Messages in this thread | | | Date | Sun, 30 May 2010 14:42:53 -0300 | From | Cesar Eduardo Barros <> | Subject | Re: [PATCH] Make kunmap_atomic() harder to misuse |
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Em 30-05-2010 00:42, Andrew Morton escreveu: > On Fri, 28 May 2010 07:53:13 -0300 Cesar Eduardo Barros<cesarb@cesarb.net> 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 ‘foo’: mm/swapfile.c:2501: error: negative width in bit-field ‘<anonymous>’
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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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