linux-kernel - Re: signals: Bug or manpage inconsistency?

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Date:   Tue, 30 May 2017 21:18:47 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man@...r.kernel.org, libc-alpha <libc-alpha@...rceware.org>
Subject: Re: signals: Bug or manpage inconsistency?

On 05/30, Linus Torvalds wrote:
>
> On Tue, May 30, 2017 at 10:04 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > I can't comment, I never tried to understand the rationality behind the current
> > behaviour. But at least the sending path should never drop a blocked SIG_DFL
> > signal, there is no other way to ensure you won't miss a signal during exec.
>
> Note that both SIG_DFL _and_ SIG_IGN are possible after exec,

Yes, if it was already ignored before exec. But ignoring the compatibility the
only important case is when it is SIG_DFL because of flush_signal_handlers().

> SIG_IGN doesn't mean "ignore signal forever". It means "ignore signals
> right now", and I think that our current signal blocking semantics are
> likely the correct ones,

I am not saying it is incorrect, but I agree with Thomas in that this
sigismember(t->blocked) in sig_ignored() doesn't look really nice.

> exactly because it means "when you start
> blocking signals, the kernel will not drop them".

if the process is singe-threaded or the signal is private, or it is blocked
by all threads. Otherwise it will wakeup another thread for no reason, the
signal will be dropped in get_signal().

And again, this doesn't look consistent with do_sigaction(). It even has a
comment which explains that we want to flush the ignored signals, blocked
or not.

Nevermind, I am not trying to argue, and

> So again, I really wouldn't want to change existing semantics unless
> there is a big real reason for it. Our current semantics are not
> wrong.

I certainly agree.

Oleg.