[bug report] staging: erofs: tidy up decompression frontend

[Gao Xiang]

On Tue, Aug 27, 2019 at 12:53:47PM +0300, Dan Carpenter wrote:
> On Tue, Aug 27, 2019 at 05:36:29PM +0800, Gao Xiang wrote:
> > Hi Dan,
> > 
> > Thanks for your report.
> > 
> > On Tue, Aug 27, 2019 at 12:03:55PM +0300, Dan Carpenter wrote:
> > > Hello Gao Xiang,
> > > 
> > > This is a semi-automatic email about new static checker warnings.
> > > 
> > > The patch 97e86a858bc3: "staging: erofs: tidy up decompression
> > > frontend" from Jul 31, 2019, leads to the following Smatch complaint:
> > > 
> > >     fs/erofs/zdata.c:670 z_erofs_do_read_page()
> > >     error: we previously assumed 'clt->cl' could be null (see line 596)
> > > 
> > > fs/erofs/zdata.c
> > >    595			/* didn't get a valid collection previously (very rare) */
> > >    596			if (!clt->cl)
> > >                             ^^^^^^^^
> > > New NULL check.
> > > 
> > >    597				goto restart_now;
> > >    598			goto hitted;
> > >    599		}
> > >    600	
> > >    601		/* go ahead the next map_blocks */
> > >    602		debugln("%s: [out-of-range] pos %llu", __func__, offset + cur);
> > >    603	
> > >    604		if (z_erofs_collector_end(clt))
> > >    605			fe->backmost = false;
> > >    606	
> > >    607		map->m_la = offset + cur;
> > >    608		map->m_llen = 0;
> > >    609		err = z_erofs_map_blocks_iter(inode, map, 0);
> > >    610		if (unlikely(err))
> > >    611			goto err_out;
> > >    612	
> > >    613	restart_now:
> > >    614		if (unlikely(!(map->m_flags & EROFS_MAP_MAPPED)))
> > >    615			goto hitted;
> > >    616	
> > >    617		err = z_erofs_collector_begin(clt, inode, map);
> > 
> > At a glance, clt->cl will be all initialized in all successful paths
> > in z_erofs_collector_begin, or it all fall back into err_out...
> > I have no idea what is wrong here...
> > 
> > Some detailed path from Smatch for NIL dereferences?
> > 
> 
> Ah.  Sorry for that.  It's a false positive.  I will investigate and
> fix Smatch.

Yeah.. I was little confused, since this patch mostly renames many names...
and the main logic is unchanged for months... and for this case there are 2 paths...

 1) hit line 614 --> goto hitted --> hit line 633 --> goto next_part; (will skip line 670);
 2) hit line 617 --> go into z_erofs_collector_begin -->
    all successful paths will assign clt->cl, so clt->cl != NULL...

Though z_erofs_do_read_page is currently somewhat complicated (mostly due to some
historical fixes in order to backport friendly), I will simplify this function
in the later version (..and with care in case of introducing new bugs) :-)

Thanks,
Gao Xiang

> 
> regards,
> dan carpenter
>