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SubjectRe: [BUG] ptraced process waiting on syscall may return kernel internal errnos
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.

Also, whith this change do_sigaction()->recalc_sigpending_and_wake() doesn't
make sense any longer, yes?

> @@ -385,7 +391,8 @@ int dequeue_signal(struct task_struct *t
> }
> }
> }
> - recalc_sigpending_tsk(tsk);
> + if (likely(tsk == current))
> + recalc_sigpending();

In theory, flush_signals(t) needs a similar change. However, it is always
called with t == current. Perhaps it makes sense to make it flush_signals(void) ?
Do you see any valid usage of flush_signals(t) when t != current ?

(Actually, imho the same is true for dequeue_signal(). Except for signalfd.c
dequeue_signal() should operate on current. Perhaps it would be a bit cleaner
to have dequeue_signal_tsk(tsk) and dequeue_signal(void), the latter does
recalc_sigpending).

Oleg.

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