linux-kernel - Re: [BUG]: mm/vmalloc: uninitialized variable access in pcpu_get_vm

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Message-ID: <CAK8P3a3sjuyeQBUprGFGCXUSDAJN_+c+2z=pCR5J05rByBVByQ@mail.gmail.com>
Date:   Mon, 17 Jun 2019 16:44:49 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Roman Penyaev <rpenyaev@...e.de>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG]: mm/vmalloc: uninitialized variable access in pcpu_get_vm_areas

On Mon, Jun 17, 2019 at 4:12 PM Uladzislau Rezki <urezki@...il.com> wrote:
>
> On Mon, Jun 17, 2019 at 02:14:11PM +0200, Arnd Bergmann wrote:
> > gcc points out some obviously broken code in linux-next
> >
> > mm/vmalloc.c: In function 'pcpu_get_vm_areas':
> > mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> >     insert_vmap_area_augment(lva, &va->rb_node,
> >     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >      &free_vmap_area_root, &free_vmap_area_list);
> >      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > mm/vmalloc.c:916:20: note: 'lva' was declared here
> >   struct vmap_area *lva;
> >                     ^~~
> >
> > Remove the obviously broken code. This is almost certainly
> > not the correct solution, but it's what I have applied locally
> > to get a clean build again.
> >
> > Please fix this properly.
> >

> >
> Please do not apply this. It will just break everything.

As I wrote in my description, this was purely meant as a bug
report, not a patch to be applied.

> As Roman pointed we can just set lva = NULL; in the beginning to make GCC happy.
> For some reason GCC decides that it can be used uninitialized, but that
> is not true.

I got confused by the similarly named FL_FIT_TYPE/NE_FIT_TYPE
constants and misread this as only getting run in the case where it is
not initialized, but you are right that it always is initialized here.

I see now that the actual cause of the warning is the 'while' loop in
augment_tree_propagate_from(). gcc is unable to keep track of
the state of the 'lva' variable beyond that and prints a bogus warning.

        Arnd