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net: handle multi-part netlink responses

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Handle multi-part netlink responses to prevent truncated results from
large routing tables.

Previously, we only made a single recv call, which led to incomplete
results when the kernel split the message into multiple responses (which
happens frequently with NLM_F_DUMP).

Also guard against a potential hanging issue where the code would
indefinitely wait for NLMSG_DONE for non-multi-part responses by
detecting the NLM_F_MULTI flag and only continue waiting when necessary.
This commit is contained in:
willcl-ark 2025-04-01 14:15:50 +01:00
parent 42e99ad773
commit 4c53178256
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GPG key ID: CE6EC49945C17EA6
1 changed files with 68 additions and 37 deletions
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31
src/common/netif.cpp
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@ -36,6 +36,9 @@ namespace {
// will fail, so we skip that.
#if defined(__linux__) || (defined(__FreeBSD__) && __FreeBSD_version >= 1400000)
// Good for responses containing ~ 10,000-15,000 routes.
static constexpr ssize_t NETLINK_MAX_RESPONSE_SIZE{1'048'576};
std::optional<CNetAddr> QueryDefaultGatewayImpl(sa_family_t family)
{
// Create a netlink socket.
@ -84,6 +87,10 @@ std::optional<CNetAddr> QueryDefaultGatewayImpl(sa_family_t family)
// Receive response.
char response[4096];
ssize_t total_bytes_read{0};
bool done{false};
bool multi_part{false};
while (!done) {
int64_t recv_result;
do {
recv_result = sock->Recv(response, sizeof(response), 0);
@ -93,10 +100,27 @@ std::optional<CNetAddr> QueryDefaultGatewayImpl(sa_family_t family)
return std::nullopt;
}
total_bytes_read += recv_result;
if (total_bytes_read > NETLINK_MAX_RESPONSE_SIZE) {
LogPrintLevel(BCLog::NET, BCLog::Level::Warning, "Netlink response exceeded size limit (%zu bytes, family=%d)\n", NETLINK_MAX_RESPONSE_SIZE, family);
return std::nullopt;
}
bool processed_one{false};
for (nlmsghdr* hdr = (nlmsghdr*)response; NLMSG_OK(hdr, recv_result); hdr = NLMSG_NEXT(hdr, recv_result)) {
rtmsg* r = (rtmsg*)NLMSG_DATA(hdr);
int remaining_len = RTM_PAYLOAD(hdr);
processed_one = true;
if (hdr->nlmsg_flags & NLM_F_MULTI) {
multi_part = true;
}
if (hdr->nlmsg_type == NLMSG_DONE) {
done = true;
break;
}
if (hdr->nlmsg_type != RTM_NEWROUTE) {
continue; // Skip non-route messages
}
@ -131,6 +155,13 @@ std::optional<CNetAddr> QueryDefaultGatewayImpl(sa_family_t family)
}
}
// If we processed at least one message and multi flag not set, or if
// we received no valid messages, then we're done.
if ((processed_one && !multi_part) || !processed_one) {
done = true;
}
}
return std::nullopt;
}