linux-kernel - Re: [PATCH] UBI: Fix crash in try_recover

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Date:   Mon, 17 Oct 2016 13:10:55 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     Richard Weinberger <richard@....at>,
        Brian Norris <computersforpeace@...il.com>,
        Artem Bityutskiy <dedekind1@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        MTD Maling List <linux-mtd@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] UBI: Fix crash in try_recover_peb()

Hi Boris,

On Mon, Oct 17, 2016 at 1:02 PM, Boris Brezillon
<boris.brezillon@...e-electrons.com> wrote:
> On Mon, 17 Oct 2016 10:00:25 +0200
> Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> On Thu, Oct 13, 2016 at 4:24 PM, Boris Brezillon
>> <boris.brezillon@...e-electrons.com> wrote:
>> > On Thu, 13 Oct 2016 16:05:36 +0200
>> > Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> >
>> >>     drivers/mtd/ubi/eba.c: In function ‘try_recover_peb’:
>> >>     drivers/mtd/ubi/eba.c:744: warning: ‘vid_hdr’ is used uninitialized in this function
>> >>
>> >> The pointer vid_hdr is indeed not initialized, leading to a crash when
>> >> it is dereferenced.
>> >>
>> >> Fix this by obtaining the pointer from the VID buffer, like is done
>> >> everywhere else.
>> >
>> > Indeed, I don't know how I missed that one :-/.
>> >
>> >>
>> >> Fixes: 3291b52f9ff0acc8 ("UBI: introduce the VID buffer concept")
>> >> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>> >
>> > One minor comment below, otherwise
>> >
>> > Reviewed-by: Boris Brezillon <boris.brezillon@...e-electrons.com>
>>
>> Thanks!
>>
>> >> ---
>> >> Completely untested. And I know nothing about UBI ;-)
>> >> ---
>> >>  drivers/mtd/ubi/eba.c | 1 +
>> >>  1 file changed, 1 insertion(+)
>> >>
>> >> diff --git a/drivers/mtd/ubi/eba.c b/drivers/mtd/ubi/eba.c
>> >> index 95c4048a371e87b6..388e46be6ad92805 100644
>> >> --- a/drivers/mtd/ubi/eba.c
>> >> +++ b/drivers/mtd/ubi/eba.c
>> >> @@ -741,6 +741,7 @@ static int try_recover_peb(struct ubi_volume *vol, int pnum, int lnum,
>> >>               goto out_put;
>> >>       }
>> >>
>> >> +     vid_hdr = ubi_get_vid_hdr(vidb);
>> >
>> > Can you move this assignment at variable declaration time?
>>
>> I can do that. However, that moves the call to ubi_get_vid_hdr() _before_
>> the call to ubi_io_read_vid_hdr().
>>
>> While that would still work (ubi_get_vid_hdr() would just return a pointer to
>> the not-yet-read data), I think it's better to not move it, to prevent people
>> from accidentally trying to use it before the data has been read.
>
> Well, maybe. I just thought it would be safer to assign it as early as
> possible to avoid another 'uninitialized var' bug if move code around
> again (which will happen soon ;-)).

I prefer seeing the warning again (I _will_ notice if it pops up again!) over
failing at runtime because the data structure it points to hasn't been
initialized yet, which the compiler can't warn me about...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds