Date:   Fri, 21 Jul 2023 15:29:35 +0800
From:   Baokun Li <libaokun1@...wei.com>
To:     "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
        <linux-ext4@...r.kernel.org>
CC:     <tytso@....edu>, <adilger.kernel@...ger.ca>, <jack@...e.cz>,
        <ojaswin@...ux.ibm.com>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>, <yangerkun@...wei.com>,
        <yukuai3@...wei.com>, Baokun Li <libaokun1@...wei.com>
Subject: Re: [PATCH 2/4] ext4: fix BUG in ext4_mb_new_inode_pa() due to
 overflow

On 2023/7/21 3:30, Ritesh Harjani (IBM) wrote:
>
>>> I would like to carefully review all such paths. I will soon review and
>>> get back.
>> Okay, thank you very much for your careful review.
>> The 2nd and 3rd cases of adjusting the best extent are impossible to
>> overflow,
>> so only the first case is converted here.
> I noticed them too during review. I think it would be safe to make the
> changes in other two places as well such that in future we never
> trip over such overlooked overflow bugs.
>
>>>
>>>> +		BUG_ON(new_bex_end >
>>>> +			fex_end(sbi, &ac->ac_g_ex, &ac->ac_orig_goal_len));
>>> I am not sure whether using fex_end or pa_end is any helpful.
>>> I think we can just typecast if needed and keep it simple rather
>>> than adding helpers functions for addition operation.
>>> (because of the fact that fex_end() can take a third parameter which
>>> sometimes you pass as NULL. Hence it doesn't look clean, IMO)
>> I added the helper functions here for two reasons:
>> 1. restricting the type of the return value.
>> 2. This avoids the ugly line breaks in most cases.
>>
>> The fex_end() indeed doesn't look as clean as the pa_end(), because we
>> might use
>> the start of the free extent plus some other length to get a new end,
>> like right in
>> ext4_mb_new_inode_pa(), which makes me have to add another extra length
>> argument, but I think it's worth it, and even with the addition of a
>> parameter
>> that will probably be unused, it still looks a lot shorter than the
>> original code.
> IMO, we don't need pa_end() and fex_end() at all. In several places in
> ext4 we always have taken care by directly typecasting to avoid
> overflows. Also it reads much simpler rather to typecast in place than
> having a helper function which is also not very elegant due to a third
> parameter. Hence I think we should drop those helpers.
I still think helper is useful, but my previous thinking is problematic. 
I shouldn't
make fex_end() adapt to ext4_mb_new_inode_pa(), but should make
ext4_mb_new_inode_pa() adapt to fex_end(). After dropping the third argument
of fex_end(), modify the ext4_mb_new_inode_pa() function as follows:


diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a2475b8c9fb5..7492ba9062f4 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -5072,8 +5072,11 @@ ext4_mb_new_inode_pa(struct 
ext4_allocation_context *ac)
         pa = ac->ac_pa;

         if (ac->ac_b_ex.fe_len < ac->ac_orig_goal_len) {
-               int new_bex_start;
-               int new_bex_end;
+               struct ext4_free_extent ex = {
+                       .fe_logical = ac->ac_g_ex.fe_logical;
+                       .fe_len = ac->ac_orig_goal_len;
+               }
+               loff_t orig_goal_end = fex_end(sbi, &ex);

                 /* we can't allocate as much as normalizer wants.
                  * so, found space must get proper lstart
@@ -5092,29 +5095,23 @@ ext4_mb_new_inode_pa(struct 
ext4_allocation_context *ac)
                  *    still cover original start
                  * 3. Else, keep the best ex at start of original request.
                  */
-               new_bex_end = ac->ac_g_ex.fe_logical +
-                       EXT4_C2B(sbi, ac->ac_orig_goal_len);
-               new_bex_start = new_bex_end - EXT4_C2B(sbi, 
ac->ac_b_ex.fe_len);
-               if (ac->ac_o_ex.fe_logical >= new_bex_start)
-                       goto adjust_bex;
+               ex.fe_len = ac->ac_b_ex.fe_len;

-               new_bex_start = ac->ac_g_ex.fe_logical;
-               new_bex_end =
-                       new_bex_start + EXT4_C2B(sbi, ac->ac_b_ex.fe_len);
-               if (ac->ac_o_ex.fe_logical < new_bex_end)
+               ex.fe_logical = orig_goal_end - EXT4_C2B(sbi, ex.fe_len);
+               if (ac->ac_o_ex.fe_logical >= ex.fe_logical)
                         goto adjust_bex;

-               new_bex_start = ac->ac_o_ex.fe_logical;
-               new_bex_end =
-                       new_bex_start + EXT4_C2B(sbi, ac->ac_b_ex.fe_len);
+               ex.fe_logical = ac->ac_g_ex.fe_logical;
+               if (ac->ac_o_ex.fe_logical < fex_end(sbi, &ex))
+                       goto adjust_bex;

+               ex.fe_logical = ac->ac_o_ex.fe_logical;
  adjust_bex:
-               ac->ac_b_ex.fe_logical = new_bex_start;
+               ac->ac_b_ex.fe_logical = ex.fe_logical;

                 BUG_ON(ac->ac_o_ex.fe_logical < ac->ac_b_ex.fe_logical);
                 BUG_ON(ac->ac_o_ex.fe_len > ac->ac_b_ex.fe_len);
-               BUG_ON(new_bex_end > (ac->ac_g_ex.fe_logical +
-                                     EXT4_C2B(sbi, ac->ac_orig_goal_len)));
+               BUG_ON(fex_end(sbi, &ex) > orig_goal_end);
         }

         pa->pa_lstart = ac->ac_b_ex.fe_logical;


What do you think of this modification?

> Thanks once again for catching the overflows and coming up with a
> easy reproducer. I am surprised that this bug was never caught with LTP,
> fstests, smatch static checker.
> How did you find it? :)
>
> -ritesh
This problem is found in the internal test.

Thank you for your review!
-- 
With Best Regards,
Baokun Li
.

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