linux-kernel - Re: [PATCH 1/2] pci: endpoint: Free func

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Message-ID: <alpine.LNX.2.20.1802231828250.16055@5HSWXM1.fritz.box>
Date:   Fri, 23 Feb 2018 18:38:42 +0100 (CET)
From:   Rolf Evers-Fischer <embedded24@...rs-fischer.de>
To:     Kishon Vijay Abraham I <kishon@...com>
cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rolf Evers-Fischer <embedded24@...rs-fischer.de>,
        bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Rolf Evers-Fischer <rolf.evers.fischer@...iv.com>
Subject: Re: [PATCH 1/2] pci: endpoint: Free func_name after last usage

Hi Kishon,

On Fri, 23 Feb 2018, Kishon Vijay Abraham I wrote:
> On Friday 23 February 2018 03:06 PM, Lorenzo Pieralisi wrote:
> > On Fri, Feb 23, 2018 at 11:40:49AM +0530, Kishon Vijay Abraham I wrote:
> >> Hi Lorenzo,
> >>
> >> On Thursday 22 February 2018 11:49 PM, Lorenzo Pieralisi wrote:
> >>> On Wed, Feb 21, 2018 at 01:47:06PM +0100, Rolf Evers-Fischer wrote:
> >>>> From: Rolf Evers-Fischer <rolf.evers.fischer@...iv.com>
> >>>>
> >>>> This commit decreases the number of jump labels and ensures
> >>>> that the next commit doesn't increase the number of occurrences
> >>>> of 'kfree(func_name)'.
> >>>>
> >>>> Change-Id: I0d1b6fd652395b85f82b11c43bf9b7db512854d1
> >>>> Signed-off-by: Rolf Evers-Fischer <rolf.evers.fischer@...iv.com>
> >>>> Signed-off-by: Rolf Evers-Fischer <embedded24@...rs-fischer.de>
> >>>> ---
> >>>>  drivers/pci/endpoint/pci-epf-core.c | 7 ++-----
> >>>>  1 file changed, 2 insertions(+), 5 deletions(-)
> >>>>
> >>>> diff --git a/drivers/pci/endpoint/pci-epf-core.c b/drivers/pci/endpoint/pci-epf-core.c
> >>>> index 766ce1dca2ec..23d0e128d1a5 100644
> >>>> --- a/drivers/pci/endpoint/pci-epf-core.c
> >>>> +++ b/drivers/pci/endpoint/pci-epf-core.c
> >>>> @@ -220,9 +220,10 @@ struct pci_epf *pci_epf_create(const char *name)
> >>>>  	*buf = '\0';
> >>>>  
> >>>>  	epf->name = kstrdup(func_name, GFP_KERNEL);
> >>>> +	kfree(func_name);
> >>>
> >>> I am certainly missing something but what if we reworked the code
> >>> and just:
> >>>
> >>> kstrdup(name, GFP_KERNEL);
> >>>
> >>> once instead of allocating another local copy (that we then have to
> >>> free) ?
> >>
> >> name will be something like pci_epf_test.0 and in epf->name we want to just
> >> have pci_epf_test.
> >>>
> >>> Reworded: why
> >>>
> >>> epf->name = func_name;
> >>
> >> memory should be allocated for epf->name before it can be initialized. IMO
> >> without kstrdup, there will be a null pointer exception.
> > 
> > I understand that but the point is that func_name *was* allocated with
> > kstrdup() already I would like to understand why we need to do it twice
> > (and kfree the first allocation).
> 
> func_name would be allocated for a size greater than what will be in epf->name.
> It won't be significant though.
> 
> Thanks
> Kishon
> 
I have just an idea, how to use only one copy, but with the shorter 
length. What about changing the code like this?
  int len = strchrnul(name, '.') - name;
  epf->name = kstrndup(name, len, GFP_KERNEL);

In this case we are only allocating the smaller amount of memory and we 
don't need the 'buf' and 'func_name' pointers anymore. They will be 
replaced with 'len' and some pointer arithmetic.

Kind regards,
 Rolf