linux-kernel - Re: [PATCH 1/1] system call notification with self

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Date:	Tue, 9 Sep 2008 16:43:02 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Pierre Morel <pmorel@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Roland McGrath <roland@...hat.com>,
	Heiko Carstens <heicars2@...ux.vnet.ibm.com>,
	sameske@...ux.vnet.ibm.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, gregkh@...e.de,
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Cedric Le Goater <clg@...ibm.com>,
	Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [PATCH 1/1] system call notification with self_ptrace

On 09/08, Pierre Morel wrote:
>
> --- linux-2.6.26.orig/arch/s390/kernel/signal.c
> +++ linux-2.6.26/arch/s390/kernel/signal.c
> @@ -409,6 +409,11 @@ handle_signal(unsigned long sig, struct 
> 		spin_unlock_irq(&current->sighand->siglock);
> 	}
>
> +	if (current->instrumentation) {
> +		clear_thread_flag(TIF_SYSCALL_TRACE);
> +		current->instrumentation &= ~PTS_SELF;
> +	}
> +
> 	return ret;
> }

I still think this patch shouldn't change handle_signal().

Once again. The signal handler for SIGSYS can first do
sys_ptrace(PTRACE_SELF_OFF) (which is filtered out), and then use any
other syscall, so this change is not needed, afaics.

The overhead of the additional PTRACE_SELF_OFF syscall is very small,
especially compared to signal delivery. I don't think this functionality
will be widely used, but this change adds the unconditional overhead
to handle_signal().

Btw, the check above looks wrong, shouldn't it be

	if (current->instrumentation & PTS_SELF)

?

And. According to the prior discussion, this requires to hook every
signal handler in user space, otherwise we can miss syscall. But every
hook should start with PTRACE_SELF_ON, so I can't see any gain.

> +#define PTS_INSTRUMENTED	0x00000001
> +#define PTS_SELF	0x00000002

I don't really understand why do we need 2 flags, see also below,

> --- linux-2.6.26.orig/kernel/ptrace.c
> +++ linux-2.6.26/kernel/ptrace.c
> @@ -543,6 +543,38 @@ asmlinkage long sys_ptrace(long request,
> 	 * This lock_kernel fixes a subtle race with suid exec
> 	 */
> 	lock_kernel();
> +	if (request == PTRACE_SELF_ON) {
> +		task_lock(current);
> +		if (current->ptrace) {
> +			task_unlock(current);
> +			ret = -EPERM;
> +			goto out;
> +		}
> +		set_thread_flag(TIF_SYSCALL_TRACE);
> +		current->instrumentation |= PTS_INSTRUMENTED|PTS_SELF;
> +		task_unlock(current);
> +		ret = 0;
> +		goto out;

The code looks strange. How about

	if (request == PTRACE_SELF_ON) {
		ret = -EPERM;
		task_lock(current);
		if (!current->ptrace) {
			set_thread_flag(TIF_SYSCALL_TRACE);
			current->instrumentation |= PTS_INSTRUMENTED|PTS_SELF;
			ret = 0;
		}
		task_unlock(current);
		goto out;
	}

?

I don't understand how task_lock() can help. This code runs under
lock_kernel(), and without this lock the code is racy anyway.

> +	}
> +	if (request == PTRACE_SELF_OFF) {
> +		task_lock(current);
> +		if (current->ptrace) {
> +			task_unlock(current);
> +			ret = -EPERM;
> +			goto out;
> +		}
> +		clear_thread_flag(TIF_SYSCALL_TRACE);
> +		current->instrumentation &= ~PTS_SELF;

So. PTRACE_SELF_OFF doesn't clear PTS_INSTRUMENTED? How can the task
reset ->instrumentation ?

> +	if (current->instrumentation) {
> +		ret = -EPERM;
> +		goto out;
> +	}

So, PTRACE_SELF_XXX disables the "normal" ptrace. Not sure this is good.

Oleg.

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