Re: [PATCH 3/3] ptrace,syscall_user_dispatch: add a getter/setter for sud configuration

From: Gregory Price <gregory.price@memverge.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Gregory Price <gourry.memverge@gmail.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org,
	krisman@collabora.com, tglx@linutronix.de, luto@kernel.org,
	peterz@infradead.org, ebiederm@xmission.com,
	akpm@linux-foundation.org, adobriyan@gmail.com, corbet@lwn.net,
	shuah@kernel.org
Subject: Re: [PATCH 3/3] ptrace,syscall_user_dispatch: add a getter/setter for sud configuration
Date: Mon, 23 Jan 2023 13:12:33 -0500	[thread overview]
Message-ID: <Y87OEdDXwZG8pmmE@memverge.com> (raw)
In-Reply-To: <20230123154101.GA6268@redhat.com>

On Mon, Jan 23, 2023 at 04:41:02PM +0100, Oleg Nesterov wrote:
> On 01/22, Gregory Price wrote:
> >
> > +int syscall_user_dispatch_get_config(struct task_struct *task, unsigned long size,
> > +		void __user *data)
> > +{
> > +	struct syscall_user_dispatch *sd = &task->syscall_dispatch;
> > +	struct syscall_user_dispatch_config config;
> > +
> > +	if (size != sizeof(struct syscall_user_dispatch_config))
> > +		return -EINVAL;
> > +
> > +	if (test_syscall_work(SYSCALL_USER_DISPATCH))
> > +		config.mode = PR_SYS_DISPATCH_ON;
> > +	else
> > +		config.mode = PR_SYS_DISPATCH_OFF;
> 
> Stupid question...
> 
> Why do we need 2/3 (which reports SYSCALL_USER_DISPATCH in proc/pid/status) then?
> 
> Oleg.
>

Actually a good question.

My original though was: CRIU uses proc/status to determine whether to
use seccomp dumping, so i may as well implement the same thing.

On further thought, I think you're right.  We can just always read and
set these settings regardless of the original state because SUD is not
seccomp.

1. if GENERIC_ENTRY is not compiled, and TIF_SYSCALL_USER_DISPATCH is
   not available, these settings get ignored anyway.
2. if disabled, offset/len/selector is guaranteed to be off
3. if you try to set something other than the above then this will fail
   anyway (see: set_syscall_user_dispatch)

ergo
4. It's always say to read/write these settings.  As with anything else
   you can certainly cause the user program to crash by setting garbage
	 but that's to be expected.

So i think dropping 2/3 in the list is good.  If you concur i'll do
that.