linux-kernel - Re: [PATCH]kconfig/menu.c: fix uninitialized variable warning

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Date:	Thu, 19 Sep 2013 22:04:30 +0200
From:	"Yann E. MORIN" <yann.morin.1998@...e.fr>
To:	Benjamin Poirier <bpoirier@...e.de>
Cc:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>, mmarek@...e.cz,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH]kconfig/menu.c: fix uninitialized variable warning

Benjamin, All,

On 2013-09-19 15:13 -0400, Benjamin Poirier spake thusly:
> On 2013/09/19 19:27, Yann E. MORIN wrote:
> > Benjamin, Madhavan, All,
> > 
> > On 2013-09-19 11:22 -0400, Benjamin Poirier spake thusly:
> > > On 2013/09/19 12:58, Madhavan Srinivasan wrote:
> > > > In file included from scripts/kconfig/zconf.tab.c:2537:0:
> > > > scripts/kconfig/menu.c: In function ‘get_symbol_str’:
> > > > scripts/kconfig/menu.c:586:18: warning: ‘jump’ may be used uninitialized in
> > > > this function [-Wmaybe-uninitialized]
> > >                    ^^^^^
> > > 
> > > from gcc(1):
> > > 	"These warnings are made optional because GCC is not smart
> > > 	enough to see all the reasons why the code might be correct
> > > 	despite appearing to have an error."
> > > 
> > > I do not see this warning with gcc 4.7.2 or 4.8.1. Which version are you
> > > using?
> > > 
> > > The code does:
> > > 
> > > static void get_prompt_str(struct gstr *r, struct property *prop,
> > > [...]
> > > 	if (head && location) {
> > > 		jump = xmalloc(sizeof(struct jump_key));
> > > [... different basic block ...]
> > > 			if (head && location && menu == location)
> > > 				jump->offset = r->len - 1;
> > > 
> > > If the second "if" is true, then the first one was true as well, and
> > > "jump" is initilized.
> > 
> > Exactly what I was going to say.
> > 
> > However:
> >   - I believe we should strive for a warning-free code whenever possible
> >   - while still getting help from gcc to pinpoint potential issues.
> > 
> > Clearly, gcc is wrong here. Setting jump to NULL will cause fault if we
> > try to dereference it. Since this should never happen given the code as
> > it currently is, I'm slightly in favour of acking this patch.
> > 
> > Any other reason not to apply it?
> 
> Did you manage to get that warning as well? I didn't. If it's only from
> old versions of gcc I'd feel better leaving the warning there than
> masking it, in case a future code change does really introduce a use
> before initialize.

No, I was not able to reproduce it with either gcc-4.4.7, gcc-4.6.4 or
gcc-4.7.3 on my Ubuntu 13.04. Neither gcc-4.5 nor 4.8 are packaged, so
I could not test them.

Anyway, -Wmaybe-uninitialized is new with 4.7. So, the warning can't
happen with gcc < 4.7.

Madhavan, what gcc version are you using?

Regards,
Yann E. MORIN.

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