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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Leon Romanovsky <leon@kernel.org>,
	Peilin Ye <yepeilin.cs@gmail.com>,
	Santosh Shilimkar <santosh.shilimkar@oracle.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-kernel-mentees@lists.linuxfoundation.org,
	netdev@vger.kernel.org, linux-rdma@vger.kernel.org,
	rds-devel@oss.oracle.com, linux-kernel@vger.kernel.org
Subject: Re: [Linux-kernel-mentees] [PATCH net] rds: Prevent kernel-infoleak in rds_notify_queue_get()
Date: Fri, 31 Jul 2020 15:27:12 -0300
Message-ID: <20200731182712.GI24045@ziepe.ca> (raw)
In-Reply-To: <20200731171924.GA2014207@kroah.com>

On Fri, Jul 31, 2020 at 07:19:24PM +0200, Greg Kroah-Hartman wrote:

> > I tried for a bit and didn't find a way to get even old gcc 4.4 to not
> > initialize the holes.
> 
> Odd, so it is just the "= {0};" that does not zero out the holes?

Nope, it seems to work fine too. I tried a number of situations and I
could not get the compiler to not zero holes, even back to gcc 4.4

It is not just accidental either, take this:

	struct rds_rdma_notify {
		unsigned long user_token;
		unsigned char status;
		unsigned long user_token1 __attribute__((aligned(32)));
	} foo = {0};

Which has quite a big hole, clang generates:

	movq	$0, 56(%rdi)
	movq	$0, 48(%rdi)
	movq	$0, 40(%rdi)
	movq	$0, 32(%rdi)
	movq	$0, 24(%rdi)
	movq	$0, 16(%rdi)
	movq	$0, 8(%rdi)
	movq	$0, (%rdi)

Deliberate extra instructions to fill both holes. gcc 10 does the
same, older gcc's do create a rep stosq over the whole thing.

Some fiddling with godbolt shows quite a variety of output, but I
didn't see anything that looks like a compiler not filling
padding. Even godbolt's gcc 4.1 filled the padding, which is super old.

In several cases it seems the aggregate initializer produced better
code than memset, in other cases it didn't

Without an actual example where this doesn't work right it is hard to
say anything more..

Jason

  reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-30 19:20 Peilin Ye
2020-07-30 19:29 ` santosh.shilimkar
2020-07-31  4:53 ` Leon Romanovsky
2020-07-31  5:33   ` Greg Kroah-Hartman
2020-07-31  5:33     ` Greg Kroah-Hartman
     [not found]       ` <CAHp75Vdr2HC_ogNhBCxxGut9=Z6pQMFiA0w-268OQv+5unYOTg@mail.gmail.com>
2020-07-31  7:00         ` Leon Romanovsky
2020-07-31  7:05           ` Andy Shevchenko
2020-07-31 14:04       ` Jason Gunthorpe
2020-07-31 14:21         ` Greg Kroah-Hartman
2020-07-31 14:36           ` Jason Gunthorpe
2020-07-31 17:19             ` Greg Kroah-Hartman
2020-07-31 18:27               ` Jason Gunthorpe [this message]
2020-08-01  8:00                 ` Dan Carpenter
2020-08-01 14:40                   ` Jason Gunthorpe
2020-08-03  9:34                     ` Dan Carpenter
2020-08-01  5:38               ` Leon Romanovsky
2020-08-02 22:10                 ` Jason Gunthorpe
2020-08-02 22:23                   ` Joe Perches
2020-08-02 22:28                     ` Jason Gunthorpe
2020-08-02 22:45                       ` Joe Perches
2020-08-03  4:58                         ` Leon Romanovsky
2020-08-03 23:06                         ` Jason Gunthorpe
2020-08-08 22:57                           ` Jack Leadford
2020-08-09  7:04                             ` Leon Romanovsky
2020-08-14 17:07                             ` Jason Gunthorpe
2020-07-31  9:59   ` Dan Carpenter
2020-07-31 11:14     ` Håkon Bugge
2020-07-31 11:59       ` Greg Kroah-Hartman
2020-07-31 12:03         ` Håkon Bugge
2020-07-31 23:54 ` David Miller

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