netdev - RE: PATCH: netdev: add a cast NLMSG_OK to avoid a GCC warning in users' code

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CB922FF@AcuExch.aculab.com>
Date:	Fri, 11 Sep 2015 08:42:27 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	"'D. Hugh Redelmeier'" <hugh@...osa.com>,
	David Miller <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: PATCH: netdev: add a cast NLMSG_OK to avoid a GCC warning in
 users' code

From: D. Hugh Redelmeier
> Sent: 09 September 2015 21:24
...
> 2) if you use the type "unsigned int" on a 32-bit machine, you get the
>    warning for an earlier conjunct:
> 
> #define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
> 			   (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
> 			   (nlh)->nlmsg_len <= (len))
> 
>    (len) >= (int)sizeof(struct nlmsghdr)  <=== unsigned >= int
> 
> 3) on a 32-bit machine, size_t is likely "unsigned int" so the
>    same problem as (2) should arise.
> 
> 4) on a 64-bit machine with 64-bit ints, the same problems are likely.
>    I don't have one to test on.
> 
> Casting to "short" or "unsigned short" works, but I don't know that
> the value is guaranteed to fit in either of them.

Why not cast (nlh)->nl_msg_len instead?
Or subtract the two values and compare against zero?
Perhaps:
	(typeof (len))(nlh)->nlmsg_len <= (len)
which is almost certainly safe unless 'len' is 'signed char'.

	David

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