Re: [BUG]: mm/vmalloc: uninitialized variable access in pcpu_get_vm_areas

From: Arnd Bergmann <arnd@arndb.de>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: Roman Gushchin <guro@fb.com>, Michal Hocko <mhocko@suse.com>,
	Matthew Wilcox <willy@infradead.org>,
	Thomas Garnier <thgarnie@google.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Joel Fernandes <joelaf@google.com>,
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
	Tejun Heo <tj@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Roman Penyaev <rpenyaev@suse.de>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Mike Rapoport <rppt@linux.ibm.com>, Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [BUG]: mm/vmalloc: uninitialized variable access in pcpu_get_vm_areas
Date: Mon, 17 Jun 2019 16:44:49 +0200	[thread overview]
Message-ID: <CAK8P3a3sjuyeQBUprGFGCXUSDAJN_+c+2z=pCR5J05rByBVByQ@mail.gmail.com> (raw)
In-Reply-To: <20190617141244.5x22nrylw7hodafp@pc636>

On Mon, Jun 17, 2019 at 4:12 PM Uladzislau Rezki <urezki@gmail.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