[PATCH bpf v3] tools: bpftool: fix truncated netlink dumps

[Jakub Kicinski <kuba@kernel.org>]

Netlink requires that the recv buffer used during dumps is at least
min(PAGE_SIZE, 8k) (see the man page). Otherwise the messages will
get truncated. Make sure bpftool follows this requirement, avoid
missing information on systems with large pages.

Acked-by: Quentin Monnet <qmo@kernel.org>
Fixes: 7084566a236f ("tools/bpftool: Remove libbpf_internal.h usage in bpftool")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
v3:
 - add missing new line
v2: https://lore.kernel.org/20260215000656.323486-1-kuba@kernel.org
 - make the same change in libbpf_netlink_recv()
   note that libbpf_netlink_recv() does a PEEK and then realloc
   if the message is larger, so it's technically already safe
   but probably better if we are consistent
 - fprintf(stderr) -> p_err()
 - adjust msg
v1: https://lore.kernel.org/bpf/20251205011823.751868-1-kuba@kernel.org/

CC: ast@kernel.org
CC: daniel@iogearbox.net
CC: andrii@kernel.org
CC: martin.lau@linux.dev
CC: eddyz87@gmail.com
CC: song@kernel.org
CC: yonghong.song@linux.dev
CC: john.fastabend@gmail.com
CC: kpsingh@kernel.org
CC: sdf@fomichev.me
CC: haoluo@google.com
CC: jolsa@kernel.org
CC: bpf@vger.kernel.org
---
 tools/bpf/bpftool/net.c | 5 ++++-
 tools/lib/bpf/netlink.c | 4 +++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/net.c b/tools/bpf/bpftool/net.c
index f25d66c8395e..974189da8a91 100644
--- a/tools/bpf/bpftool/net.c
+++ b/tools/bpf/bpftool/net.c
@@ -156,7 +156,7 @@ static int netlink_recv(int sock, __u32 nl_pid, __u32 seq,
 	bool multipart = true;
 	struct nlmsgerr *err;
 	struct nlmsghdr *nh;
-	char buf[4096];
+	char buf[8192];
 	int len, ret;
 
 	while (multipart) {
@@ -201,6 +201,9 @@ static int netlink_recv(int sock, __u32 nl_pid, __u32 seq,
 					return ret;
 			}
 		}
+
+		if (len)
+			p_err("Invalid message or trailing data in Netlink response: %d bytes left", len);
 	}
 	ret = 0;
 done:
diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index c997e69d507f..c9a78fb16f11 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -143,7 +143,7 @@ static int libbpf_netlink_recv(int sock, __u32 nl_pid, int seq,
 	struct nlmsghdr *nh;
 	int len, ret;
 
-	ret = alloc_iov(&iov, 4096);
+	ret = alloc_iov(&iov, 8192);
 	if (ret)
 		goto done;
 
@@ -212,6 +212,8 @@ static int libbpf_netlink_recv(int sock, __u32 nl_pid, int seq,
 				}
 			}
 		}
+		if (len)
+			pr_warn("Invalid message or trailing data in Netlink response: %d bytes left\n", len);
 	}
 	ret = 0;
 done:
-- 
2.53.0