W dniu 21 kwietnia 2011 07:11 użytkownik Arnd Bergmann <arnd@xxxxxxxx> napisał: > On Wednesday 20 April 2011 21:46:04 Michał Mirosław wrote: >> 2011/4/20 Arnd Bergmann <arnd@xxxxxxxx>: >> > No, please don't try to invent random new ways of doing this. >> > Your example relies on the assumption that the task is calling >> > the entry point for its native word size. Some architectures >> > intentionally allow calling the 32 bit entry point from 64 bit >> > tasks and vice versa, e.g. for user space emulators converting >> > to a different ABI, and in that case is_compat_task() produces >> > the wrong result. Don't ever rely on that. >> >> This doesn't make sense to me. If you call 32-bit entry point from >> 64-bit process, you can't reliably pass pointers through the call >> (unless you limit 64-bit process to 32-bit address space). >> >> Do you know a working example of something using this kind of cross-call? > > There are people that use 32 bit programs on x86_64 in 64 bit mode > and switch on the ADDR_LIMIT_32BIT personality, IIRC. > This gives you more registers and lets you do 64 bit arithmetic > while not using any more memory to store long pointers. > There are a few problems with this, and the new x32 ABI will make it > cleaner. Ok. Thanks for the pointers. >> >> I'm okay with the anon union + ``compat_ptr(*(u32 *))`` part of your >> >> solution. If everyone else thinks it is reasonable, I'll submit a v7 >> >> with it. >> > No need for a union or a ptr_size member in the struct. Just use >> > a single __u64 and let the user cast the pointer to that. This >> > will work on all architectures. >> >> Union is just hiding this cast (it will be done in kernel) and allows >> cleaner code for userspace (there's a single kernel and possibly >> multiple applications that will implement this call). > > As I explained, it doesn't work. Please read my earlier mails. If you're referring to your mail: Subject: Re: [PATCH v4] mmc: Add ioctl to let userspace apps send ACMDs Date: Wed, 13 Apr 2011 01:00:39 +0200 Message-Id: <201104130100.39810.arnd@xxxxxxxx> Then I already provided an example of implementation that's independent of endianness and avoids casts on userspace. >> >> However, I still think it should be implemented in compat_ioctl() >> >> because compat_blkdev_ioctl() expects it. Either that, or I add to the >> >> big switch in compat_blkdev_driver_ioctl(), and spreading this change >> >> out to block/compat_ioctl.c does not seem like The Right Thing to me. >> > Yes, you definitely need to fill the .compat_ioctl member. We don't want >> > new entries in the switch statement, in particular none that are specific >> > to a single driver. >> Hmm, you're right. fs/compat_ioctl.c falls back to plain .ioctl if >> .compat_ioctl == NULL. > No, it doesn't. I'll need to look at the code again then. Best Regards, Michał Mirosław -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
