netlink: avoid memset of 0 bytes sparse warning

netlink: avoid memset of 0 bytes sparse warning

[Patrick McHardy]

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commit d30196c4cab8442451794d25a8d8a072dadca2eb
Author: Patrick McHardy <kaber@trash.net>
Date:   Wed Nov 19 14:55:11 2008 +0100

    netlink: avoid memset of 0 bytes sparse warning
    
    A netlink attribute padding of zero triggers this sparse warning:
    
    include/linux/netlink.h:245:8: warning: memset with byte count of 0
    
    Avoid the memset when the size parameter is constant and requires no padding.
    
    Signed-off-by: Patrick McHardy <kaber@trash.net>

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 9ff1b54..51b09a1 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -242,7 +242,8 @@ __nlmsg_put(struct sk_buff *skb, u32 pid, u32 seq, int type, int len, int flags)
 	nlh->nlmsg_flags = flags;
 	nlh->nlmsg_pid = pid;
 	nlh->nlmsg_seq = seq;
-	memset(NLMSG_DATA(nlh) + len, 0, NLMSG_ALIGN(size) - size);
+	if (!__builtin_constant_p(size) || NLMSG_ALIGN(size) - size != 0)
+		memset(NLMSG_DATA(nlh) + len, 0, NLMSG_ALIGN(size) - size);
 	return nlh;
 }
 

Re: netlink: avoid memset of 0 bytes sparse warning

[David Miller]

From: Patrick McHardy <kaber@trash.net>
Date: Wed, 19 Nov 2008 14:56:12 +0100

>     netlink: avoid memset of 0 bytes sparse warning
>     
>     A netlink attribute padding of zero triggers this sparse warning:
>     
>     include/linux/netlink.h:245:8: warning: memset with byte count of 0
>     
>     Avoid the memset when the size parameter is constant and requires no padding.
>     
>     Signed-off-by: Patrick McHardy <kaber@trash.net>

Applied to net-next-2.6, but I totally disagree with this sparse warning.

GCC even elides the memset entirely if it can determine the length
is constant and 0.