Re: [PATCH 1/1] ext4: fix crash on BUG_ON in ext4_alloc_group_tables

[Baokun Li <libaokun1@huawei.com>]

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,

Could you help test if the following changes work?


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),