Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault

[Masami Hiramatsu]

On Sat, 23 Feb 2019 20:38:03 -0800
Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> On Sat, Feb 23, 2019 at 4:44 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > On Sat, 23 Feb 2019 12:47:46 +0900
> > Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >
> > > Since kprobes handler runs in IRQ context, we can not use access_ok() in it.
> > > (only on x86 + CONFIG_DEBUG_ATOMIC_SLEEP=y)
> >
> > Is it really IRQ context or exception context? That is, one
> > (interrupts) happen for any task, but exceptions happen because of the
> > software that is executed (like a breakpoint). Although you can have a
> > kprobe trigger in an interrupt handler (where user access wouldn't make
> > sense anyway). But there should be no problem with user access from an
> > exception handler.
> >
> 
> Can we just get rid of this might_sleep()?  access_ok() doesn't sleep
> as far as I know.

Hmm, which might_sleep() would you pointed? What I talked was a 
WARN_ON_ONCE(!in_task()) in access_ok() on x86 (only!), and in_task() just
checks preempt_count.

I guess PeterZ assumed that access_ok() is used only with user space access
APIs (e.g. copy_from_user) which can cause page-fault and locks mm (and might
sleep :)), but now we are trying to use access_ok() with new functions which
disables page-fault and just return -EFAULT.

Thank you,


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>