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

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Message-ID: <CAEivzxdnAt3WbVmMLpb+HCBSrwkX6vesMvK3onc+Zc9wzv1EtA@mail.gmail.com>
Date: Thu, 26 Sep 2024 11:23:24 +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 10:50 AM Baokun Li <libaokun1@...wei.com> wrote:
>
> On 2024/9/26 0:17, Aleksandr Mikhalitsyn wrote:
> > On Wed, Sep 25, 2024 at 5:57 PM Jan Kara <jack@...e.cz> 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
> > Hi Jan,
> >
> > First of all, thanks for your reaction/review on this one ;-)
> >
> > You are absolutely right, have just checked with our reproducer and
> > this modification:
> >
> > diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> > index e04eb08b9060..530a918f0cab 100644
> > --- a/fs/ext4/resize.c
> > +++ b/fs/ext4/resize.c
> > @@ -258,6 +258,8 @@ static struct ext4_new_flex_group_data
> > *alloc_flex_gd(unsigned int flexbg_size,
> >                  flex_gd->resize_bg = 1 << max(fls(last_group - o_group + 1),
> >                                                fls(n_group - last_group));
> >
> > +       BUG_ON(flex_gd->resize_bg > flexbg_size);
> > +
> >          flex_gd->groups = kmalloc_array(flex_gd->resize_bg,
> >                                          sizeof(struct ext4_new_group_data),
> >                                          GFP_NOFS);
> >
> > and yes, it crashes on this BUG_ON. So it looks like instead of making
> > flex_gd->resize_bg to be smaller
> > than flexbg_size in most cases we can actually have an opposite effect
> > here. I guess we really need to fix alloc_flex_gd() too.
> >
> >> 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?
> > At the same time, if I understand the code right, as we can have
> > flex_gd->resize_bg != flexbg_size after
> > 5d1935ac02ca5a ("ext4: avoid online resizing failures due to oversized
> > flex bg") and
> > 665d3e0af4d3 ("ext4: reduce unnecessary memory allocation in alloc_flex_gd()")
> > we should always refer to flex_gd->resize_bg value which means that
> > ext4_alloc_group_tables() fix is needed too.
> > Am I correct in my understanding?
>
> Hi Alex,

Hi Baokun,

>
> These two are not exactly equivalent.
>
> The flex_gd->resize_bg is only used to determine how many block groups we
> allocate memory to, i.e., the maximum number of block groups per resize.
> And the flexbg_size is used to make some judgement on flexible block
> groups, for example, the BUG_ON triggered in the issue is to make sure
> src_group and last_group must be in the same flexible block group.

Huge thanks for explaining this!

Then I guess it's better if you send a patch with your fix.
Feel free to add my Tested-by tag.

Question to you and Jan. Do you guys think that it makes sense to try
to create a minimal reproducer for this problem without Incus/LXD involved?
(only e2fsprogs, lvm tools, etc)

I guess this test can be put in the xfstests test suite, right?

Kind regards,
Alex

>
>
> Regards,
> Baokun
>