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usockets: fail the connection when send() reports too many bytes - #46452
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Updated 11:05 PM PT - Oct 21st, 2026
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WalkthroughThe change adds an ChangesSocket send-result fault handling
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title clearly and concisely describes the main change: rejecting send results that exceed the requested byte count and failing the connection. |
| Description check | |
The description includes both required sections. It explains the problem, cause, fix, scope, limitations, and verification results with sufficient technical detail. |
| Linked Issues check | |
Check skipped because no linked issues were found for this pull request. |
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Review comments at @test/js/bun/http/serve-syscall-fault.test.ts:
- Around line 84-85: Separate the `/excess` fault coverage so the `send` case
arms only `send` and a distinct response case arms only `writev`; assert the
expected `ECONNRESET` behavior for each case.
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📒 Files selected for processing: 10
packages/bun-usockets/src/bsd.cpackages/bun-usockets/src/fault_inject.cpackages/bun-usockets/src/internal/fault_inject.hsrc/js/internal-for-testing.tssrc/runtime/socket/socket_body.rssrc/uws_sys/lib.rstest/js/bun/http/serve-syscall-fault.test.tstest/js/bun/util/socket-fault-injection.test.tstest/js/node/net/net-syscall-fault.test.tstest/js/node/tls/tls-syscall-fault.test.ts
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…wn through usockets
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Findings marked 🟡 are optional suggestions and need no follow-up push.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
packages/bun-usockets/src/bsd.c— macOS UDP users can receive a garbage datagram count from sendMany (a syscall number on x86_64, the fd on arm64) and resend or miscount packets. bsd_sendmmsg's sendmsg_x fast path at bsd.c:125-126 returns any non-negative result straight to the caller, and udp_socket.rs:1484 hands that number to JS as the count sent. The new bsd_sent guard covers only stream sends. Fix: clamp the datagram-path results (sendmsg_x at bsd.c:126, the sendmsg fallback at bsd.c:135) to sendbuf->num and treat a larger value as an error, the same way bsd_sent treats an over-count on streams.Why this was flagged
If pf on a macOS host consumes a UDP packet, udp6_output's ip6_output returns EJUSTRETURN and XNU leaves the result register untouched, the same mechanism the PR describes for send(). bsd_sendmmsg at bsd.c:123-126 calls sendmsg_x and returns ret unchanged whenever ret >= 0; on x86_64 that is the syscall number (tens of millions), on arm64 the descriptor. udp_socket.rs:1480-1484 passes res to get_us_error (which only treats -1 as failure, udp_socket.rs:67-70) and returns it to JS as the number of datagrams sent. A caller of socket.sendMany that uses the count to decide which payloads to retry either resends every packet (arm64 fd below num) or sees an impossible count (x86_64). Base branch has the same code, but this PR is the one that identified the quirk and chose to guard only bsd_send/bsd_writev. Remedy: in bsd_sendmmsg, if ret > sendbuf->num return -1 with ECONNABORTED (or clamp to num).
Verification: pre-existing, partial fix. Trigger: macOS, a connected
Bun.udpSocketwhile pf consumes the datagram, so the result register holds the syscall number (x86_64) or the fd (arm64).packages/bun-usockets/src/bsd.c:125-126: any non-negative value is returned as the datagram count with no clamp tosendbuf->num. The newbsd_sent()guard (bsd.c:961-973) is only wired intobsd_send/bsd_writev.
| static ssize_t bsd_sent(LIBUS_SOCKET_DESCRIPTOR fd, ssize_t sent, size_t requested) { | ||
| if (sent < 0 || (size_t)sent <= requested) { |
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🟣 pre-existing, not blocking: pre-existing, partial fix: on macOS x86_64 a single write of 32 MiB or more still silently corrupts the stream when the kernel hits the EJUSTRETURN quirk, exactly as on the base branch. bsd_sent at packages/bun-usockets/src/bsd.c:962 only rejects a result above requested, but the bogus x86_64 result is the syscall number, 33554565 for sendto and 33554460 for sendmsg, which is below requested once a caller passes that many bytes; AsyncSocket.h:368 and us_socket_t.rs:131 pass up to INT_MAX. Fix: cap the byte count handed to send() and sendmsg() in bsd_send and bsd_writev on macOS below 33554460, which callers already treat as a short write and resume on writable. [also at: packages/bun-usockets/src/bsd.c:964 - pre-existing: on Apple Silicon Macs the new check almost never fires, so the EJUSTRETURN quirk still silently corrupts the stream instead of failing the connection. bsd_sent at packages/bun-usockets/src/bsd.c:962 only rejects sent > requested.]
