Date: Sat, 31 Mar 2012 01:46:31 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Richard Yao <ryao@cs.stonybrook.edu> Cc: freebsd-stable@freebsd.org Subject: Re: Text relocations in kernel modules Message-ID: <20120330224631.GJ2358@deviant.kiev.zoral.com.ua> In-Reply-To: <4F76350F.8000708@cs.stonybrook.edu> References: <4F75E404.8000104@cs.stonybrook.edu> <4F75EF86.6090909@cs.stonybrook.edu> <20120330190713.GG2358@deviant.kiev.zoral.com.ua> <4F760C9E.6060405@cs.stonybrook.edu> <20120330194649.GH2358@deviant.kiev.zoral.com.ua> <4F761371.7020606@cs.stonybrook.edu> <20120330203605.GI2358@deviant.kiev.zoral.com.ua> <4F76350F.8000708@cs.stonybrook.edu>
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--//MD+r6UUJvz0X/C Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Mar 30, 2012 at 06:34:55PM -0400, Richard Yao wrote: > On 03/30/12 16:36, Konstantin Belousov wrote: > > First, there _are_ relocations against text in the amd64 modules, but I > > suspect that your scripts do not detect this. Most likely, scripts look > > for DT_TEXTREL dynamic tag, and tags are only present in the executables > > or shared objects, not in the object files. The amd64 modules are object > > files, so you just mis-interpret the situation. >=20 > readelf is a part of binutils. It is not a script. Here is the version > that Gentoo/FreeBSD uses: So you completely missed what I told you. >=20 > # readelf --version > GNU readelf (GNU Binutils) 2.20.1.20100303 > Copyright 2009 Free Software Foundation, Inc. > This program is free software; you may redistribute it under the terms of > the GNU General Public License version 3 or (at your option) any later > version. > This program has absolutely no warranty. >=20 > In addition, this is what it says when I ask it to look at virtio_blk.ko: >=20 > # readelf -d /boot/modules/virtio_blk.ko >=20 > Dynamic section at offset 0x2f6c contains 13 entries: > Tag Type Name/Value > 0x00000004 (HASH) 0xd4 > 0x6ffffef5 (GNU_HASH) 0x480 > 0x00000005 (STRTAB) 0x9d0 > 0x00000006 (SYMTAB) 0x4e0 > 0x0000000a (STRSZ) 1295 (bytes) > 0x0000000b (SYMENT) 16 (bytes) > 0x00000011 (REL) 0xee0 > 0x00000012 (RELSZ) 1664 (bytes) > 0x00000013 (RELENT) 8 (bytes) > 0x00000016 (TEXTREL) 0x0 > 0x0000001e (FLAGS) TEXTREL > 0x6ffffffa (RELCOUNT) 87 > 0x00000000 (NULL) 0x0 >=20 > Running the same command on amd64 FreeBSD's version returns nothing. I > have attached the result of `readelf -a ...` on both the i386 version > and the amd64 version. Reread what I wrote to you. Also, it pays off learning how ELF works before making conclusion from the absence of the output of readelf -d. Amd64 modules _are not_ shared objects. >=20 > > Second, from what you wrote, I see the issue in either wrong policy > > being established in your project, or (another) mis-interpretation of > > the policy. Indeed, having text relocations in the shared objects is > > bad, because said relocations hinder text pages sharing. Relocated page > > is modified, so COW mechanism causes it to become private to process. >=20 > I believe that relocations also cause the linker to work harder when the > modules themselves are loaded the first time. They can also cause bugs > when code is ported to another architecture. I can only answer that this is your fantasy, however ample. I see that conversation is going nowhere, I will not reply further. >=20 > > On the other hand, there is only one instance of the loaded kernel modu= le, > > its text segment (or section, for amd64) is not shared, so modifications > > to the text pages do not cause increased memory use. More, not compiling > > modules with -fPIC (absence of -fPIC is what makes the text relocations= to > > appear in the final link result) makes the code faster, esp. on i386. >=20 > Compiling with -fPIC breaks the build. >=20 > > So, there is nothing to report, and fix is outside the FreeBSD domain: > > either fix your policy by not stating that text relocation in kernel > > module is banned, or just find that policy only applicable to usermode > > objects. >=20 > Linux has no such text relocations in its modules. I have checked on > both i386 and amd64. I have difficulty believing that FreeBSD needs text > relocations when Linux does not. >=20 > I am fairly certain that this is going to interfere with ASLR in the > kernel, which is a security issue. It is definitely something to report. --//MD+r6UUJvz0X/C Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAk92N8cACgkQC3+MBN1Mb4i32wCeILV0GitT3U/Z5bgGfdyecQ8K MdcAoM9n3gz5fZBqoupD0SJdSGDOEvcf =CXW4 -----END PGP SIGNATURE----- --//MD+r6UUJvz0X/C--
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