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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


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