linux-kernel - Re: [PATCH] mm/mremap: fix uninitialized return code

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Message-ID: <07467d4e-4882-4649-8845-c53cb2cc243c@lucifer.local>
Date: Wed, 5 Mar 2025 18:34:52 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Arnd Bergmann <arnd@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pedro.falcato@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Jeff Xu <jeffxu@...omium.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mremap: fix uninitialized return code

On Wed, Mar 05, 2025 at 07:25:38PM +0100, Arnd Bergmann wrote:
> On Wed, Mar 5, 2025, at 18:29, Lorenzo Stoakes wrote:
> > On Wed, Mar 05, 2025 at 06:27:56PM +0100, Arnd Bergmann wrote:
> >> From: Arnd Bergmann <arnd@...db.de>
> >>
> >> The 'err' variable is set in a conditinal branch and is not
> >> set otherwise:
> >>
> >> mm/mremap.c:1017:7: error: variable 'err' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> >>  1017 |                 if (vma->vm_start != old_addr)
> >>
> >> Set it to zero before the initial value is set.
> >>
> >> Fixes: 3129f7896afb ("mm/mremap: initial refactor of move_vma()")
> >> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> >
> > Thanks for this, however this has already been fixed via Andrew doing a
> > fix-patch which should land in mm-unstable,-next soon (possibly tmr?).
> >
> > This is entirely my bad btw, for some reason my compiler locally didn't
> > catch this even with CONFIG_WERROR on which _really_ surprises me...
>
> Unfortunately gcc never shows warnings about conditionally uninitialized
> variables. There is a -Wmaybe-uninitialized, but that has so many
> false positives that it is completely useless and it is turned
> off globally.

OK I notice by building with LLVM=1 this is immediately reported locally,
so from now on I'll make sure to do a clang build too.

At least there is some means by which I can sensibly determine this!

Cheers!

>
>       Arnd