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SubjectRe: [BUG] ptraced process waiting on syscall may return kernel internal errnos
Date
Sorry for being late, I've just realized that you are discussing the freezer
here. ;-)

On Wednesday, 13 June 2007 17:15, Oleg Nesterov wrote:
> Sorry for delay, I was completely offline,
>
> On 06/06, Roland McGrath wrote:
> >
> > [PATCH] Restrict clearing TIF_SIGPENDING
> >
> > This patch should get a few birds. It prevents sigaction calls from
> > clearing TIF_SIGPENDING in other threads, which could leak -ERESTART*.
> > It fixes ptrace_stop not to clear it, which done at the syscall exit
> > stop could leak -ERESTART*. It probably removes the harm from
> > signalfd, at least assuming it never calls dequeue_signal on kernel
> > threads that might have used block_all_signals.
> >
> > Signed-off-by: Roland McGrath <roland@redhat.com>
> > ---
> > kernel/signal.c | 40 ++++++++++++++++++++++++----------------
> > 1 files changed, 24 insertions(+), 16 deletions(-)
> >
> > diff --git a/kernel/signal.c b/kernel/signal.c
> > index acdfc05..dc5797c 100644
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -105,7 +105,11 @@ static int recalc_sigpending_tsk(struct
> > set_tsk_thread_flag(t, TIF_SIGPENDING);
> > return 1;
> > }
> > - clear_tsk_thread_flag(t, TIF_SIGPENDING);
> > + /*
> > + * We must never clear the flag in another thread, or in current
> > + * when it's possible the current syscall is returning -ERESTART*.
> > + * So we don't clear it here, and only callers who know they should do.
> > + */
> > return 0;
> > }
>
> This breaks cancel_freezing(). Somehow we should clear TIF_SIGPENDING for
> kernel threads. Otherwise we may have subtle failures if try_to_freeze_tasks()
> fails.

Well, the only code path in which we'd want to call cancel_freezing() for kernel
threads is when the freezing of kernel threads. However, this only happens if
one of the kernel threads declares itself as freezable and the fails to call
try_to_freeze(), which is a bug. Thus I don't think that we need to worry
about that case too much.

Moreover, I'm not sure that it's a good idea at all to send signals to kernel
threads from the freezer, since in fact we only need to wake them up to make
them call try_to_freeze() (after we've set TIF_FREEZE for them).

Greetings,
Rafael


--
"Premature optimization is the root of all evil." - Donald Knuth
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