linux-kernel - Re: [PATCH 05/16] vfs: bogus warnings in fs/namei.c

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Message-Id: <201210091307.19225.arnd@arndb.de>
Date:	Tue, 9 Oct 2012 13:07:19 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	arm@...nel.org, Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 05/16] vfs: bogus warnings in fs/namei.c

On Tuesday 09 October 2012, Arnd Bergmann wrote:
> On Monday 08 October 2012, Jan Kara wrote:
> > On Fri 05-10-12 16:55:19, Arnd Bergmann wrote:
> > > The follow_link() function always initializes its *p argument,
> > > or returns an error, but not all versions of gcc figure this
> > > out, so we have to work around this using the uninitialized_var()
> > > macro.
> >   Well, I'm somewhat sceptical to this approach. I agree that bogus
> > warnings are not nice but later when the code is changed and possibly real
> > use without initialization is added, we won't notice it. Without changing
> > anything, we'd at least have a chance of catching it with gcc versions
> > which were clever enough to not warn with the original code. Or
> > alternatively if we unconditionally initialized the variable that would get
> > rid of the warning and made the code more future-proof (that's what I
> > usually end up doing)... I don't really care that much about the chosen
> > solution, Al is the one to decide. But I wanted to point out there are
> > downsides to your solution.
> 
> I'll drop the patch for now and won't send it from my tree then. I agree
> that uninitialized_var() is not ideal, but none of the alternatives seemed
> better.
> 
> With my latest compiler, I don't actually see the warnings any more, so
> maybe someone fixed gcc instead, or this went away after another change.
> I'll let you know if it comes back so we can discuss about a better fix then.
> 

Update: I could actually reproduce the problem now, but it only happens when
building with 'gcc -s' (i.e. CONFIG_CC_OPTIMIZE_FOR_SIZE). It does happen
with both gcc-4.6 and with gcc-4.8, and on both x86-64 and ARM. An alternative
patch that would also make it go away is the variant below, but I think that's
even worse than the first version I suggested because it makes the binary
output slightly worse by adding an unnecessary initialization when building with
'make -s'.

	Arnd

diff --git a/fs/namei.c b/fs/namei.c
index aa30d19..c3612a5 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -810,6 +810,7 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 	return error;
 
 out_put_nd_path:
+	*p = NULL;
 	path_put(&nd->path);
 	path_put(link);
 	return error;
--
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