Re: [PATCH v2 3/3] selftests/seccomp: Test SECCOMP_IOCTL_NOTIF_ADDFD

[Kees Cook <keescook@chromium.org>]

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