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Date:   Tue, 3 Jan 2023 11:17:20 -0800
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Jun Nie <jun.nie@linaro.org>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca,
        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
        tudor.ambarus@linaro.org
Subject: Re: [PATCH] ext4: reject 1k block fs on the first block of disk
Message-ID: <Y7R/QKIbYQ2TCP+W@magnolia>
References: <20221229014502.2322727-1-jun.nie@linaro.org>
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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 filesystems")
> Link: https://syzkaller.appspot.com/bug?id=79d5768e9bfe362911ac1a5057a36fc6b5c30002
> 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 ext4_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 reject
> +		 * 1K fs on block 0 as fs should start block 1. */
> +		if (dkeys[0].fmr_physical ==0 && dkeys[1].fmr_physical == 0)
> +			continue;

...and if this filesystem has 4k blocks, and therefore *does* define a
block 0?

--D

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