linux-kernel - Re: [PATCH 3/3] staging: xgifb: Initialize uninitialized variables

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Date:	Wed, 8 Sep 2010 01:49:32 -0700
From:	Greg KH <gregkh@...e.de>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	Javier Martinez Canillas <martinez.javier@...il.com>,
	Bill Pemberton <wfp5p@...ginia.edu>,
	Arnaud Patard <apatard@...driva.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] staging: xgifb: Initialize uninitialized variables

On Wed, Sep 08, 2010 at 09:16:43AM +0200, Jiri Slaby wrote:
> On 09/08/2010 02:38 AM, Greg KH wrote:
> > On Tue, Sep 07, 2010 at 10:46:27AM +0200, Jiri Slaby wrote:
> >> On 09/07/2010 07:34 AM, Javier Martinez Canillas wrote:
> >>> Current patch solves compilation warnings in staging/xgifb for using possibly uninitialized variables.
> >>>
> >>> Signed-off-by: Javier Martinez Canillas <martinez.javier@...il.com>
> >>> ---
> >>>  drivers/staging/xgifb/vb_setmode.c |    4 ++--
> >>>  1 files changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/staging/xgifb/vb_setmode.c b/drivers/staging/xgifb/vb_setmode.c
> >>> index c4db2fc..720a592 100644
> >>> --- a/drivers/staging/xgifb/vb_setmode.c
> >>> +++ b/drivers/staging/xgifb/vb_setmode.c
> >>> @@ -376,7 +376,7 @@ void InitTo330Pointer(unsigned char ChipType, struct vb_device_info *pVBInfo)
> >>>  unsigned char XGISetModeNew(struct xgi_hw_device_info *HwDeviceExtension,
> >>>  			unsigned short ModeNo)
> >>>  {
> >>> -	unsigned short ModeIdIndex;
> >>> +	unsigned short ModeIdIndex = 0;
> >>>  	/* unsigned char *pVBInfo->FBAddr = HwDeviceExtension->pjVideoMemoryAddress; */
> >>>  	struct vb_device_info VBINF;
> >>>  	struct vb_device_info *pVBInfo = &VBINF;
> >>
> >> No, please don't. This is a compiler bug.
> >>
> >> The first use in that function is:
> >> XGI_SearchModeID( ModeNo , &ModeIdIndex, pVBInfo );
> >>
> >> and that function contains:
> >> if (XYZ) {
> >>   for( *ModeIdIndex = 0 ; ; ( *ModeIdIndex )++ )
> >>     ...
> >> } else {
> >>   for( *ModeIdIndex = 0 ; ; ( *ModeIdIndex )++ )
> >>     ...
> >> }
> >>
> >> So your compiler is foolish if that emits a warning.
> > 
> > No, I don't see a way that the compiler could really figure that type of
> > logic out.  It doesn't know that XGI_SearchModeID() always will set that
> > field.  Heck, at first glance I couldn't even figure it out :)
> 
> No, the compiler could. Trust me :). Actually you've just hidden a bug
> in XGI_SearchModeID. There is #ifdef LINUX and LINUX is defined in
> main.h. But that file is not included in vb_setmode.c. Hence the
> uninitialized warnings...

Doh, you are right.

> > So I don't mind applying this patch, as I'll just constantly keep
> > getting people sending me the same change, when they could be off
> > working on other things that this driver needs.
> 
> So instead of hiding bugs, please fix the right ones.

Good point, I'll go do that right now.

thanks,

greg k-h
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