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:   Thu, 22 Oct 2020 21:06:29 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        David Laight <David.Laight@...lab.com>,
        Christoph Hellwig <hch@....de>,
        David Hildenbrand <david@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "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"

On Thu, Oct 22, 2020 at 08:24:58PM +0100, Al Viro wrote:

> Depending upon the calling conventions, compiler might do truncation in caller or
> in a callee, but it must be done _somewhere_.

Unless I'm misreading AAPCS64,
	"Unlike in the 32-bit AAPCS, named integral values must be narrowed by the callee
	 rather than the caller"
in 6.4.2 means that callee must not _not_ expect the upper 32 bits of %x0..%x7 to contain
anything valid for 32bit arguments and it must zero-extend %w0..%w7 when passing that to
something that expects a 64bit argument.  On inlining it should be the same situation as
storing unsigned int argument into unsigned long local variable and working with that - if

void f(unsigned int w)
{
	unsigned long x = w;
	printf("%lx\n", x);
}

ends up passing %x0 to printf, it's an obvious bug - it must do something like
	uxtw x0, w0
first.

What am I missing here?