linux-kernel - Is iounmap(NULL) safe or not?

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Date:   Sat,  6 May 2017 01:50:08 +0300
From:   Alexey Khoroshilov <khoroshilov@...ras.ru>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Alexey Khoroshilov <khoroshilov@...ras.ru>, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org
Subject: Is iounmap(NULL) safe or not?

Hello,

It seems thare are many places where code assumes iounmap(NULL) is safe.
Also there are several patches that state it explicitly:
  ff6defa6a8fa ("ALSA: Deletion of checks before the function call "iounmap")
  e24bb0ed8179 ("staging: dgnc: remove NULL test")

At the same time it seems PPC implementation generates a warning in this case:
  3bfafd6b136b ("netxen: avoid invalid iounmap")

  arch/powerpc/mm/pgtable_64.c:
	if ((unsigned long)addr < ioremap_bot) {
		printk(KERN_WARNING "Attempt to iounmap early bolted mapping"
		       " at 0x%p\n", addr);
		return;
	}

Could you please clarify if iounmap(NULL) safe or not.
I guess it would be less errorprone if the answer is architecture independent.

--
Thank you,
Alexey Khoroshilov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org