Re: [PATCH bpf-next 5/5] bpftool: fix a clang compilation warning

[Sedat Dilek <sedat.dilek@gmail.com>]

On Sat, Apr 10, 2021 at 6:49 PM Yonghong Song <yhs@fb.com> 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 let us add an explicit type conversion (from "int" to "unsigned int")
> for "len" in NLMSG_OK to silence this warning.
>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  tools/bpf/bpftool/net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> 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
>

Thanks for the patch.

I remember darkly I have seen this, too.

The only warning I see remaining *here* is fixed by this patch from bpf-next:

commit 7519c387e69d367075bf493de8a9ea427c9d2a1b
"selftests: xsk: Remove unused function"

- Sedat -

[1] https://git.kernel.org/bpf/bpf-next/c/7519c387e69d367075bf493de8a9ea427c9d2a1b