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

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Message-ID: <CAEivzxekNfuGw_aK2yq91OpzJfhg_RDDWO2Onm6kZ-ioh3GaUg@mail.gmail.com>
Date: Thu, 26 Sep 2024 13:32:20 +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 11:40 AM Baokun Li <libaokun1@...wei.com> wrote:
>
> On 2024/9/26 17:23, Aleksandr Mikhalitsyn wrote:
> > 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.
> Okay, I'll send a patch later.
> >
> > 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
> I think it makes sense, and it's good to have more use cases to look
> around some corners. If you have an idea, let it go.

Minimal reproducer:

mkdir -p /tmp/ext4_crash/mnt
EXT4_CRASH_IMG="/tmp/ext4_crash/disk.img"
rm -f $EXT4_CRASH_IMG
truncate $EXT4_CRASH_IMG --size 25MiB
EXT4_CRASH_DEV=$(losetup --find --nooverlap --direct-io=on --show
$EXT4_CRASH_IMG)
mkfs.ext4 -E nodiscard,lazy_itable_init=0,lazy_journal_init=0 $EXT4_CRASH_DEV
mount $EXT4_CRASH_DEV /tmp/ext4_crash/mnt
truncate $EXT4_CRASH_IMG --size 3GiB
losetup -c $EXT4_CRASH_DEV
resize2fs $EXT4_CRASH_DEV

>
> Regards, Baokun
>