when compiling it with cc -m64 it runs fine. >> * when compiling it with cc -m32 is crashes. >> * when compiling it with gcc -m64 it runs fine. >> * when compiling it with gcc -m32 it runs fine. >>=20 >> So why does the above program crash when compiled for 32-bit when = using clang, but runs fine when compiled with gcc. >=20 > The difference is between ld.bfd and ld.lld, which emit executables = with > different entry point addresses. cc -m32 -fuse-ld=3Dbfd gives an > executable that does not crash. >=20 > When linked with lld, libc and ld-elf get mapped into the region > [0x20000000,0x21000000], so the program crashes when the libc.so = mapping > is overwritten with that created by the mmap() call and the program > calls printf(). >=20 >> I'm testing this on 32-bit and 64-bit head systems. gcc is from = ports. >>=20 >> The reason I'm looking into it is that I want to get syzkaller = working on 32-bit with clang. >=20 > Do you know why SYZ_DATA_OFFSET is hard-coded the way it is? It looks > like it works more or less by accident, but at a glance I don't see = why > it has to be a fixed mapping. It looks like 0x10000000 works fine on my 32-bit VM. I added you as a reviewer on = https://github.com/google/syzkaller/pull/1809 Best regards Michael