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

David Laight
2020-10-23T13:29:03Z

From: Arnd Bergmann
> Sent: 23 October 2020 14:23
>
> On Fri, Oct 23, 2020 at 2:46 PM David Laight <David.Laight@aculab.com> 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.
>
> 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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