linux-kernel - Re: [PATCH v8 15/19] staging: r8188eu: change the type of a variable in rtw

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Date:   Mon, 20 Sep 2021 22:01:33 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pavel Skripkin <paskripkin@...il.com>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        David Laight <david.Laight@...lab.com>,
        Martin Kaiser <martin@...ser.cx>
Subject: Re: [PATCH v8 15/19] staging: r8188eu: change the type of a variable
 in rtw_read16()

On Mon, Sep 20, 2021 at 06:17:36PM +0200, Fabio M. De Francesco wrote:
> On Monday, September 20, 2021 3:10:36 PM CEST Dan Carpenter wrote:
> > On Mon, Sep 20, 2021 at 03:03:44PM +0200, Fabio M. De Francesco wrote:
> > > On Monday, September 20, 2021 1:56:47 PM CEST Dan Carpenter wrote:
> > > > On Mon, Sep 20, 2021 at 01:53:52AM +0200, Fabio M. De Francesco wrote:
> > > > > Change the type of "data" from __le32 to __le16.
> > > > > 
> > > > 
> > > > You should note in the commit message that:
> > > > 
> > > > The last two bytes of "data" are not initialized so the 
> le32_to_cpu(data)
> > > > technically reads uninitialized data.  This can likely be detected by
> > > > the KASan checker as reading uninitialized data.  But because the bytes
> > > > are discarded in the end so this will not affect runtime.
> > > > 
> > > > regards,
> > > > dan carpenter
> > > > 
> > > 
> > > Dear Dan,
> > > 
> > > Thanks for your suggestion about this specific topic. 
> > > 
> > > We thought that, since "data" is in bitwise AND with 0xffff before being 
> > > passed to the callee, it was enough to have reviewers know why we're 
> doing 
> > > that change of type with no further explanations. Actually it seems to be 
> not 
> > > enough to motivate that change.
> > > 
> > > We will surely use the note you provided. 
> > > 
> > > However, since I'm not used to blindly follow suggestions (even if I 
> trust 
> > > your words with no doubts at all) without complete understanding of what 
> I'm 
> > > doing, I will need to understand what KASan is before copy-paste your 
> note.
> > 
> > Google is your friend!
> 
> Yes, it is :)
> 
> I think you were referring to the KernelMemorySanitizer (KMSan), a detector 
> of uses of uninitialized memory (but it seems to not be upstream):
> https://github.com/google/kmsan 
> 
> Instead you wrote about the The Kernel Address Sanitizer (KASan) that seems 
> to be a dynamic memory error detector designed to find out-of-bound and use-
> after-free bugs (this is upstream):
> https://www.kernel.org/doc/html/v5.0/dev-tools/kasan.html 
> 
> Can you please confirm?

That sounds probably right.

> 
> Back to the code... uninitialised data is not a problem in the old code, it's 
> just bad design. The new code cannot affect runtime, it's just better design. 

It would be a problem for KMSan and the kbuild-bot will email you about
it.  From your commit message "Change the type of "data" from __le32 to
__le16." it's not clear you understand why the kbuild-bot will email you
and why it's correct to do so.

regards,
dan carpenter