Re: BUG: bad usercopy in memdup_user

[Laura Abbott]

On 12/19/2017 01:36 PM, 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.
>
> So no, it won't work.
>
> When we find something like this, we should either remove it, fix the
> permissions, or switch to %px.
>
> In this case, there's obviously no permission issue: it's an error
> report. So it's either "remove it, or switch to %px".
>
> I'm personally not clear on whether the pointer really makes any sense
> at all. But if it does, it should just be changed to %px, since it's a
> bug report.
>
> But honestly, what do people expect that the pointer value will
> actually tell you if it is unhashed?
>
> I suspect that an "offset and size within the kernel object" value
> might make sense.  But what does the _pointer_ tell you?
>
> I've noticed this with pretty much every report. People get upset
> about the hashing, but don't seem to actually be able to ever tell
> what the f*ck they would use the non-hashed pointer value for.
>
> I've asked for this before: whenever somebody complains about the
> hashing, you had better tell exactly what the unhashed value would
> have given you, and how it would have helped debug the problem.
>
> Because if you can't tell that, then dammit, then we should just
> _remove_ the stupid %p.
>
> Instead, people ask for "can I get everything unhashed" even when they
> can't give a reason for it.
>
>              Linus

It's most useful in the "I really have no idea what's going on" case.
Sometimes just narrowing down if a pointer was kernel or vmalloc or
kmap or whatever is the only starting point. I agree that
size plus offset from object would be helpful.

Thanks,
Laura