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

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Message-ID: <20200529184606.GB11153@ircssh-2.c.rugged-nimbus-611.internal>
Date:   Fri, 29 May 2020 18:46:07 +0000
From:   Sargun Dhillon <sargun@...gun.me>
To:     Kees Cook <keescook@...omium.org>
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 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.