linux-kernel - Re: [PATCH v4 0/5] x86: two-phase syscall tracing and seccomp fastpath

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Message-ID: <20140729192056.GA6308@redhat.com>
Date:	Tue, 29 Jul 2014 21:20:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
	linux-arch@...r.kernel.org, linux-security-module@...r.kernel.org,
	Alexei Starovoitov <ast@...mgrid.com>, hpa@...or.com,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v4 0/5] x86: two-phase syscall tracing and seccomp
	fastpath

Andy, to avoid the confusion: I am not trying to review this changes.
As you probably know my understanding of asm code in entry.S is very
limited.

Just a couple of questions to ensure I understand this correctly.

On 07/28, Andy Lutomirski wrote:
>
> This is both a cleanup and a speedup.  It reduces overhead due to
> installing a trivial seccomp filter by 87%.  The speedup comes from
> avoiding the full syscall tracing mechanism for filters that don't
> return SECCOMP_RET_TRACE.

And only after I look at 5/5 I _seem_ to actually understand where
this speedup comes from.

So. Currently tracesys: path always lead to "iret" after syscall, with
this change we can avoid it if phase_1() returns zero, correct?

And, this also removes the special TIF_SYSCALL_AUDIT-only case in entry.S,
cool.

I am wondering if we can do something similar with do_notify_resume() ?

Stupid question. To simplify, lets forget that syscall_trace_enter()
already returns the value. Can't we simplify the asm code if we do
not export 2 functions, but make syscall_trace_enter() return
"bool slow_path_is_needed". So that "tracesys:" could do

	// pseudo code

tracesys:
	SAVE_REST
	FIXUP_TOP_OF_STACK

	call syscall_trace_enter

	if (!slow_path_is_needed) {
		addq REST_SKIP, %rsp
		jmp system_call_fastpath
	}

	...

?

Once again, I am just curious, it is not that I actually suggest to consider
this option.

Oleg.

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