Re: [RFC] TIF_NOTIFY_RESUME, arch/*/*/*signal*.c and all such

[Oleg Nesterov]

On 04/29, Al Viro wrote:
>
> On Sun, Apr 29, 2012 at 06:41:55PM +0200, Oleg Nesterov wrote:
> >
> > Besides, at least on x86 tracehook_signal_handler's logic is not exactly
> > right and should be fixed.
>
> Details, please...

See http://marc.info/?t=127550678000005 and
https://bugzilla.kernel.org/show_bug.cgi?id=16061

	- the tracee reports a signal SIG

	- the tracer does ptrace(SINGLESTEP, SIG), this approves the signal
	  and also sets TF

	- the tracee dequeues the signal, changes its IP to sig_handler().

	  then it notices TIF_SINGLESTEP and notifies the tracer without
	  return to user-space _and_ without clearing TF or TIF_SINGLESTEP

	- the tracer does ptrace(SINGLESTEP) again, but now enable_single_step()
	  looses TIF_FORCED_TF.

	- the tracer does ptrace(CONT), but user_disable_single_step() doesn't
	  clear TF since TIF_FORCED_TF is not set

	- the tracee returns to user-space with X86_EFLAGS_TF in eflags

Somehow we forgot about this bug...

Oleg.

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