Re: mm/vmalloc.c:3689 vread_iter() error: we previously assumed 'vm' could be null (see line 3667)

[Baoquan He]

On 10/18/23 at 08:52am, Andrew Morton wrote:
> On Wed, 18 Oct 2023 23:15:31 +0800 Baoquan He <bhe@xxxxxxxxxx> wrote:
> 
> > From: Baoquan He <bhe@xxxxxxxxxx>
> > Date: Wed, 18 Oct 2023 22:50:14 +0800
> > Subject: [PATCH] mm/vmalloc: fix the unchecked dereference warning in vread_iter()
> > Content-type: text/plain
> > 
> > LKP reported smatch warning as below:
> > 
> > ===================
> > smatch warnings:
> > mm/vmalloc.c:3689 vread_iter() error: we previously assumed 'vm' could be null (see line 3667)
> > ......
> > 06c8994626d1b7  @3667 size = vm ? get_vm_area_size(vm) : va_size(va);
> > ......
> > 06c8994626d1b7  @3689 else if (!(vm->flags & VM_IOREMAP))
> >                                  ^^^^^^^^^
> > Unchecked dereference
> > =====================
> > 
> > So add checking on whether 'vm' is not null when dereferencing it in
> > vread_iter(). This mutes smatch complaint.
> > 
> > ...
> >
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3813,7 +3813,7 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
> >  
> >  		if (flags & VMAP_RAM)
> >  			copied = vmap_ram_vread_iter(iter, addr, n, flags);
> > -		else if (!(vm->flags & VM_IOREMAP))
> > +		else if (!(vm && (vm->flags & VM_IOREMAP)))
> >  			copied = aligned_vread_iter(iter, addr, n);
> >  		else /* IOREMAP area is treated as memory hole */
> >  			copied = zero_iter(iter, n);
> 
> So is this not a real runtime bug?  We're only doing this to suppress a
> smatch warning?
> 
> If so, can we please include a description of *why* this wasn't a bug? 
> What conditions ensure that vm!=NULL at this point?

I think this is not a real runtime bug. The only chance it can hapen is
when (flags == VMAP_BLOCK) is true. That has been warned and could never
happen. I updated patch log and paste v2 here. 

                /*
                 * VMAP_BLOCK indicates a sub-type of vm_map_ram area, need
                 * be set together with VMAP_RAM.
                 */
                WARN_ON(flags == VMAP_BLOCK);
 
                if (!vm && !flags)
                        continue;