linux-kernel - Re: [PATCH 1/1] ext4: fix crash on BUG_ON in ext4_alloc_group

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Message-ID: <CAEivzxc-b-QDx8AEdHEwa06Q2TYgZZkw2PWQ+K_Lyf+oyTM1Zg@mail.gmail.com>
Date: Thu, 26 Sep 2024 11:16:07 +0200
From: Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: Baokun Li <libaokun1@...wei.com>
Cc: Jan Kara <jack@...e.cz>, tytso@....edu, stable@...r.kernel.org, 
	Andreas Dilger <adilger.kernel@...ger.ca>, Stéphane Graber <stgraber@...raber.org>, 
	Christian Brauner <brauner@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, 
	Wesley Hershberger <wesley.hershberger@...onical.com>, Yang Erkun <yangerkun@...wei.com>
Subject: Re: [PATCH 1/1] ext4: fix crash on BUG_ON in ext4_alloc_group_tables

On Thu, Sep 26, 2024 at 10:29 AM Baokun Li <libaokun1@...wei.com> wrote:
>
> On 2024/9/25 23:57, Jan Kara wrote:
> > On Wed 25-09-24 16:33:24, Alexander Mikhalitsyn wrote:
> >> [   33.882936] EXT4-fs (dm-5): mounted filesystem 8aaf41b2-6ac0-4fa8-b92b-77d10e1d16ca r/w with ordered data mode. Quota mode: none.
> >> [   33.888365] EXT4-fs (dm-5): resizing filesystem from 7168 to 786432 blocks
> >> [   33.888740] ------------[ cut here ]------------
> >> [   33.888742] kernel BUG at fs/ext4/resize.c:324!
> > Ah, I was staring at this for a while before I understood what's going on
> > (it would be great to explain this in the changelog BTW).  As far as I
> > understand commit 665d3e0af4d3 ("ext4: reduce unnecessary memory allocation
> > in alloc_flex_gd()") can actually make flex_gd->resize_bg larger than
> > flexbg_size (for example when ogroup = flexbg_size, ngroup = 2*flexbg_size
> > - 1) which then confuses things. I think that was not really intended and
> > instead of fixing up ext4_alloc_group_tables() we should really change
> > the logic in alloc_flex_gd() to make sure flex_gd->resize_bg never exceeds
> > flexbg size. Baokun?
> >
> >                                                               Honza
>
> Hi Honza,
>
> Your analysis is absolutely correct. It's a bug!
> Thank you for locating this issue！
> An extra 1 should not be added when calculating resize_bg in
> alloc_flex_gd().
>
>
> Hi Aleksandr,

Hi Baokun,

>
> Could you help test if the following changes work?

I can confirm that this patch helps.

Tested-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>

Kind regards,
Alex

>
>
> Thanks,
> Baokun
>
> ---
>
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index e04eb08b9060..1f01a7632149 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -253,10 +253,12 @@ static struct ext4_new_flex_group_data
> *alloc_flex_gd(unsigned int flexbg_size,
>          /* Avoid allocating large 'groups' array if not needed */
>          last_group = o_group | (flex_gd->resize_bg - 1);
>          if (n_group <= last_group)
> -               flex_gd->resize_bg = 1 << fls(n_group - o_group + 1);
> +               flex_gd->resize_bg = 1 << fls(n_group - o_group);
>          else if (n_group - last_group < flex_gd->resize_bg)
> -               flex_gd->resize_bg = 1 << max(fls(last_group - o_group + 1),
> +               flex_gd->resize_bg = 1 << max(fls(last_group - o_group),
>                                                fls(n_group - last_group));
>
>          flex_gd->groups = kmalloc_array(flex_gd->resize_bg,
>                                          sizeof(struct ext4_new_group_data),
>