linux-kernel - Re: [PATCH 25/28] s390: pci: don't print uninitialized data for debugging

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Message-Id: <alpine.LFD.2.20.1610181034070.1706@schleppi>
Date:   Tue, 18 Oct 2016 10:53:28 +0200 (CEST)
From:   Sebastian Ott <sebott@...ux.vnet.ibm.com>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
cc:     Arnd Bergmann <arnd@...db.de>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Joerg Roedel <jroedel@...e.de>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH 25/28] s390: pci: don't print uninitialized data for
 debugging

On Tue, 18 Oct 2016, Martin Schwidefsky wrote:
> On Tue, 18 Oct 2016 00:16:13 +0200
> Arnd Bergmann <arnd@...db.de> wrote:
> 
> > gcc correctly warns about an incorrect use of the 'pa' variable
> > in case we pass an empty scatterlist to __s390_dma_map_sg:
> > 
> > arch/s390/pci/pci_dma.c: In function '__s390_dma_map_sg':
> > arch/s390/pci/pci_dma.c:309:13: warning: 'pa' may be used uninitialized in this function [-Wmaybe-uninitialized]
> > 
> > This adds a bogus initialization to the function to sanitize
> > the debug output.  I would have preferred a solution without
> > the initialization, but I only got the report from the
> > kbuild bot after turning on the warning again, and didn't
> > manage to reproduce it myself.
> > 
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  arch/s390/pci/pci_dma.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/s390/pci/pci_dma.c b/arch/s390/pci/pci_dma.c
> > index 7350c8b..6b2f72f 100644
> > --- a/arch/s390/pci/pci_dma.c
> > +++ b/arch/s390/pci/pci_dma.c
> > @@ -423,7 +423,7 @@ static int __s390_dma_map_sg(struct device *dev, struct scatterlist *sg,
> >  	dma_addr_t dma_addr_base, dma_addr;
> >  	int flags = ZPCI_PTE_VALID;
> >  	struct scatterlist *s;
> > -	unsigned long pa;
> > +	unsigned long pa = 0;
> >  	int ret;
> > 
> >  	size = PAGE_ALIGN(size);
> 
> The compiler is right. pa is set in the for-loop. If "dma_addr < dma_addr_base + size"
> is never true and __dma_purge_tlb() returns with an error, pa *is* uninitialized.
> The fix seems sensible to me.

Although that could only happen if map_sg is called with zero length sg
list entries I'm in favor getting rid of that warning.

Acked-by: Sebastian Ott <sebott@...ux.vnet.ibm.com>

Regards,
Sebastian