On Sun, Aug 21, 2016 at 11:00:24PM +1000, Bruce Evans wrote: > On Sun, 21 Aug 2016, Slawa Olhovchenkov wrote: > > > On Sun, Aug 21, 2016 at 09:32:35PM +1000, Bruce Evans wrote: > >... > >> *(foo_t *)asks for alignment bugs. We have already fixed lots of these > >> bugs for copying struct timevals in places like ping.c. Compilers warn > >> about misalignment when certain warnings are enabled, but only on arches > >> where misalignment is more than a pessimization. There is no reason why > >> td_retval would be always aligned on these arches. Alignment of 64-bit > >> types on 32-bit arches is usually so unimportant that even int32_t is > >> not required to be aligned by the ABI, and there is no point in > >> aligning td_retval specially unless you also do it for a large fraction > >> of 64-bit integers in the kernel, and there are negative points for > >> doing that. > > > > For eliminate aligment bugs need to replace all assigment more then 1 > > bytes to *td_retval by memcpy? > > The copying must be of size 1 or 2 ints unless you are making even larger > type puns than now. 1 int is obviously safe to just assign, and 2 ints > should use memcpy().
Why? I am remeber about platforms with missaligment trap when accessing int16 by odd address. Now platforms like this do not exist anymore? > There are also endianness problems. The old version was even more broken > on big endian systems. The current version needs some magic to reverse > the memcpy() of the bits. We already depend on this for some 64-bit > syscalls like lseek(). Can you explain some more? This is not transfer over network and don't read from external media. Where is problem? > Hmm, lseek() uses different magic. Instead of the memcpy(), we assign > to tdu_off in td_uretoff. td_retval is really td_uretoff.tdu_retval > obfuscated like this for compatibilty. This is slightly better than > the memcpy() since it makes tdu_off and also tdu_retval 64-bit aligned > if that is required for off_t. The same magic still occurs in userland. > On normal 32-bit arches, the 2 integers arrive in 2 registers that have > to be combined in the right order to give a 64-bit value. The magic is > just that things are arranged so that no code is needed to rearrange the > registers in the usual case where the application uses a similar ABI to > the kernel. Not the same ABI, since the application might be 32 bits > and the kernel 64 bits. This requires lots of conversions, but none for > register order for at least x86. > > Bruce _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
