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From: Al Viro <viro@zeniv.linux.org.uk>
To: David Laight <David.Laight@aculab.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jens Axboe <axboe@kernel.dk>, Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
	"linux-parisc@vger.kernel.org" <linux-parisc@vger.kernel.org>,
	"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
	"linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
	"sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
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	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
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	"keyrings@vger.kernel.org" <keyrings@vger.kernel.org>,
	"linux-security-module@vger.kernel.org" 
	<linux-security-module@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec
Date: Wed, 23 Sep 2020 15:49:17 +0100
Message-ID: <20200923144917.GM3421308@ZenIV.linux.org.uk> (raw)
In-Reply-To: <200cf2b9ce5e408f8838948fda7ce9a0@AcuMS.aculab.com>

On Wed, Sep 23, 2020 at 02:38:24PM +0000, David Laight wrote:
> From: Al Viro
> > Sent: 23 September 2020 15:17
> > 
> > On Wed, Sep 23, 2020 at 08:05:41AM +0200, Christoph Hellwig wrote:
> > 
> > > +struct iovec *iovec_from_user(const struct iovec __user *uvec,
> > > +		unsigned long nr_segs, unsigned long fast_segs,
> > 
> > Hmm...  For fast_segs unsigned long had always been ridiculous
> > (4G struct iovec on caller stack frame?), but that got me wondering about
> > nr_segs and I wish I'd thought of that when introducing import_iovec().
> > 
> > The thing is, import_iovec() takes unsigned int there.  Which is fine
> > (hell, the maximal value that can be accepted in 1024), except that
> > we do pass unsigned long syscall argument to it in some places.
> 
> It will make diddly-squit difference.
> The parameters end up in registers on most calling conventions.
> Plausibly you get an extra 'REX' byte on x86 for the 64bit value.
> What you want to avoid is explicit sign/zero extension and value
> masking after arithmetic.

Don't tell me what I want; your telepathic abilities are consistently sucky.

I am *NOT* talking about microoptimization here.  I have described
the behaviour change of syscall caused by commit 5 years ago.  Which is
generally considered a problem.  Then I asked whether that behaviour
change would fall under the "if nobody noticed, it's not a userland ABI
breakage" exception.

Could you show me the point where I have expressed concerns about
the quality of amd64 code generated for that thing, before or after
the change in question?

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-23  6:05 let import_iovec deal with compat_iovecs as well v3 Christoph Hellwig
2020-09-23  6:05 ` [PATCH 1/9] compat.h: fix a spelling error in <linux/compat.h> Christoph Hellwig
2020-09-23  6:05 ` [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c Christoph Hellwig
2020-09-23  6:05 ` [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec Christoph Hellwig
2020-09-23 14:16   ` Al Viro
2020-09-23 14:38     ` David Laight
2020-09-23 14:49       ` Al Viro [this message]
2020-09-23 14:40     ` Al Viro
2020-09-23  6:05 ` [PATCH 4/9] iov_iter: transparently handle compat iovecs in import_iovec Christoph Hellwig
2020-09-23  6:05 ` [PATCH 5/9] fs: remove various compat readv/writev helpers Christoph Hellwig
2020-09-23 14:25   ` Al Viro
2020-09-23 14:32     ` Christoph Hellwig
2020-09-23 14:59       ` Al Viro
2020-09-23 16:38         ` Al Viro
2020-09-23 17:05           ` Al Viro
2020-09-23 17:46             ` Christoph Hellwig
2020-09-23 17:08           ` Brian Gerst
2020-09-23 18:45           ` Arnd Bergmann
2020-09-23 19:47             ` Al Viro
2020-09-23 19:52               ` Arnd Bergmann
2020-09-23 21:30             ` David Laight
2020-09-23  6:05 ` [PATCH 6/9] fs: remove the compat readv/writev syscalls Christoph Hellwig
2020-09-23  6:05 ` [PATCH 7/9] fs: remove compat_sys_vmsplice Christoph Hellwig
2020-09-23  6:05 ` [PATCH 8/9] mm: remove compat_process_vm_{readv,writev} Christoph Hellwig
2020-09-23  6:05 ` [PATCH 9/9] security/keys: remove compat_keyctl_instantiate_key_iov Christoph Hellwig
2020-09-25  4:51 let import_iovec deal with compat_iovecs as well v4 Christoph Hellwig
2020-09-25  4:51 ` [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec Christoph Hellwig

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