linux-kernel - Re: [PATCH] driver core: Fix an uninitialized variable is used by __device

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Message-ID: <Zsfk-9lf1sRMgBqE@google.com>
Date: Thu, 22 Aug 2024 18:25:15 -0700
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Zijun Hu <zijun_hu@...oud.com>, "Rafael J. Wysocki" <rafael@...nel.org>,
	linux-kernel@...r.kernel.org, Zijun Hu <quic_zijuhu@...cinc.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH] driver core: Fix an uninitialized variable is used by
 __device_attach()

On Fri, Aug 23, 2024 at 09:14:12AM +0800, Greg Kroah-Hartman wrote:
> On Fri, Aug 23, 2024 at 08:46:12AM +0800, Zijun Hu wrote:
> > On 2024/8/23 08:02, Dmitry Torokhov wrote:
> > > Hi,
> > > 
> > > On Fri, Aug 23, 2024 at 07:46:09AM +0800, Zijun Hu wrote:
> > >> From: Zijun Hu <quic_zijuhu@...cinc.com>
> > >>
> > >> An uninitialized variable @data.have_async may be used as analyzed
> > >> by the following inline comments:
> > >>
> > >> static int __device_attach(struct device *dev, bool allow_async)
> > >> {
> > >> 	// if @allow_async is true.
> > >>
> > >> 	...
> > >> 	struct device_attach_data data = {
> > >> 		.dev = dev,
> > >> 		.check_async = allow_async,
> > >> 		.want_async = false,
> > >> 	};
> > >> 	// @data.have_async is not initialized.
> > > 
> > > No, in the presence of a structure initializer fields not explicitly
> > > initialized will be set to 0 by the compiler.
> > > 
> > really?
> > do all C compilers have such behavior ?
> 
> Oh wait, if this were static, then yes, it would all be set to 0, sorry,
> I misread this.
> 
> This is on the stack so it needs to be zeroed out explicitly.  We should
> set the whole thing to 0 and then set only the fields we want to
> override to ensure it's all correct.

No we do not. ISO/IEC 9899:201x 6.7.9 Initialization:

"21 If there are fewer initializers in a brace-enclosed list than there
are elements or members of an aggregate, or fewer characters in a string
literal used to initialize an array of known size than there are
elements in the array, the remainder of the aggregate shall be
initialized implicitly the same as objects that have static storage
duration."

That is why you can 0-initialize a structure by doing:

	struct s s1 = { 0 };

or even

	struct s s1 = { };

Thanks.

-- 
Dmitry