Date: Tue, 28 Aug 2012 07:31:51 -0600 From: Warner Losh <imp@bsdimp.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Andrey Zonov <zont@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: warning: cast increases required alignment of target type Message-ID: <1B08245D-D467-48A5-B18C-9C8FF6823F6F@bsdimp.com> In-Reply-To: <20120828132851.GN33100@deviant.kiev.zoral.com.ua> References: <503C8809.3050507@FreeBSD.org> <CDF0AE44-1CB2-42F7-BE60-DD2E4697A374@bsdimp.com> <20120828132851.GN33100@deviant.kiev.zoral.com.ua>
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On Aug 28, 2012, at 7:28 AM, Konstantin Belousov wrote: > On Tue, Aug 28, 2012 at 07:12:15AM -0600, Warner Losh wrote: >> >> On Aug 28, 2012, at 2:57 AM, Andrey Zonov wrote: >> >>> Hi, >>> >>> Does anyone know how to correctly fix this warning for >>> arm/ia64/mips/sparc64? >>> >>> usr.bin/elf2aout/elf2aout.c: In function 'main': >>> usr.bin/elf2aout/elf2aout.c:129: warning: cast increases required >>> alignment of target type >>> >>> I found this explanation from bde, but still don't understand how to >>> correctly fix this issue. >>> >>> ------------------------------------------------------------------------ >>> r99799 | bde | 2002-07-11 22:06:09 +0400 (Thu, 11 Jul 2002) | 10 lines >>> >>> 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. >>> >>> Is this fix correct? >> >> 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. Yes. Hence the warning. The hardware does *NOT* transparently fix misaligned accesses, which causes core dumps which is why this warning is in place. > 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. It is alignment in the source code. Telling the compiler about these alignments is what I'm suggesting. > 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. In the rtld case, we should know if things are properly aligned or not as part of the ABI. A check wouldn't hurt too much, but it is done at every program execution and too many sanity checks that are never happens can cause performance hits. Warnerhome | help
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