On Wed, Jul 1, 2020 at 11:22 AM Linus Torvalds <torva...@linux-foundation.org> wrote: > > Josh / PeterZ, > it turns out that clang seems to now have fixed the last known > nagging details with "asm goto" with outputs, so I'm looking at > actually trying to merge the support for that in the kernel. > > The main annoyance isn't actually using "asm goto" at all, the main > annoyance is just that it will all have to be conditional on whether > the compiler supports it or not. We have the config option for that > already, but it will just end up with two copies of the code depending > on that option. > > It's not a huge deal: the recent cleanups wrt the x86 uaccess code > have made the code _much_ more straightforward and legible, and I'm > not so worried about it all. > > Except that when I looked at this, I realized that I really had picked > the wrong model for how exceptions are handled wrt stac/clac. In > particular user access exceptions return with stac set, so we end up > having a code pattern where the error case will also have to do the > user_access_end() to finish the STAC region. > > Adding a user_access_end() to the user exception fault handler is trivial. > > But the thing I'm asking you for is how nasty it would be to change > objtool to have those rules? > > IOW, right now we have > > if (!user_acces_begin(...)) > goto efault; > unsafe_get/put_user(ptr, val, label); > user_access_end(); > return 0; > > label: > user_access_end(); > efaulr: > return -EFAULT; > > and I'd like to make the "label" case just go to "efault", with > objtool knowing that the exception handling already did the > user_access_end().
Do we really want the exception handling to do the CLAC? Having unsafe_get_user() do CLAC seems surprising to me, and it will break use cases like: if (!user_access_begin(...) goto out; ret = unsafe_get_user(...); user_access_end(); check ret; I have no problem with a special ex_handler_uaccess_clac, but I don't think it should be the default. --Andy