Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c"

David Hildenbrand
2020-10-23T13:09:49Z

On 23.10.20 14:46, David Laight wrote:
> From: Greg KH <gregkh@linuxfoundation.org>
>> Sent: 22 October 2020 14:51
>
> I've rammed the code into godbolt.
>
> https://godbolt.org/z/9v5PPW
>
> Definitely a clang bug.
>
> Search for [wx]24 in the clang output.
> nr_segs comes in as w2 and the initial bound checks are done on w2.
> w24 is loaded from w2 - I don't believe this changes the high bits.
> There are no references to w24, just x24.
> So the kmalloc_array() is passed 'huge' and will fail.
> The iov_iter_init also gets the 64bit value.
>
> Note that the gcc code has a sign-extend copy of w2.

Do we have a result from using "unsigned long" in the base function and
explicitly masking of the high bits? That should definitely work.

Now, I am not a compiler expert, but as I already cited, at least on
x86-64 clang expects that the high bits were cleared by the caller - in
contrast to gcc. I suspect it's the same on arm64, but again, I am no
compiler expert.

If what I said and cites for x86-64 is correct, if the function expects
an "unsigned int", it will happily use 64bit operations without further
checks where valid when assuming high bits are zero. That's why even
converting everything to "unsigned int" as proposed by me won't work on
clang - it assumes high bits are zero (as indicated by Nick).

As I am neither a compiler experts (did I mention that already? ;) ) nor
an arm64 experts, I can't tell if this is a compiler BUG or not.

Main issue seems to be garbage in high bits as originally suggested by me.

--
Thanks,

David / dhildenb