On 4/11/21 4:05 AM, Sedat Dilek wrote:
On Sat, Apr 10, 2021 at 6:49 PM 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 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@xxxxxx>
---
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.
In this particular case, through analysis, the compiler COULD decide
the comparison is okay as the range of "int" value for "len" is > 0.
But it really depends on when and how much analysis the compiler
did before issuing this particular warning. So working around at the
source code is a better choice than silencing all similar warnings. Some
of such warnings may actually reveal a real issue.
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