A small fix can ride a push you are already making; otherwise a short reply is enough.
Why this was flagged
On macOS x86_64, when ip6_output() returns EJUSTRETURN for a pf-consumed packet, the sendto syscall returns 33554565 and sendmsg returns 33554460, as the PR describes. bsd_sent at packages/bun-usockets/src/bsd.c:962 passes any result with (size_t)sent <= requested straight back to the caller. Bun.serve hands a whole Response body to us_socket_write in one call (packages/bun-uws/src/AsyncSocket.h:368, length up to INT_MAX) and node:net hands a whole socket.write() buffer to us_socket_write_check_error (src/uws_sys/us_socket_t.rs:131). For a 40 MiB body requested is 41943040, so a 33554565 result is accepted as a partial write; AsyncSocket.h:383 then buffers only the bytes past that offset and node:net advances bytes_written the same way, while the kernel actually took at most what fit in the send buffer. The peer receives a stream with roughly 30 MiB missing and no error is raised. The base branch does the same here; this PR fixes the case where the result exceeds the request but leaves the large-write case.
Verification: Only a result strictly greater than requested is converted to ECONNABORTED + shutdown at packages/bun-usockets/src/bsd.c:962-964. packages/bun-uws/src/AsyncSocket.h:365-368 only splits writes above INT_MAX, and src/uws_sys/us_socket_t.rs:131 clamps node:net writes to i32::MAX. AsyncSocket.h:383 buffers src + written onward, so bytes before offset 33554565 are silently skipped from the stream.
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Not changed here: a count at or below the length cannot be told from a short write. A cap would cover send on x86_64 only; sendmsg would need the iovecs trimmed, and arm64 has no bound to cap to. Listed under "Not covered" in the description.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Moving the rejection from bsd.c into us_internal_reject_excess_send resolves what I raised on the last push: the write side alone is shut down through us_internal_socket_raw_shutdown, so the poll type and WRITABLE interest change with it and the kernel receive queue is kept. Beyond the inline findings, I also checked that ECONNABORTED is already in us_internal_send_errno_is_peer_gone, so us_socket_write_check_error, us_internal_socket_raw_write and the TLS path report it fatal with no retry; that the second us_internal_socket_raw_shutdown from openssl.c:533 is a no-op once the poll type is SHUT_DOWN; and that the loop.c close runs after the read drain, so bytes the peer already sent still reach on_data before the ECONNRESET close.
Extended reasoning...
The change adds an over-reported send guard at the usockets socket layer, a new per-socket flag closed by the next poll dispatch, a fault-injection action to drive it, and tests across Bun.serve, Bun.connect, node:net, node:tls, fetch and WebSocket. It touches no auth, crypto or parsing surface; the security-relevant effect is failing a connection whose byte accounting is unknown rather than continuing. The previous run's bsd.c objections are addressed in the code, and the remaining inline findings concern the node:net buffered-write error code and test coverage gaps, which the author should weigh.
