linux-kernel - Re: [PATCH] netlink: fix gcc -Wconversion compilation warning

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Message-ID: <m1oc8jlg92.fsf@fess.ebiederm.org>
Date:	Fri, 17 Dec 2010 22:58:01 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	David Miller <davem@...emloft.net>
Cc:	kirill@...temov.name, netdev@...r.kernel.org, pablo@...filter.org,
	ldv@...linux.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netlink: fix gcc -Wconversion compilation warning

David Miller <davem@...emloft.net> writes:

> From: ebiederm@...ssion.com (Eric W. Biederman)
> Date: Mon, 13 Dec 2010 13:35:25 -0800
>
>> "Kirill A. Shutsemov" <kirill@...temov.name> writes:
>> 
>>> From: Dmitry V. Levin <ldv@...linux.org>
>>>
>>> $ cat << EOF | gcc -Wconversion -xc -S -o/dev/null -
>>> unsigned f(void) {return NLMSG_HDRLEN;}
>>> EOF
>>> <stdin>: In function 'f':
>>> <stdin>:3:26: warning: negative integer implicitly converted to unsigned type
>>>
>> This doesn't look like a bad fix, but I believe things will fail if
>> we give NLMSG_ALIGN an unsigned long like size_t.  Say like sizeof.
>
> What are you talking about? That's exactly his test case,
> look at what NLMSG_HDRLEN is defined to, it's exactly the
> case you're worried "will fail", it passes sizeof() to
> NLMSG_ALIGN.
>
> I think I'll apply Kirill's original patch, it's good enough
> and simpler.

Probably.  The case I was worried about was masks that become
0xffffffxx on 64bit instead of 0xffffffffffffxx.  Especially
when mixing those with ints.

It is possible to get some really weird things.

Eric
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