Date: Tue, 28 Aug 2012 16:28:51 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Warner Losh <imp@bsdimp.com> Cc: Andrey Zonov <zont@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: warning: cast increases required alignment of target type Message-ID: <20120828132851.GN33100@deviant.kiev.zoral.com.ua> In-Reply-To: <CDF0AE44-1CB2-42F7-BE60-DD2E4697A374@bsdimp.com> References: <503C8809.3050507@FreeBSD.org> <CDF0AE44-1CB2-42F7-BE60-DD2E4697A374@bsdimp.com>
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--rNtzt+pFA3UwHi4l Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Aug 28, 2012 at 07:12:15AM -0600, Warner Losh wrote: >=20 > On Aug 28, 2012, at 2:57 AM, Andrey Zonov wrote: >=20 > > Hi, > >=20 > > Does anyone know how to correctly fix this warning for > > arm/ia64/mips/sparc64? > >=20 > > usr.bin/elf2aout/elf2aout.c: In function 'main': > > usr.bin/elf2aout/elf2aout.c:129: warning: cast increases required > > alignment of target type > >=20 > > I found this explanation from bde, but still don't understand how to > > correctly fix this issue. > >=20 > > ------------------------------------------------------------------------ > > r99799 | bde | 2002-07-11 22:06:09 +0400 (Thu, 11 Jul 2002) | 10 lines > >=20 > > Set NO_WERROR to ignore the following warning which is emitted on > > alphas: > > .../elf2aout.c:130: warning: cast increases required alignment of > > target type > > The warning is about casting ((char *)e + phoff) to a struct pointer, > > where e is aligned but phoff might be garbage, so I think the warning > > should be emitted on most machines (even on i386's, alignment checking > > might be on) and the correct fix would involve validation phoff before > > using it. > >=20 > > Is this fix correct? >=20 > No. You need to tell the compiler that e has the alignment you think > it has so that it can check to make sure that you are actually right. > Just casting like this defeats the purpose of the check and will break > on other architectures. Does compiler ever checks the alignment ? I thought that it is hardware that performs (transparent) checks, like elfags.AC bit on x86. IMO hardware checks are good enough, since having program headers not aligned as required by ELF standard/ABI means that incoming ELF file is corrupted. Note that the same code is present in libexec/rtld-elf/map_object.c:map_object(), and there it might indeed make sense to introduce manual check for the alignment, to avoid hw triggering trap. Instead, rtld could gracefully refuse to load the mis-formed object. --rNtzt+pFA3UwHi4l Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlA8x5IACgkQC3+MBN1Mb4hQkQCg8otcSRnQ3zwehPQZ/04vvYk5 rq8AoNiZm03DkLPOMCjl4SZHV8YpeVMU =RdCC -----END PGP SIGNATURE----- --rNtzt+pFA3UwHi4l--
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