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Subject[PATCH] netlink: drop (int) cast on length arg in NLMSG_OK
The NLMSG_OK macro compares three things:
- the len arg from the user
- a size_t: sizeof(struct nlmsghdr)
- an int: sizeof(struct nlmsghdr) casted
- an u32: the nlmsghdr->nlmsg_len member

When building with -Wsign-compare, this macro triggers a signed compare
warning. This is because it compares len to an int, and then compares
it to a u32. If len is signed, we get a warning due to the last test.
If len is unsigned, we get a warning due to the first test. Like in
socketutils.c:145:8: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]

Lets drop the int cast on the first sizeof. This way, once the user
casts len to an unsigned value, everything shakes out correctly.

Signed-off-by: Mike Frysinger <>
include/uapi/linux/netlink.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 1a85940..3eb4e24 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -84,7 +84,7 @@ struct nlmsghdr {
#define NLMSG_DATA(nlh) ((void*)(((char*)nlh) + NLMSG_LENGTH(0)))
#define NLMSG_NEXT(nlh,len) ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
(struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
-#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
+#define NLMSG_OK(nlh,len) ((len) >= sizeof(struct nlmsghdr) && \
(nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
(nlh)->nlmsg_len <= (len))
#define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))

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