Should EXIT_DEAD be visible to userspace ?

[Dave Jones <davej@codemonkey.org.uk>]

One of our users reported a crash in some userspace tooling this
morning, which scrapes /proc/pid to gather stack traces, process states
etc of everything running at the time.

The crash occurred because it fell over an unexpected task state,
which was 'X'.   According to the procps man-pages, this state should
never be seen, but here it clearly was.

The kernel running at the time was kinda old (5.2) but I don't see much
change in the EXIT_DEAD space that would explain something that got
fixed subsequently.   It's also probably going to be difficult to
reproduce unfortunately.

So my question is, is procps wrong and code should expect to see X state
processes in proc ?  The code in question is being hardened to handle
unexpected inputs, but I'm curious if the kernel is leaking some state
that it shouldn't.

	Dave