Re: [PATCH] xfs: do not call xfs_buf_hash_destroy on a NULL pag

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Jan 20, 2017 at 01:26:12PM -0600, Eric Sandeen wrote:
> On 1/20/17 8:26 AM, Colin King wrote:
> > From: Colin Ian King <colin.king@canonical.com>
> > 
> > If pag cannot be allocated, the current error exit path will trip
> > a null pointer deference error when calling xfs_buf_hash_destroy
> > with a null pag.  Fix this by adding a new error exit lable and
> > jumping to this, avoiding the hash destroy and unnecessary kmem_free
> > on pag.
> > 
> > Fixes CoverityScan CID#1397628 ("Dereference after null check")
> > 
> > Signed-off-by: Colin Ian King <colin.king@canonical.com>
> 
> Hm, I think this leaves the code with issues.
> 
> > ---
> >  fs/xfs/xfs_mount.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> > index 9b9540d..4e66cd19 100644
> > --- a/fs/xfs/xfs_mount.c
> > +++ b/fs/xfs/xfs_mount.c
> > @@ -207,7 +207,7 @@ xfs_initialize_perag(
> >  
> >  		pag = kmem_zalloc(sizeof(*pag), KM_MAYFAIL);
> >  		if (!pag)
> > -			goto out_unwind;
> > +			goto out_unwind_pags;
> 
> So let's say we got to index == 3 at the top of the loop, and
> this fails.
> 
> We succeeded in initializing 0, 1, and 2, but 3 failed.
> 
> So we go to out_unwind_pags with index == 3...
> 
> >  		pag->pag_agno = index;
> >  		pag->pag_mount = mp;
> >  		spin_lock_init(&pag->pag_ici_lock);
> > @@ -242,6 +242,7 @@ xfs_initialize_perag(
> >  out_unwind:
> >  	xfs_buf_hash_destroy(pag);
> >  	kmem_free(pag);
> > +out_unwind_pags:
> 
> ... where index == 3, and:
> 
> >  	for (; index > first_initialised; index--) {
> >  		pag = radix_tree_delete(&mp->m_perag_tree, index);
> 
> this should fail, because it never got inserted, and...
> 
> >  		xfs_buf_hash_destroy(pag);
> 
> this still tries to destroy a NULL pag, no?
> 
> There also seems to be an existing issue w/the code where ag 0 is
> never torn down in the error case, because first_initialized doesn't
> stay set to 0:
> 
>                 if (!first_initialised)
>                         first_initialised = index;
> 
> And we don't even tear down ag 1, because:
> 
> >  	for (; index > first_initialised; index--) {
> >  		pag = radix_tree_delete(&mp->m_perag_tree, index);
> 
> when the loop reaches the first initialized AG, it stops.
> 
> So we seem to always leak at least 2 if we managed to get far enough
> to initialize them.

Ugh, yeah, the the whole error exit from that function is fubar...
Anyone want to clean this up?

--D

> 
> -Eric
> 
> > 
> --
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