Messages in this thread Patch in this message | | | Date | Fri, 21 Jul 2023 15:29:35 +0800 | Subject | Re: [PATCH 2/4] ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow | From | Baokun Li <> |
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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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