From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id 640066B01BD for ; Sun, 30 May 2010 13:42:59 -0400 (EDT) Message-ID: <4C02A39D.3050303@cesarb.net> Date: Sun, 30 May 2010 14:42:53 -0300 From: Cesar Eduardo Barros MIME-Version: 1.0 Subject: Re: [PATCH] Make kunmap_atomic() harder to misuse References: <1275043993-26557-1-git-send-email-cesarb@cesarb.net> <20100529204256.b92b1ff6.akpm@linux-foundation.org> In-Reply-To: <20100529204256.b92b1ff6.akpm@linux-foundation.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Russell King , Ralf Baechle , David Howells , Koichi Yasutake , Benjamin Herrenschmidt , Paul Mackerras , "David S. Miller" , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, Arnd Bergmann , Rusty Russell List-ID: Em 30-05-2010 00:42, Andrew Morton escreveu: > On Fri, 28 May 2010 07:53:13 -0300 Cesar Eduardo Barros wrote: >> Make it much harder to misuse, by moving it to level 9 on Rusty's >> list[4] ("The compiler/linker won't let you get it wrong"). This is done >> by refusing to build if the pointer passed to it is convertible to a >> struct page * but it is not a void * (verified by trying to convert it >> to a pointer to a dummy struct). >> >> The real kunmap_atomic() is renamed to kunmap_atomic_notypecheck() >> (which is what you would call in case for some strange reason calling it >> with a pointer to a struct page is not incorrect in your code). >> > > Fair enough, that's a 99% fix. A long time ago I made kmap_atomic() > return a char * (iirc) and kunmap_atomic() is passed a char*. It > worked, but I ended up throwing it away. I don't precisely remember > why - I think it was intrusiveness and general hassle rather than > anything fundamental. I vaguely recall reading something about that on LWN a long time ago.[1] The advantage of my __builtin_types_compatible_p approach is that it does not have to change the callers at all (except in the extremly unlikely case that someone actually meant to call it with a struct page *, which is something I did not find when looking at the whole kernel with spatch[2]). The disadvantage of my approach is that gcc's error message is absolutely atrocious: mm/swapfile.c: In function a??fooa??: mm/swapfile.c:2501: error: negative width in bit-field a??a?? But that is a problem with BUILD_BUG_ON, not this code. >> +/* Prevent people trying to call kunmap_atomic() as if it were kunmap() */ >> +struct __kunmap_atomic_dummy {}; >> +#define kunmap_atomic(addr, idx) do { \ >> + BUILD_BUG_ON( \ >> + __builtin_types_compatible_p(typeof(addr), struct page *)&& \ >> + !__builtin_types_compatible_p(typeof(addr), struct __kunmap_atomic_dummy *)); \ >> + kunmap_atomic_notypecheck((addr), (idx)); \ >> + } while (0) > > We have a little __same_type() helper for this. __must_be_array() > should be using it, too. It would be great (shortening the long lines a lot), except that in this case it is a complete misnomer, which would probably confuse people reading the code. If __same_type(typeof(addr), void *) worked, I would not need a dummy struct; but __same_type is actually looking for compatible types, not same type (perhaps for non-pointers it actually means "same type"). In the first part of the condition, I am actually looking for "same type", but even there __same_type(void *, struct page *) would return true (which is why I need the second part). And now I am having second thoughts about the line breaks here; I should have also broken between the parameters of __builtin_types_compatible_p, to avoid long lines. If you want, I can resend the patch with it reindented. [1] Yep, there it is: https://lwn.net/Articles/111226/ [2] @@ struct page *page; expression E; @@ * kunmap_atomic(page, E) -- Cesar Eduardo Barros cesarb@cesarb.net cesar.barros@gmail.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org