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SubjectRe: signals: Bug or manpage inconsistency?
On Tue, 30 May 2017, Thomas Gleixner wrote:
> The commit which added the queuing of blocked and ignored signals is in the
> history tree with a pretty useless changelog.
>
> https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
>
> commit 98fc8ab9e74389e0c7001052597f61336dc62833
> Author: Linus Torvalds <torvalds@penguin.transmeta.com>
> Date: Tue Feb 11 20:49:03 2003 -0800
>
> Don't wake up processes unnecessarily for ignored signals
>
> It rewrites sig_ignored() and adds the following to it:
>
> + /*
> + * Blocked signals are never ignored, since the
> + * signal handler may change by the time it is
> + * unblocked.
> + */
> + if (sigismember(&t->blocked, sig))
> + return 0;
>
> I have no idea how that is related to $subject of the commit and why this
> decision was made.
>
> Linus, any recollection?

So I found at least some explanation by studying the spec some more.

There are two variants of ignored signals:

1) handler is SIG_IGN

2) handler is SIG_DFL and default action is 'ignore'

These are the signals in SIG_KERNEL_IGNORE_MASK

#define SIG_KERNEL_IGNORE_MASK (\
rt_sigmask(SIGCONT) | rt_sigmask(SIGCHLD) | \
rt_sigmask(SIGWINCH) | rt_sigmask(SIGURG) )

These signals are not allowed to be discarded when the signal is
blocked.

So my understanding of the spec is:

#1 Can discard the signals as long as SIG_IGN is set whether the signal
is blocked or not

#2 Must queue them if the signal is blocked, otherwise discard

I changed the logic according to this with the patch below and a quick test
run of lpt and glibc test cases produces no failures.

Thoughts?

Thanks,

tglx

8<--------------------
Subject: signals: Reduce scope of blocked signals in sig_handler_ignored()
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 30 May 2017 18:01:33 +0200

Add proper changelog and a big fat comment in the code.

Not-yet-signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
kernel/signal.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

Index: b/kernel/signal.c
===================================================================
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -70,6 +70,13 @@ static int sig_handler_ignored(void __us
(handler == SIG_DFL && sig_kernel_ignore(sig));
}

+static int sig_handler_is_sigign(struct task_struct *t, int sig)
+{
+ void __user *handler = sig_handler(t, sig);
+
+ return handler == SIG_IGN;
+}
+
static int sig_task_ignored(struct task_struct *t, int sig, bool force)
{
void __user *handler;
@@ -91,7 +98,7 @@ static int sig_ignored(struct task_struc
* unblocked.
*/
if (sigismember(&t->blocked, sig) || sigismember(&t->real_blocked, sig))
- return 0;
+ return sig_handler_is_sigign(t, sig);

if (!sig_task_ignored(t, sig, force))
return 0;
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