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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 18 Oct 2023 23:15:31 +0800 Baoquan He <bhe@redhat.com> wrote:

> From: Baoquan He <bhe@redhat.com>
> 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?