Still open from earlier reviews (1):
- Unresolved: 1 minor or pre-existing.
| ssize_t sent = bsd_write2(us_poll_fd(&s->p), header, header_length, payload, payload_length); | ||
| int written = us_internal_reject_excess_send(s, sent, (size_t) header_length + (size_t) payload_length); | ||
| if (written != header_length + payload_length) { | ||
| s->flags.last_write_failed = 1; | ||
| us_internal_rearm_writable(s); |
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🟡 (optional) node:net users whose socket already has bytes buffered get a 'read ECONNRESET' close instead of the 'write ECONNABORTED' the PR delivers on the first-attempt path. us_socket_write2 at socket.c:446-453 has no fatal out-param, so a rejected send comes back as 0 and the Rust writev path (socket_body.rs:2865) treats it as backpressure; the writable dispatch that would have surfaced the errno through internal_flush is now skipped by the !s->send_rejected test at loop.c:646, and loop.c:916 closes with ECONNRESET. Fix: surface the rejection on every node:net write path, e.g. have write_or_end_buffered check is_shutdown() after a short write2 and return -ECONNABORTED like write_maybe_corked, so the write callback and 'error' carry syscall write and ECONNABORTED whichever path the write took.
Why this was flagged
A node:net socket (plain TCP, unix) has data in buffered_data_for_node_net from an earlier backpressured write and the user calls write() again; write_or_end_buffered takes the writev fast path at src/runtime/socket/socket_body.rs:2861-2869 and calls us_socket_write2. On a send that reports more bytes than given, us_internal_reject_excess_send (socket.c:409-425) sets send_rejected, shuts the write side down and returns -1; us_socket_write2 (socket.c:446-453) returns 0 with no error indication, so Rust appends the chunk to the buffer and returns wrote=0 and JS write() returns false. On the next event the writable dispatch is skipped by !s->send_rejected at loop.c:646, so the 'write' errno is never produced; loop.c:916 then closes with LIBUS_ECONNRESET, which on_close turns into a read ECONNRESET error that destroys the stream. The first-attempt excess case (net-syscall-fault.test.ts:239) reports write ECONNABORTED, so only this path diverges from the PR's stated contract. The base branch mis-handled the over-count here, so this is a cost of the fix rather than a regression, but the test matrix does not cover the buffered write2 path.
Verification: nit. Trigger: a plain node:net socket that holds bytes in buffered_data_for_node_net gets the excess-send result on its next write(). us_socket_write2 (packages/bun-usockets/src/socket.c:446-453) has no fatal out-param and returns 0; the writable dispatch is skipped by !s->send_rejected (loop.c:646) and the socket is closed with LIBUS_ECONNRESET at loop.c:916-918. Not pre-existing.
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Left as is: on that path the socket still fails with an error and nothing more is sent, only the code differs (ECONNRESET from the close instead of ECONNABORTED from the write).
| if action == fi::ACTION_EXCESS && syscall != fi::SEND && syscall != fi::WRITEV { | ||
| return Err(global.throw(format_args!( | ||
| "rule.action \"excess\" is only supported for syscall \"send\" or \"writev\"" | ||
| ))); | ||
| } |
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🟡 nit (optional): the new "excess" per-syscall validation has no test, unlike its siblings. socket_body.rs:5370-5374 rejects "excess" for any syscall other than send/writev, but test/js/bun/util/socket-fault-injection.test.ts only covers the equivalent guards for "short" (line 26) and "zero" (line 37). Fix: add a case there asserting fault.set({ syscall, action: "excess" }) throws /only supported for syscall/ for recv, sendmsg, recvmsg, connect, accept, ssl_loop_buffer and ssl_write, and returns true for send and writev.
Why this was flagged
The setter in src/runtime/socket/socket_body.rs:5370-5374 adds a guard rejecting action == fi::ACTION_EXCESS unless syscall is fi::SEND or fi::WRITEV, with the message rule.action "excess" is only supported for syscall "send" or "writev". The existing control-surface test file test/js/bun/util/socket-fault-injection.test.ts has dedicated tests for the sibling guards (set() rejects 'short' at line 26, set() rejects 'zero' at line 37) but the diff adds nothing for "excess". If the guard regresses (e.g. arming "excess" on recv silently never fires because us_fault_hit's US_FAULT_EXCESS branch is only reached from bsd_send/bsd_writev), no test catches it. Coverage nit only; no runtime defect on the merged code.
Verification: nit. Triggering condition: whenever the new per-syscall guard for "excess" regresses or is dropped, nothing in the suite catches it. The diff adds the guard at src/runtime/socket/socket_body.rs:5370-5374. test/js/bun/util/socket-fault-injection.test.ts has sibling tests at line 26 and line 37 but is not in the 15 changed files. The rejection branch of the new validator is never exercised.
