linux-kernel - Re: [PATCH 1/3] uml: drivers/net

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Date:	Fri, 8 Jul 2011 12:44:53 +0200
From:	Richard Weinberger <richard@....at>
To:	Vitaliy Ivanov <vitalivanov@...il.com>
Cc:	Jeff Dike <jdike@...toit.com>,
	user-mode-linux-devel@...ts.sourceforge.net,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] uml: drivers/net_user.c memory leak fix

Am Freitag 08 Juli 2011, 12:30:56 schrieb Vitaliy Ivanov:
> On Thu, Jul 7, 2011 at 8:27 PM, Richard Weinberger <richard@....at> wrote:
> > Am Donnerstag 07 Juli 2011, 18:36:02 schrieb Vitaliy Ivanov:
> >> >From 9b9f36f46aa708c3245f5ded83f96421966b2edf Mon Sep 17 00:00:00 2001
> >> 
> >> From: Vitaliy Ivanov <vitalivanov@...il.com>
> >> Date: Thu, 7 Jul 2011 19:23:13 +0300
> >> Subject: [PATCH 1/3] uml: drivers/net_user.c memory leak fix
> >> 
> >> Perform memory cleanup on exit.
> >> On receiving invalid 'pid' we still should clean 'output' variable.
> >> 
> >> Signed-off-by: Vitaliy Ivanov <vitalivanov@...il.com>
> >> ---
> >>  arch/um/drivers/net_user.c |    5 +++--
> >>  1 files changed, 3 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/arch/um/drivers/net_user.c b/arch/um/drivers/net_user.c
> >> index 9415dd9..989b653 100644
> >> --- a/arch/um/drivers/net_user.c
> >> +++ b/arch/um/drivers/net_user.c
> >> @@ -228,10 +228,11 @@ static void change(char *dev, char *what, unsigned
> >> char *addr, "buffer\n");
> >> 
> >>       pid = change_tramp(argv, output, output_len);
> >> -     if (pid < 0) return;
> >> 
> >>       if (output != NULL) {
> >> -             printk("%s", output);
> >> +             if (pid >= 0) {
> >> +                     printk("%s", output);
> >> +             }
> >>               kfree(output);
> >>       }
> >>  }
> > 
> > This control logic is a bit strange.
> > When change_tramp() fails we should not printk() the output variable.
> > 
> > if (pid < 0){
> >  free(output);
> >  return;
> > }
> > 
> > Would be much cleaner.
> 
> I just didn't want to clone this free-return stuff. So, what you
> proposing is like this:
> ------------
> ...
>         output = uml_kmalloc(output_len, UM_GFP_KERNEL);
>         if (output == NULL)
>                 printk(UM_KERN_ERR "change : failed to allocate output "
>                        "buffer\n");
> 
>         pid = change_tramp(argv, output, output_len);
>         if (pid < 0) {
>                 free(output);               <---------- I'm not sure
> but 'output' can be NULL here.
>                 return;
>         }
> 
>         if (output != NULL) {
>                 printk("%s", output);
>                 kfree(output);
>         }
> }
> ------------
> 
> I was trying to print 'output' only in case change_tramp is
> successful. That's the old logic. And at the same time perform free
> only in case output is not NULL.

Why?
Freeing a NULL pointer is perfectly fine.

Thanks,
//richard
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