Re: [PATCH v2] btrfs: fix a potential hole-punching failure

[David Sterba]

On Thu, Mar 25, 2021 at 09:56:22AM +0800, bingjingc wrote:
> From: BingJing Chang <bingjingc@xxxxxxxxxxxx>
> 
> In commit d77815461f04 ("btrfs: Avoid trucating page or punching hole
> in a already existed hole."), existed holes can be skipped by calling
> find_first_non_hole() to adjust *start and *len. However, if the given
> len is invalid and large, when an EXTENT_MAP_HOLE extent is found, the
> *len will not be set to zero because (em->start + em->len) is less than
> (*start + *len). Then the ret will be 1 but the *len will not be set to
> 0. The propagated non-zero ret will result in fallocate failure.
> 
> In the while-loop of btrfs_replace_file_extents(), len is not updated
> every time before it calls find_first_non_hole(). That is, after
> btrfs_drop_extents() successfully drops the last non-hole file extent,
> it may fail with -ENOSPC when attempting to drop a file extent item
> representing a hole. The problem can happen. After it calls
> find_first_non_hole(), the cur_offset will be adjusted to be larger
> than or equal to end. However, since the len is not set to zero. The
> break-loop condition (ret && !len) will not meet. After it leaves the
> while-loop, fallocate will return 1, which is an unexpected return
> value.
> 
> We're not able to construct a reproducible way to let
> btrfs_drop_extents() fail with -ENOSPC after it drops the last non-hole
> file extent but with remaining holes left. However, it's quite easy to
> fix. We just need to update and check the len every time before we call
> find_first_non_hole(). To make the while loop more readable, we also
> pull the variable updates to the bottom of loop like this:
> while (cur_offset < end) {
>         ...
>         // update cur_offset & len
>         // advance cur_offset & len in hole-punching case if needed
> }
> 
> Reported-by: Robbie Ko <robbieko@xxxxxxxxxxxx>
> Fixes: d77815461f04 ("btrfs: Avoid trucating page or punching hole in a
> already existed hole.")
> Reviewed-by: Robbie Ko <robbieko@xxxxxxxxxxxx>
> Reviewed-by: Chung-Chiang Cheng <cccheng@xxxxxxxxxxxx>
> Signed-off-by: BingJing Chang <bingjingc@xxxxxxxxxxxx>

Thanks, added to misc-next.