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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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@arndb.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@arndb.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