On Mon, Apr 12, 2021 at 7:29 AM Yonghong Song <yhs@xxxxxx> wrote:
>
> With clang compiler:
> make -j60 LLVM=1 LLVM_IAS=1 <=== compile kernel
> # build selftests/bpf or bpftool
> make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1
> make -j60 -C tools/bpf/bpftool LLVM=1 LLVM_IAS=1
> the following compilation warning showed up,
> net.c:160:37: warning: comparison of integers of different signs: '__u32' (aka 'unsigned int') and 'int' [-Wsign-compare]
> for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, len);
> ^~~~~~~~~~~~~~~~~
> .../tools/include/uapi/linux/netlink.h:99:24: note: expanded from macro 'NLMSG_OK'
> (nlh)->nlmsg_len <= (len))
> ~~~~~~~~~~~~~~~~ ^ ~~~
>
> In this particular case, "len" is defined as "int" and (nlh)->nlmsg_len is "unsigned int".
> The macro NLMSG_OK is defined as below in uapi/linux/netlink.h.
> #define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
> (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
> (nlh)->nlmsg_len <= (len))
>
> The clang compiler complains the comparision "(nlh)->nlmsg_len <= (len))",
> but in bpftool/net.c, it is already ensured that "len > 0" must be true.
> So theoretically the compiler could deduce that comparison of
> "(nlh)->nlmsg_len" and "len" is okay, but this really depends on compiler
> internals. Let us add an explicit type conversion (from "int" to "unsigned int")
> for "len" in NLMSG_OK to silence this warning right now.
>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
> tools/bpf/bpftool/net.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
>
> diff --git a/tools/bpf/bpftool/net.c b/tools/bpf/bpftool/net.c
> index ff3aa0cf3997..f836d115d7d6 100644
> --- a/tools/bpf/bpftool/net.c
> +++ b/tools/bpf/bpftool/net.c
> @@ -157,7 +157,7 @@ static int netlink_recv(int sock, __u32 nl_pid, __u32 seq,
> if (len == 0)
> break;
>
> - for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, len);
> + for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, (unsigned int)len);
> nh = NLMSG_NEXT(nh, len)) {
> if (nh->nlmsg_pid != nl_pid) {
> ret = -LIBBPF_ERRNO__WRNGPID;
> --
> 2.30.2
>
