linux-kernel - Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov

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Date:   Fri, 23 Oct 2020 16:33:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Greg KH' <gregkh@...uxfoundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Christoph Hellwig <hch@....de>,
        "kernel-team@...roid.com" <kernel-team@...roid.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-aio@...ck.org" <linux-aio@...ck.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move
 rw_copy_check_uvector() into lib/iov_iter.c"

On 23.10.20 15:09, David Hildenbrand wrote:
> On 23.10.20 14:46, David Laight wrote:
>> From: Greg KH <gregkh@...uxfoundation.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.
> 

I just checked against upstream code generated by clang 10 and it
properly discards the upper 32bit via a mov w23 w2.

So at least clang 10 indeed properly assumes we could have garbage and
masks it off.

Maybe the issue is somewhere else, unrelated to nr_pages ... or clang 11
behaves differently.

-- 
Thanks,

David / dhildenb