On Thu, Oct 19, 2023 at 10:28:21AM +0800, Baoquan He wrote: > 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; > > Thanks. If you want you could just ignore the warning. It's a one time warning so we won't send the mail again and if people have questions about it, they can just look it up on lore. The truth is when I was reviewing this code the first time I got mixed up between flags and vm->flags so that's part of why I reported it. Smatch ignores the WARN_ON(). Historically WARN_ON() has been useless for indicating whether something can happen or not. These days, WARN_ON() is treated as a syzkaller bug so we prefer pr_warn() if something can actually happen. We still see a lot of WARN_ON()s happening in real life so I'm not eager to make Smatch treat them like a BUG_ON(). Also, sadly, even if we changed the WARN_ON() to a BUG_ON() it still wouldn't silence the warning because Smatch is not quite clever enough to parse that. regards, dan carpenter
