Re: [PATCH 2/4] ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow

[Baokun Li]

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
.