linux-kernel - Re: [PATCH v3 6/8] x86: Split syscall_trace

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140729173136.GA2808@redhat.com>
Date:	Tue, 29 Jul 2014 19:31:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 6/8] x86: Split syscall_trace_enter into two phases

On 07/29, Andy Lutomirski wrote:
>
> On Tue, Jul 29, 2014 at 9:54 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > Yes, just to trigger the slow path, I guess.
> >
> >> I'll update the code to call user_exit iff TIF_NOHZ is
> >> set.
> >
> > Or perhaps it would be better to not add another user of this (strange) flag
> > and just call user_exit() unconditionally(). But, yes, you need to use
> > from "work = flags & (_TIF_WORK_SYSCALL_ENTRY & ~TIF_NOHZ)" then.\
>
> user_exit looks slow enough to me that a branch to try to avoid it may
> be worthwhile.  I bet that explicitly checking the flag is
> actually both faster and clearer.

I don't think so (unless I am confused again), note that user_exit() uses
jump label. But this doesn't matter. I meant that we should avoid TIF_NOHZ
if possible because I think it should die somehow (currently I do not know
how ;). And because it is ugly to check the same condition twice:

	if (work & TIF_NOHZ) {
		// user_exit()
		if (context_tracking_is_enabled())
			context_tracking_user_exit();
	}

TIF_NOHZ is set if and only if context_tracking_is_enabled() is true.
So I think that

	work = current_thread_info()->flags & (_TIF_WORK_SYSCALL_ENTRY & ~TIF_NOHZ);

	user_exit();

looks a bit better. But I won't argue.

> That's what I did for v4.

I am going to read it today. Not that I think I can help or find something
wrong.

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/