linux-kernel - Re: [PATCH] ptrace: add ability to retrieve signals without removing them from a queue

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Date:	Tue, 19 Feb 2013 20:34:24 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	mtk.manpages@...il.com, Andrey Vagin <avagin@...nvz.org>,
	linux-kernel@...r.kernel.org, criu@...nvz.org,
	linux-api@...r.kernel.org, Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] ptrace: add ability to retrieve signals without
	removing them from a queue

On 02/19, Pavel Emelyanov wrote:
>
> On 02/19/2013 04:15 PM, Michael Kerrisk (man-pages) wrote:
> > On Wed, Feb 13, 2013 at 4:16 PM, Andrey Vagin <avagin@...nvz.org> wrote:
> >> This patch adds a new ptrace request PTRACE_PEEKSIGINFO.
> >>
> >> This request is used to retrieve information about a signal with the
> >> specified sequence number. A siginfo_t structure is copied from the child
> >> to location data in the parent.
> >>
> >> The low 16 bits of addr contains a sequence number of signal in a queue:
> >
> > I think 16 bits is probably not enough.... Already, on the "out of the
> > box" system that I have at hand, one can queue more than 2^16-1
> > signals:
> >
> >     $ cat /proc/$$/status | grep SigQ
> >     SigQ:	2/126065
>
> Yup :( Well, actually it would be enough to have only 1 bit as "flags"
> and the rest (31 at least) for the number. But taking into account
> Oleg's wish to have an ability to extend the amount of flags

I am not sure this is really needed, and we can add more PTRACE_PEEK_
codes later. I am fine either way, we can even add PEEK_PRIVATE/SHARED
right now.

But, given that every PEEK does list_for_each() until it finds the
necessary sequence number, I am wondering how this O(n**2) will work
if you want to dump 126065 signals ;)

Oleg.

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