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SubjectRe: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious page faults

* Masami Hiramatsu <mhiramat@redhat.com> wrote:

> - if (notify_page_fault(regs))
> - return;
> if (unlikely(kmmio_fault(regs, address)))
> return;
>
> @@ -634,6 +632,9 @@ void __kprobes do_page_fault(struct pt_r
> if (spurious_fault(address, error_code))
> return;
>
> + /* kprobes don't want to hook the spurious faults. */
> + if (notify_page_fault(regs))
> + return;
> /*
> * Don't take the mm semaphore here. If we fixup a prefetch
> * fault we could otherwise deadlock.
> @@ -641,6 +642,9 @@ void __kprobes do_page_fault(struct pt_r
> goto bad_area_nosemaphore;
> }
>
> + /* kprobes don't want to hook the spurious faults. */
> + if (notify_page_fault(regs))
> + return;

I dont know - this spreads that callback to two places now. Any
reason why kprobes cannot call spurious_fault(), if there's a
probe active?

Also, moving that would remove the planned cleanup of merging these
two into one call:

if (notify_page_fault(regs))
return;
if (unlikely(kmmio_fault(regs, address)))
return;

We should reduce the probing cross section, not increase it,
especially in such a critical codepath as the pagefault handler.

Btw., why cannot kprobes install a dynamic probe to the fault
handler itself? That way the default path would have no such
callbacks and checks at all.

Ingo


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