Re: [PATCH v2] btrfs: reflink: Initialize return value in btrfs_extent_same()

[Sidong Yang <realwakka@gmail.com>]

On Mon, Aug 23, 2021 at 10:44:08AM +0100, Filipe Manana wrote:
> On Fri, Aug 20, 2021 at 1:42 AM Sidong Yang <realwakka@gmail.com> wrote:
> >
> > btrfs_extent_same() cannot be called with zero length. This patch add
> > code that initialize ret as -EINVAL to make it safe.
> 
> I suppose the motivation of the patch is to fix a warning from smatch,
> or other similar tools, about 'ret' not being initialized when olen is
> 0.

Yes, Actually I used smatch you said.

> Initializing 'ret' to some value surely makes the warning go away,
> even though it's not possible for olen to be 0 at btrfs_extent_same(),
> as that
> is filtered up in the call chain.
> 
> However setting to -EINVAL by default is confusing and counter
> intuitive because dedupe operations are supposed to return 0 (success)
> for a 0 length range.

Yeah, I think it depends on btrfs_extent_same()'s concept. It does
nothing when 0 length. It's okay if we consider it's normal operation
and it seems natural.

>
> So 'ret' should be initialized to 0 to avoid any confusion.

Agree. I want to know other people's thoughts.

Thanks,
Sidong
 
> Thanks.
> 
> >
> > Signed-off-by: Sidong Yang <realwakka@gmail.com>
> > ---
> > v2:
> >  - Removed assert and added initializing ret
> > ---
> >  fs/btrfs/reflink.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/fs/btrfs/reflink.c b/fs/btrfs/reflink.c
> > index 9b0814318e72..864f42198c5c 100644
> > --- a/fs/btrfs/reflink.c
> > +++ b/fs/btrfs/reflink.c
> > @@ -653,6 +653,7 @@ static int btrfs_extent_same(struct inode *src, u64 loff, u64 olen,
> >         u64 i, tail_len, chunk_count;
> >         struct btrfs_root *root_dst = BTRFS_I(dst)->root;
> >
> > +       ret = -EINVAL;
> >         spin_lock(&root_dst->root_item_lock);
> >         if (root_dst->send_in_progress) {
> >                 btrfs_warn_rl(root_dst->fs_info,
> > --
> > 2.25.1
> >
> 
> 
> -- 
> Filipe David Manana,
> 
> “Whether you think you can, or you think you can't — you're right.”