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

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Date:	Tue, 14 Dec 2010 13:27:59 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Pablo Neira Ayuso <pablo@...filter.org>,
	"Dmitry V. Levin" <ldv@...linux.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netlink: fix gcc -Wconversion compilation warning

On Mon, Dec 13, 2010 at 01:35:25PM -0800, Eric W. Biederman wrote:
> "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.
> 
> Admittedly it has to be a huge size but still if we are going
> to go fixing things...
> 
> And then there is the silliness that NLMSG_HDRLEN forces itself
> to be an int, when it start out as a size_t.
> 
> So I think NLMSG_ALIGN either needs to operation exclusively
> on unsigned longs aka size_t, or it needs to be type preserving.
> 
> Do you have time to look at this a bit more?

Something like this? Untested.
---
>From 8b86073dc9697c0eff93c003d6331c1e4aeda60e Mon Sep 17 00:00:00 2001
From: Kirill A. Shutemov <kirill@...temov.name>
Date: Tue, 14 Dec 2010 12:50:39 +0200
Subject: [PATCH] netlink: fix gcc -Wconversion compilation warning

$ 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

Define NLMSG_ALIGNTO as size_t, NLMSG_ALIGN always returns size_t and
drop non-sense casting in NLMSG_HDRLEN. Now NLMSG_HDRLEN returns size_t.

Signed-off-by: Dmitry V. Levin <ldv@...linux.org>
Signed-off-by: Kirill A. Shutemov <kirill@...temov.name>
---
 include/linux/netlink.h |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 1235669..db7d373 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -70,9 +70,9 @@ struct nlmsghdr {
    Check		NLM_F_EXCL
  */
 
-#define NLMSG_ALIGNTO	4
-#define NLMSG_ALIGN(len) ( ((len)+NLMSG_ALIGNTO-1) & ~(NLMSG_ALIGNTO-1) )
-#define NLMSG_HDRLEN	 ((int) NLMSG_ALIGN(sizeof(struct nlmsghdr)))
+#define NLMSG_ALIGNTO	((size_t) 4)
+#define NLMSG_ALIGN(len) ( ((size_t)(len)+NLMSG_ALIGNTO-1) & ~(NLMSG_ALIGNTO-1) )
+#define NLMSG_HDRLEN	 NLMSG_ALIGN(sizeof(struct nlmsghdr))
 #define NLMSG_LENGTH(len) ((len)+NLMSG_ALIGN(NLMSG_HDRLEN))
 #define NLMSG_SPACE(len) NLMSG_ALIGN(NLMSG_LENGTH(len))
 #define NLMSG_DATA(nlh)  ((void*)(((char*)nlh) + NLMSG_LENGTH(0)))
-- 
 Kirill A. Shutemov
--
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