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

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Message-ID: <8e758668cffa4713969df33299180c64@AcuMS.aculab.com>
Date:   Fri, 23 Oct 2020 13:28:57 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...db.de>
CC:     Greg KH <gregkh@...uxfoundation.org>,
        David Hildenbrand <david@...hat.com>,
        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>,
        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"

From: Arnd Bergmann
> Sent: 23 October 2020 14:23
> 
> On Fri, Oct 23, 2020 at 2:46 PM David Laight <David.Laight@...lab.com> 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.
> 
> You believe wrong, "mov w24, w2" is a zero-extending operation.

Ah ok, but gcc uses utxw for the same task.
I guess they could be the same opcode.

Last time I wrote ARM thumb didn't really exist - never mind 64bit

	David

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