Filter route messages for tables dhcpcd ignores, so foreign-table churn can't overflow netlink - #709
Filter route messages for tables dhcpcd ignores, so foreign-table churn can't overflow netlink#709sithglan wants to merge 2 commits into
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Included review availability: Your plan includes up to 2 reviews per rolling hour; 1 remains after this review. WalkthroughLinux netlink socket creation now accepts an optional filter. The link socket filters route messages to ChangesLinux netlink filtering
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: ⚪ Minimal · up to The change filters route messages for tables dhcpcd ignores, improving startup when large foreign routing tables create netlink churn; no actionable merge-blocking risk remains beyond normal checks and review. Sequence Diagram(s)sequenceDiagram
participant LinuxInterfaceSetup
participant if_linksocket
participant KernelNetlink
LinuxInterfaceSetup->>if_linksocket: open link socket with filtering enabled
if_linksocket->>KernelNetlink: attach route-message BPF filter
if_linksocket->>KernelNetlink: bind socket
LinuxInterfaceSetup->>if_linksocket: open route and generic sockets without filtering
LinuxInterfaceSetup->>KernelNetlink: enable strict route dump checking when available
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@src/if-linux.c`:
- Around line 459-460: Update the BPF route-message filter to accept only
RT_TABLE_MAIN, removing the RT_TABLE_UNSPEC acceptance path. Revise the nearby
comment to reflect that extended-table messages are filtered out, while
preserving the existing handling for RT_TABLE_MAIN.
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src/if-linux.csrc/if.h
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dhcpcd only ever acts on routes in the main table -- if_copyrt() opens with "if (rtm->rtm_table != RT_TABLE_MAIN) return -1;". On a host that also runs a routing daemon holding a full BGP feed in its own kernel table, that discard happens far too late: the message has already been allocated, queued and charged against SO_RCVBUF. Netlink charges roughly 480 bytes of socket accounting per route notification, so the 1 MiB granted by 100_default-link-rcvbuf.patch holds only ~2180 messages, and even net.core.rmem_max (16 MiB here) holds only ~35000. A single BGP re-convergence emits several hundred thousand. Overflow is not merely likely, it is arithmetically guaranteed, and no receive buffer size can prevent it. On overflow, dhcpcd_linkoverflow() discards the *entire* queue, including the RTM_NEWLINK and RTM_NEWADDR messages for the interface that just came up, then re-learns state that is stale again milliseconds later because the flood is still in progress. On a PPPoE line whose 24 hour forced disconnect makes the routing daemon reconverge at exactly the moment dhcpcd needs to see ppp0 appear, DHCPv6 prefix delegation is delayed by minutes. Apply dhcpcd's existing main-table test in the kernel instead, with a socket filter. A broadcast rejected by sk_filter() is never queued and never charged against SO_RCVBUF, so foreign-table churn can no longer overflow us however large it is. Because the filter mirrors if_copyrt() exactly, no message dhcpcd would have acted upon is affected; link, address and main-table route messages are all still delivered. rtm_table is only 8 bits wide. Table ids that do not fit are reported as RT_TABLE_COMPAT with the real id in RTA_TABLE, which cBPF cannot walk. That needs no special case: the filter accepts nothing but RT_TABLE_MAIN, which is precisely the test if_copyrt() makes, so the widest tables are kept out of the queue along with the rest. Measured on the affected host: 60000 table-100 routes installed while dhcpcd is behind on netlink gives 1 overflow and 5185 discarded messages per run without this patch (3 of 3 runs) and no overflow with it (3 of 3 runs). Note: this patch shifts GCC's inlining decisions enough to expose a pre-existing false positive, "-Wstringop-overflow: writing 16 bytes into a region of size 12" for the add_attr_l() call in if_address6(). That write is bounds checked against sizeof(struct nlma) (88 bytes) and lands at offset 28..44 of it; GCC mis-reports the destination object as the 16 byte nlmsghdr member because that is the address add_attr_l() receives. It is the usual NLMSG_TAIL idiom, and upstream already annotates the analogous site in if_copyrt() for Coverity.
