| Message ID | 1616637382-27311-1-git-send-email-bingjingc@synology.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] btrfs: fix a potential hole-punching failure | expand |
On Thu, Mar 25, 2021 at 3:42 AM bingjingc <bingjingc@synology.com> wrote: > > From: BingJing Chang <bingjingc@synology.com> > > 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@synology.com> > Fixes: d77815461f04 ("btrfs: Avoid trucating page or punching hole in a > already existed hole.") > Reviewed-by: Robbie Ko <robbieko@synology.com> > Reviewed-by: Chung-Chiang Cheng <cccheng@synology.com> > Signed-off-by: BingJing Chang <bingjingc@synology.com> Reviewed-by: Filipe Manana <fdmanana@suse.com> Looks good, thanks. > --- > fs/btrfs/file.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c > index 0e155f0..dccb017 100644 > --- a/fs/btrfs/file.c > +++ b/fs/btrfs/file.c > @@ -2735,8 +2735,6 @@ int btrfs_replace_file_extents(struct inode *inode, struct btrfs_path *path, > extent_info->file_offset += replace_len; > } > > - cur_offset = drop_args.drop_end; > - > ret = btrfs_update_inode(trans, root, BTRFS_I(inode)); > if (ret) > break; > @@ -2756,7 +2754,9 @@ int btrfs_replace_file_extents(struct inode *inode, struct btrfs_path *path, > BUG_ON(ret); /* shouldn't happen */ > trans->block_rsv = rsv; > > - if (!extent_info) { > + cur_offset = drop_args.drop_end; > + len = end - cur_offset; > + if (!extent_info && len) { > ret = find_first_non_hole(BTRFS_I(inode), &cur_offset, > &len); > if (unlikely(ret < 0)) > -- > 2.7.4 >
On Thu, Mar 25, 2021 at 09:56:22AM +0800, bingjingc wrote: > From: BingJing Chang <bingjingc@synology.com> > > 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@synology.com> > Fixes: d77815461f04 ("btrfs: Avoid trucating page or punching hole in a > already existed hole.") > Reviewed-by: Robbie Ko <robbieko@synology.com> > Reviewed-by: Chung-Chiang Cheng <cccheng@synology.com> > Signed-off-by: BingJing Chang <bingjingc@synology.com> Thanks, added to misc-next.
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index 0e155f0..dccb017 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -2735,8 +2735,6 @@ int btrfs_replace_file_extents(struct inode *inode, struct btrfs_path *path, extent_info->file_offset += replace_len; } - cur_offset = drop_args.drop_end; - ret = btrfs_update_inode(trans, root, BTRFS_I(inode)); if (ret) break; @@ -2756,7 +2754,9 @@ int btrfs_replace_file_extents(struct inode *inode, struct btrfs_path *path, BUG_ON(ret); /* shouldn't happen */ trans->block_rsv = rsv; - if (!extent_info) { + cur_offset = drop_args.drop_end; + len = end - cur_offset; + if (!extent_info && len) { ret = find_first_non_hole(BTRFS_I(inode), &cur_offset, &len); if (unlikely(ret < 0))