Re: [PATCH v4 2/3] virtiofsd: optionally return inode pointer from lo_do_lookup()

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Feb 03, 2021 at 03:20:14PM +0100, Greg Kurz wrote:
> On Wed,  3 Feb 2021 11:37:18 +0000
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> > lo_do_lookup() finds an existing inode or allocates a new one. It
> > increments nlookup so that the inode stays alive until the client
> > releases it.
> > 
> > Existing callers don't need the struct lo_inode so the function doesn't
> > return it. Extend the function to optionally return the inode. The next
> > commit will need it.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  tools/virtiofsd/passthrough_ll.c | 29 +++++++++++++++++++++--------
> >  1 file changed, 21 insertions(+), 8 deletions(-)
> > 
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > index e63cbd3fb7..c87a1f3d72 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -831,11 +831,13 @@ static int do_statx(struct lo_data *lo, int dirfd, const char *pathname,
> >  }
> >  
> >  /*
> > - * Increments nlookup and caller must release refcount using
> > - * lo_inode_put(&parent).
> > + * Increments nlookup on the inode on success. unref_inode_lolocked() must be
> > + * called eventually to decrement nlookup again. If inodep is non-NULL, the
> > + * inode pointer is stored and the caller must call lo_inode_put().
> >   */
> >  static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
> > -                        struct fuse_entry_param *e)
> > +                        struct fuse_entry_param *e,
> > +                        struct lo_inode **inodep)
> >  {
> >      int newfd;
> >      int res;
> > @@ -845,6 +847,10 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
> >      struct lo_inode *inode = NULL;
> >      struct lo_inode *dir = lo_inode(req, parent);
> >  
> > +    if (inodep) {
> > +        *inodep = NULL;
> > +    }
> > +
> 
> Is this side-effect needed ? If lo_do_lookup() returns an error, it
> rather seems that the caller shouldn't expect anything to be written
> here, i.e. the content of *inodep still belongs to the caller and
> whatever value it previously put in there (as patch 3/3 does) should
> be preserved IMHO.
> 
> Apart from that LGTM.

I like this approach because it prevents accessing uninitialized memory
in the caller:

  struct lo_inode *inode;

  if (lo_do_lookup(..., &inodep) != 0) {
    goto err;
  }
  ...

  err:
  lo_inode_put(&inode); <-- uninitialized in the error case!

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