From 3581d458c39bc5e58ceeeaf51796625bc978d2eb Mon Sep 17 00:00:00 2001
From: Alexey Dobriyan <adobriyan@gmail.com>
Date: Fri, 17 Jul 2015 16:24:09 -0700
Subject: [PATCH] /proc/$PID/cmdline: fixup empty ARGV case

/proc/*/cmdline code checks if it should look at ENVP area by checking
last byte of ARGV area:

	rv = access_remote_vm(mm, arg_end - 1, &c, 1, 0);
	if (rv <= 0)
		goto out_free_page;

If ARGV is somehow made empty (by doing execve(..., NULL, ...) or
manually setting ->arg_start and ->arg_end to equal values), the decision
will be based on byte which doesn't even belong to ARGV/ENVP.

So, quickly check if ARGV area is empty and report 0 to match previous
behaviour.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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