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Left out on purpose: it would only repeat the list in the setter.
| test("send → more bytes than it was given: rejects with ECONNRESET", async () => { | ||
| using server = Bun.serve({ | ||
| port: 0, | ||
| hostname: "127.0.0.1", | ||
| tls: { key: certs.key, cert: certs.cert }, | ||
| fetch: () => new Response("x"), | ||
| }); | ||
| const r = await runClientFetch(server.url.href, { syscall: "send", action: "excess" }, { ca: certs.cert }); |
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🟡 nit (optional): this fetch test only exercises the TLS ClientHello, so an over-reported send on the HTTP request itself stays untested on the fetch client. The rule at fetch-syscall-fault.test.ts:234 has no after and no fd, and runClientFetch arms it before fetch() starts, so the first bsd_send in the child, the handshake flight, consumes the single-shot rule. Fix: fire the rule on a post-handshake send (an after count that skips the handshake flights, or arm it from inside the request so it lands on the request record), keeping the ECONNRESET expectation, so the HTTP client's in-flight-request path is what is covered, as the node:tls and wss siblings already do.
Why this was flagged
runClientFetch (test/js/web/fetch/fetch-syscall-fault.test.ts:13-21) calls fault.set(rule) in the child before fetch() runs; the rule at line 234 is { syscall: "send", action: "excess" }, so after_n_calls is 0 and repeat is 1 (src/runtime/socket/socket_body.rs:5430-5431). us_fault_hit (packages/bun-usockets/src/fault_inject.c:80-88) fires on the first bsd_send with no fd filter, which is the TLS ClientHello written through ssl_raw_write (packages/bun-usockets/src/crypto/openssl.c:527). That path ends in ssl_close_after_rejected_send with ssl_handshake_state == HANDSHAKE_PENDING (openssl.c:2453-2455), so the ECONNRESET the test asserts comes from ssl_trigger_handshake_econnreset, not from a request write failing while fetch has a request in flight. The sibling tests in test/js/node/tls/tls-syscall-fault.test.ts:129-149 and test/js/web/websocket/websocket-syscall-fault.test.ts:209-222 arm the rule after the connection is open, so they do cover application data; the fetch client's own handling of a rejected request send is covered by no test.
Verification: nit. Whenever this test runs, the single-shot rule is consumed by the TLS ClientHello, so the fetch client's HTTP request write is never exercised under an over-reported send. test/js/web/fetch/fetch-syscall-fault.test.ts:234 arms { syscall: "send", action: "excess" } with no after/fd, and runClientFetch (lines 18-25) calls fault.set(rule) before fetch() starts.
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Left as is: the record path after the handshake is covered by the node:tls and wss: cases, and an after count tied to the number of handshake flights would break when that number changes.
What does this PR do?
On macOS,
send()can return a number larger than the length it was given. Bun trusted that number: a TLS write crashes inssl_emitand a plain socket write panics. Such a send now fails withECONNABORTEDand the connection is closed.Cause. XNU leaves the result register untouched when a system call ends in
EJUSTRETURN(x86_64, arm64), whichip6_output()returns for a packet pf consumed. x86_64 reads back the system call number, arm64 the descriptor. golang/go#61060 reports the same.Fix. How much the kernel took is unknown, so the stream cannot continue.
us_internal_reject_excess_send()insocket.cchecks the five calls tobsd_send,bsd_writevandbsd_write2. On such a count it:loop.cnow closes withECONNRESETafter the event's callbacks.The
ssl_emitcrash came with #44618.Not covered:
bsd_sendmsg(unix sockets), UDP and QUIC sends are unchanged.How did you verify your code works?
A new fault injection action,
excess, makessendorwritevreport more than it was given. Debug build on Linux x64, one part of the fix undone per column:node:tls,fetchover https,wss:ssl_emitnode:netwriteBun.connect, peer never answers the FINwritereturns 6Bun.serveresponse (send,writev)The seven touched test files pass. Not run on macOS.