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

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Message-ID: <48b997b0-a27a-43ca-a7cc-abbab9bb9eb5@icloud.com>
Date: Fri, 23 Aug 2024 18:52:47 +0800
From: Zijun Hu <zijun_hu@...oud.com>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: "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 2024/8/23 09:25, Dmitry Torokhov wrote:
> 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 = { };
> 
For above both initialization: it appears to initialize the whole struct.
but For the initialization approach we discuss, it appears to
initialize partial struct, it is easy to mislead developers.

> Thanks.
>