linux-kernel - Re: [PATCH] USB: dummy-hcd: Fix uninitialized array use in init()

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Message-ID: <20201206112405.GB3006@minh>
Date:   Sun, 6 Dec 2020 18:24:05 +0700
From:   Bui Quang Minh <minhquangbui99@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Corentin Labbe <clabbe@...libre.com>,
        Jules Irenge <jbi.octave@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: dummy-hcd: Fix uninitialized array use in init()

On Sat, Dec 05, 2020 at 10:15:11AM -0500, Alan Stern wrote:
> On Sat, Dec 05, 2020 at 07:47:01PM +0700, Minh Bùi Quang wrote:
> > Vào Th 6, 4 thg 12, 2020 vào lúc 23:12 Alan Stern
> > <stern@...land.harvard.edu> đã viết:
> > > Does this initialization end up using less memory than an explicit
> > > memset() call?
> > 
> > You mean speed?
> 
> No, I mean memory space.
> 
> A memset call requires a certain amount of instruction space (to push 
> the arguments and make the call) but no static data space.  
> Initialization requires some instruction space (to copy the data) and 
> static data space as well (to hold the data that is to be copied).
> 
> Alan Stern
> 

Thank you for your clarification, I didn't think about it before.

As I check when compiling the code, with MAX_NUM_UDC=32 the initialization
becomes

        xor    eax,eax
        mov    ecx,0x40
        rep stos DWORD PTR es:[rdi],eax

With MAX_NUM_UDC=2, the initialization becomes

        mov    QWORD PTR [rbp-0x30],0x0
        mov    QWORD PTR [rbp-0x28],0x0

As I see, initialization does not require additional static data space.
Am I right?

Thanks,
Quang Minh