linux-kernel - Re: [PATCH v2 3/3] selftests/seccomp: Test SECCOMP_IOCTL_NOTIF

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Date:   Fri, 29 May 2020 13:09:12 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     christian.brauner@...ntu.com,
        containers@...ts.linux-foundation.org, cyphar@...har.com,
        jannh@...gle.com, jeffv@...gle.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, palmer@...gle.com, rsesek@...gle.com,
        tycho@...ho.ws, Matt Denton <mpdenton@...gle.com>
Subject: Re: [PATCH v2 3/3] selftests/seccomp: Test SECCOMP_IOCTL_NOTIF_ADDFD

On Fri, May 29, 2020 at 06:46:07PM +0000, Sargun Dhillon wrote:
> On Fri, May 29, 2020 at 12:41:51AM -0700, Kees Cook wrote:
> > On Thu, May 28, 2020 at 04:08:58AM -0700, Sargun Dhillon wrote:
> > > +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
> > > +
> > > +	nextid = req.id + 1;
> > > +
> > > +	/* Wait for getppid to be called for the second time */
> > > +	sleep(1);
> > 
> > I always rebel at finding "sleep" in tests. ;) Is this needed? IIUC,
> > userspace will immediately see EINPROGRESS after the NOTIF_SEND
> > finishes, yes?
> > 
> > Otherwise, yes, this looks good.
> > 
> > -- 
> > Kees Cook
> I'm open to better suggestions, but there's a race where if getppid
> is not called before the second SECCOMP_IOCTL_NOTIF_ADDFD is called,
> you will just get an ENOENT, since the notification ID is not found.
> 
> The other approach is to "poll" the child, and wait for it to enter
> the second syscall. Calling receive beforehand doesn't work because
> it moves the state of the notification in the kernel to received,
> and then the kernel doesn't error with EINPROGRESS.

For tests, I prefer polling. How about adding a busy-loop
(with a iteration-bounded small usleep) that just calls
SECCOMP_IOCTL_NOTIF_ID_VALID until it's valid?

-- 
Kees Cook