[PATCH bpf-next] libbpf: Use dynamically allocated buffer when receiving netlink messages

[Toke Høiland-Jørgensen <toke@xxxxxxxxxx>]

When receiving netlink messages, libbpf was using a statically allocated
stack buffer of 4k bytes. This happened to work fine on systems with a 4k
page size, but on systems with larger page sizes it can lead to truncated
messages. The user-visible impact of this was that libbpf would insist no
XDP program was attached to some interfaces because that bit of the netlink
message got chopped off.

Fix this by switching to a dynamically allocated buffer; we borrow the
approach from iproute2 of using recvmsg() with MSG_PEEK|MSG_TRUNC to get
the actual size of the pending message before receiving it, adjusting the
buffer as necessary. While we're at it, also add retries on interrupted
system calls around the recvmsg() call.

Reported-by: Zhiqian Guan <zhguan@xxxxxxxxxx>
Fixes: 8bbb77b7c7a2 ("libbpf: Add various netlink helpers")
Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
---
 tools/lib/bpf/netlink.c | 55 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 52 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index c39c37f99d5c..9a6e95206bf0 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -87,22 +87,70 @@ enum {
 	NL_DONE,
 };
 
+static int __libbpf_netlink_recvmsg(int sock, struct msghdr *mhdr, int flags)
+{
+	int len;
+
+	do {
+		len = recvmsg(sock, mhdr, flags);
+	} while (len < 0 && (errno == EINTR || errno == EAGAIN));
+
+	if (len < 0)
+		return -errno;
+	return len;
+}
+
+static int libbpf_netlink_recvmsg(int sock, struct msghdr *mhdr, char **buf)
+{
+	struct iovec *iov = mhdr->msg_iov;
+	void *nbuf;
+	int len;
+
+	len = __libbpf_netlink_recvmsg(sock, mhdr, MSG_PEEK | MSG_TRUNC);
+	if (len < 0)
+		return len;
+
+	if (len < 4096)
+		len = 4096;
+
+	if (len > iov->iov_len) {
+		nbuf = realloc(iov->iov_base, len);
+		if (!nbuf) {
+			free(iov->iov_base);
+			return -ENOMEM;
+		}
+		iov->iov_base = nbuf;
+		iov->iov_len = len;
+	}
+
+	len = __libbpf_netlink_recvmsg(sock, mhdr, 0);
+	if (len > 0)
+		*buf = iov->iov_base;
+	return len;
+}
+
 static int libbpf_netlink_recv(int sock, __u32 nl_pid, int seq,
 			       __dump_nlmsg_t _fn, libbpf_dump_nlmsg_t fn,
 			       void *cookie)
 {
+	struct iovec iov = {};
+	struct msghdr mhdr = {
+		.msg_iov = &iov,
+		.msg_iovlen = 1,
+	};
 	bool multipart = true;
 	struct nlmsgerr *err;
 	struct nlmsghdr *nh;
-	char buf[4096];
 	int len, ret;
+	char *buf;
+
 
 	while (multipart) {
 start:
 		multipart = false;
-		len = recv(sock, buf, sizeof(buf), 0);
+		len = libbpf_netlink_recvmsg(sock, &mhdr, &buf);
 		if (len < 0) {
-			ret = -errno;
+			ret = len;
 			goto done;
 		}
 
@@ -151,6 +199,7 @@ static int libbpf_netlink_recv(int sock, __u32 nl_pid, int seq,
 	}
 	ret = 0;
 done:
+	free(iov.iov_base);
 	return ret;
 }
 
-- 
2.35.1