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References: <20221229014502.2322727-1-jun.nie@linaro.org> <Y7R/QKIbYQ2TCP+W@magnolia>
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From:   Jun Nie <jun.nie@linaro.org>
Date:   Wed, 4 Jan 2023 09:58:03 +0800
Message-ID: <CABymUCPCT9KbMQDUTxwf6A+Cg9fWJNkefbMHD7SZD3Fc7FMFHg@mail.gmail.com>
Subject: Re: [PATCH] ext4: reject 1k block fs on the first block of disk
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca,
        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
        tudor.ambarus@linaro.org
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Darrick J. Wong <djwong@kernel.org> =E4=BA=8E2023=E5=B9=B41=E6=9C=884=E6=97=
=A5=E5=91=A8=E4=B8=89 03:17=E5=86=99=E9=81=93=EF=BC=9A
>
> On Thu, Dec 29, 2022 at 09:45:02AM +0800, Jun Nie wrote:
> > For 1k-block filesystems, the filesystem starts at block 1, not block 0=
.
> > If start_fsb is 0, it will be bump up to s_first_data_block. Then
> > ext4_get_group_no_and_offset don't know what to do and return garbage
> > results (blockgroup 2^32-1). The underflow make index
> > exceed es->s_groups_count in ext4_get_group_info() and trigger the BUG_=
ON.
> >
> > Fixes: 4a4956249dac0 ("ext4: fix off-by-one fsmap error on 1k block fil=
esystems")
> > Link: https://syzkaller.appspot.com/bug?id=3D79d5768e9bfe362911ac1a5057=
a36fc6b5c30002
> > Reported-by: syzbot+6be2b977c89f79b6b153@syzkaller.appspotmail.com
> > Signed-off-by: Jun Nie <jun.nie@linaro.org>
> > ---
> >  fs/ext4/fsmap.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/fs/ext4/fsmap.c b/fs/ext4/fsmap.c
> > index 4493ef0c715e..1aef127b0634 100644
> > --- a/fs/ext4/fsmap.c
> > +++ b/fs/ext4/fsmap.c
> > @@ -702,6 +702,12 @@ int ext4_getfsmap(struct super_block *sb, struct e=
xt4_fsmap_head *head,
> >               if (handlers[i].gfd_dev > head->fmh_keys[0].fmr_device)
> >                       memset(&dkeys[0], 0, sizeof(struct ext4_fsmap));
> >
> > +             /*
> > +              * Re-check the range after above limit operation and rej=
ect
> > +              * 1K fs on block 0 as fs should start block 1. */
> > +             if (dkeys[0].fmr_physical =3D=3D0 && dkeys[1].fmr_physica=
l =3D=3D 0)
> > +                     continue;
>
> ...and if this filesystem has 4k blocks, and therefore *does* define a
> block 0?

Yes, this is a real corner case test :-)

>
> --D
>
> > +
> >               info.gfi_dev =3D handlers[i].gfd_dev;
> >               info.gfi_last =3D false;
> >               info.gfi_agno =3D -1;
> > --
> > 2.34.1
> >