[If this is the wrong way or place to submit this patch, please set me
straight.]
Summary: please add a cast to avoid a GCC warning on 32-bit userland.
Using netlink.h's NLMSG_OK correctly will cause GCC to issue a warning
on systems with 32-bit userland. The definition can easily be changed
to avoid this.
GCC's warning is to flag comparisons where C's implicit "Usual
Arithmetic Conversions" could lead to a surprising result.
Consider this context:
int i;
unsigned u;
The C standard says that i<u is evaluated as (unsigned)i < u.
This can easily be a surprise because a negative value of i will be
silently treated as a very large positive value.
In include/uapi/linux/netlink.h:
#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
(nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
(nlh)->nlmsg_len <= (len))
This comparison looks suspicious to GCC:
(nlh)->nlmsg_len <= (len)
nlmsg_len is of type __u32. If int is 32 bits, this compare will be
done as
(nlh)->nlmsg_len <= (unsigned)(len)
We know that this is actually safe because the first conjunct
determined that len isn't negative but the GCC apparently doesn't know.
A change that would calm GCC and also be correct would be to add
a cast to unsigned:
(nlh)->nlmsg_len <= (unsigned)(len))
But I imagine that len might well actually have type ssize_t. It is
often the result of a call to recvfrom(2), which is a ssize_t. So I
think that this would be safer:
(nlh)->nlmsg_len <= (size_t)(len))
I know of no system where size_t is narrower than unsigned.
This problem came up when building a userland component of libreswan in a
32-bit environment with a recent GCC and was reported by Lennart Sorensen.
Here is a patch to the Linus tree:
================
diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 6f3fe16..dd15537 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -86,7 +86,7 @@ struct nlmsghdr {
(struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
(nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
- (nlh)->nlmsg_len <= (len))
+ (nlh)->nlmsg_len <= (size_t)(len))
#define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))
#define NLMSG_NOOP 0x1 /* Nothing. */
================
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