[PATCH bpf] bpftool: Use libbpf error code for flow dissector query

[Woojin Ji <random6.xyz@gmail.com>]

bpf_prog_query() returns a negative errno on failure. query_flow_dissector()
currently closes the namespace fd and then reads errno to decide whether
-EINVAL means that the running kernel does not support flow dissector queries.

That errno check controls behavior, not just diagnostics: -EINVAL is handled
as a non-fatal old-kernel case, while any other error makes bpftool net fail.
Reading errno after close() is fragile, because close() can overwrite errno
before the check. Use the libbpf-returned error code instead so the
compatibility branch is based on the BPF_PROG_QUERY result itself.

Keep the existing errno reset in the non-fatal path to preserve batch mode
behavior. The success path is unchanged.

Fixes: 7f0c57fec80f ("bpftool: show flow_dissector attachment status")
Assisted-by: ChatGPT:gpt-5.5
Signed-off-by: Woojin Ji <random6.xyz@gmail.com>
---
 tools/bpf/bpftool/net.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/net.c b/tools/bpf/bpftool/net.c
index 974189da8a91..dba28755d284 100644
--- a/tools/bpf/bpftool/net.c
+++ b/tools/bpf/bpftool/net.c
@@ -603,14 +603,14 @@ static int query_flow_dissector(struct bpf_attach_info *attach_info)
 			     &attach_flags, prog_ids, &prog_cnt);
 	close(fd);
 	if (err) {
-		if (errno == EINVAL) {
+		if (err == -EINVAL) {
 			/* Older kernel's don't support querying
 			 * flow dissector programs.
 			 */
 			errno = 0;
 			return 0;
 		}
-		p_err("can't query prog: %s", strerror(errno));
+		p_err("can't query prog: %s", strerror(-err));
 		return -1;
 	}
 
-- 
2.54.0