if_initrt() asks for RT_TABLE_MAIN and if_addrflags6()/if_addressexists() ask for a single interface index, but without NETLINK_GET_STRICT_CHK the kernel ignores those filters and dumps everything; dhcpcd then discards the excess in if_copyrt() and in the dump callbacks. The result is correct either way, so this is purely a matter of where the filtering happens. On a host whose routing daemon holds a full BGP feed in its own kernel table the difference is enormous. Measured on the affected router, issuing exactly the RTM_GETROUTE dump if_initrt() issues: WITHOUT strict-check messages=254406 kept=13 discarded=254393 0.518 s WITH strict-check messages=13 kept=13 discarded=0 0.000 s rt_build() calls if_initrt() from 23 sites, including ipv6nd.c on every Router Advertisement, so this ran several times a second while the link was coming back up after the daily PPP disconnect. Counting recvmsg(2) calls for a dhcpcd startup against a 60000 route foreign table: 1296 before, 21 after. The option is set once, on the socket, at creation time rather than around each dump. That is not a stylistic choice: setsockopt(2) is not in the privsep seccomp allowlist in privsep-linux.c, so toggling it per dump kills the manager with SIGSYS once the sandbox is in force. Verified on this kernel that strict checking does not disturb the other traffic on this socket: RTM_NEWADDR, RTM_NEWROUTE, RTM_DELROUTE and RTM_DELADDR all still succeed, and RTM_GETADDR dumps return exactly the addresses the callbacks would have kept. ENOPROTOOPT is ignored so kernels older than 4.20 keep the previous behaviour.
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This looks really good thanks. I think the comments are too wordy and need reducing. Also it's not clear that NETLINK_GET_STRICT_CHK does not apply to the BPF filter, but to the structure options we already set which goes to explain why I saw them not working years ago when I first added them. I would like this to sit for a release or two as the dust settles from the 10.5.x security releases and then I'll update the comments and merge in. |
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Thanks for looking at it. Happy to do the comment trimming myself rather than leave it to you — say the On the second point, you're right and I should have made it explicit in the
That is almost certainly what you hit years ago: before Linux 4.20 the dump One caveat worth a line in the comment: strict checking also validates the No objection at all to letting it sit — it has been running here in production |
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One thing that isn't obvious from the diff: this PR fixes two separate 1. Time from
2. CPU burned on foreign-table route churn — "Drop foreign-table route This one is not about startup at all. The routing daemon holds a full IPv6 table To give a sense of the volume, this is a completely ordinary five seconds on the
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2 participants
I have a full IPv6 table with ~250 000 routes in route table 100. Without these two patches dhcpcd takes 56-244 seconds until the IPv6 address is available. With these patches I'm down to 4 - 5 seconds.
2026-07-03 02:35:20 → 02:36:53 92.3s
2026-07-04 02:35:24 → 02:37:02 97.5s
2026-07-05 02:35:28 → 02:37:10 102.6s
2026-07-06 02:35:31 → 02:36:51 80.2s
2026-07-07 05:49:04 → 05:50:32 87.8s
2026-07-08 05:49:08 → 05:50:19 70.2s
2026-07-09 05:49:13 → 05:50:15 61.4s
2026-07-10 05:49:16 → 05:50:51 94.9s
2026-07-11 05:49:19 → 05:50:38 78.6s
2026-07-12 05:49:22 → 05:50:50 87.4s
2026-07-13 05:49:27 → 05:50:48 81.8s
2026-07-14 05:49:31 → 05:50:39 68.2s
2026-07-15 05:49:34 → 05:51:11 96.6s
2026-07-16 05:49:37 → 05:51:23 106.1s
2026-07-17 05:49:41 → 05:51:20 99.5s
2026-07-18 05:49:45 → 05:51:57 132.0s
2026-07-19 05:49:48 → 05:51:05 76.8s
2026-07-20 05:49:52 → 05:53:38 225.3s
2026-07-21 05:49:57 → 05:51:06 68.7s
2026-07-22 05:50:01 → 05:53:18 196.9s
2026-07-23 08:54:04 → 08:55:37 92.8s
2026-07-24 08:54:07 → 08:55:59 112.0s
2026-07-25 08:54:11 → 08:55:36 84.8s
2026-07-26 08:54:15 → 08:58:04 228.6s
2026-07-27 08:54:19 → 08:55:37 78.2s
2026-07-28 08:54:22 → 08:55:38 75.5s
2026-07-29 08:54:27 → 08:55:47 80.6s
2026-07-30 08:54:30 → 08:57:49 198.7s
2026-07-31 08:54:33 → 08:55:32 59.1s
2026-08-02 06:12:05 → 06:13:09 64.4s
2026-08-03 06:12:08 → 06:13:13 64.7s
2026-08-04 06:12:12 → 06:16:17 244.4s
2026-08-05 06:12:17 → 06:13:13 56.7s
2026-08-06 06:12:21 → 06:13:33 72.5s
2026-08-07 06:12:24 → 06:15:03 158.7s
2026-08-08 06:36:04 → 06:37:21 77.1s
2026-08-09 06:36:09 → 06:37:07 58.0s
2026-08-10 06:36:12 → 06:37:25 73.2s
2026-08-11 06:36:15 → 06:38:06 110.4s
2026-08-15 08:18:48 → 08:20:01 72.5s
2026-08-16 08:18:53 → 08:22:19 206.0s
n = 41; min 56.7s median 84.8s mean 104.2s max 244.4s
over 60s: 38/41 over 120s: 8/41