Re: [PATCH v2 4/6] iommufd: Deliver fault messages to user space

[Jason Gunthorpe]

On Thu, Dec 07, 2023 at 05:34:10PM +0100, Joel Granados wrote:
> > @@ -58,6 +255,8 @@ static void hw_pagetable_fault_free(struct hw_pgtable_fault *fault)
> >  	WARN_ON(!list_empty(&fault->deliver));
> >  	WARN_ON(!list_empty(&fault->response));
> >  
> > +	fput(fault->fault_file);
> > +	put_unused_fd(fault->fault_fd);

> I have resolved this in a naive way by just not calling the
> put_unused_fd function.

That is correct.

put_unused_fd() should only be called on error paths prior to the
syscall return.

The design of a FD must follow this pattern

 syscall():
   fdno = get_unused_fd_flags(O_CLOEXEC);
   filep = [..]
 
   // syscall MUST succeed after this statement:
   fd_install(fdno, filep);
   return 0;

  err:
    put_unused_fd(fdno)
    return -ERRNO

Also the refcounting looks a little strange, the filep reference is
consumed by fd_install, so what is that fput pairing with in fault_free?

Jason