Re: BUG: bad usercopy in memdup_user

[Matthew Wilcox]

On Tue, Dec 19, 2017 at 01:36:46PM -0800, Linus Torvalds wrote:
> On Tue, Dec 19, 2017 at 5:22 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > Could we have a way to know that the printed address is hashed and not just
> > a pointer getting completely scrogged?  Perhaps prefix it with ... a hash!
> > So this line would look like:
> 
> The problem with that is that it will break tools that parse things.

Yeah, but the problem is that until people know to expect hashes, it
breaks people.  I spent most of a day last week puzzling over a value
coming from a VM_BUG_ON that was explicitly tested for and couldn't
happen.

> When we find something like this, we should either remove it, fix the
> permissions, or switch to %px.

Right; I sent a patch to fix VM_BUG_ON earlier today after reading
this thread.

> But honestly, what do people expect that the pointer value will
> actually tell you if it is unhashed?

It would have been meaningful to me.  For a start, I would have seen
that the bottom two bits were clear, so this was actually a pointer and
not something masquerading as a pointer.