Accept FIOASYNC ioctl and F_SETOWN/F_GETOWN fcntl

[Max042004]

Problem

Running nginx (aarch64 Linux, e.g. nginx:alpine) under elfuse in its default master/worker mode never serves requests: the listen socket binds, the host kernel completes TCP handshakes, but every request hangs and times out. With master_process off; (single process, no fork) nginx serves correctly, which masks the real issue.

Root cause

nginx's ngx_spawn_process arms the master→worker channel socket right before fork() with:

ioctl(channel[0], FIOASYNC, &on);     /* enable SIGIO-driven I/O   */
fcntl(channel[0], F_SETOWN, ngx_pid); /* who receives that SIGIO   */

and treats failure of either as fatal — it logs an alert, ngx_close_channel(), and return NGX_INVALID_PID without ever calling fork(). elfuse answered FIOASYNC with ENOTTY and F_SETOWN with EINVAL, so the master silently ended up with zero workers. A verbose trace shows no clone(220), no accept4(242), no epoll_pwait(22) after the FIOASYNC failure — the master just spins in rt_sigsuspend, and the accepted connections are never serviced.

Fix

elfuse does not forward host SIGIO into the guest, and nginx workers receive both client I/O and channel commands via epoll rather than SIGIO, so both calls are safe to accept as no-ops:

• sys_ioctl: FIOASYNC reads the int arg (for EFAULT parity) and returns success without arming host O_ASYNC.
• sys_fcntl: F_SETOWN / F_SETOWN_EX accept and track no owner; F_GETOWN / F_GETOWN_EX report "no owner". glibc implements fcntl(F_GETOWN) on top of F_GETOWN_EX, so the EX form writes a struct f_owner_ex{type=F_OWNER_PID, pid=0} to stay coherent.

With this, nginx -g "daemon off;" forks its worker and serves HTTP on the stock config: GET/HEAD/404, keep-alive, and 100-way concurrency all return 200 with byte-identical bodies.

Testing

• New tests/test-ioctl-fioasync.c (registered in tests/manifest.txt) replays nginx's pre-fork channel arming on a socketpair and a TCP socket, asserting FIOASYNC, F_SETOWN, and F_GETOWN all succeed.
• make check: the full guest driver suite passes (the only failure observed was the pre-existing, unrelated flaky test-fork — "unexpected exit reason 0x3" — which is racy on main and untouched by this change, which only adds switch cases in sys_ioctl/sys_fcntl).

---

Summary by cubic

Accepts FIOASYNC ioctl and F_SETOWN/F_GETOWN (and EX variants) fcntl as safe no-ops so nginx master/worker mode forks workers and serves requests under elfuse. Fixes the hang where requests timed out because no workers were spawned.

• Bug Fixes
  • sys_ioctl: handle LINUX_FIOASYNC as a no-op (reads int arg; no host SIGIO). Adds constant in abi.h.
  • sys_fcntl: accept F_SETOWN/F_SETOWN_EX as no-ops; F_GETOWN returns 0; F_GETOWN_EX writes {type=F_OWNER_PID, pid=0}.
  • Tests: add tests/test-ioctl-fioasync.c and manifest entry; checks socketpair and TCP socket paths.

^{Written for commit b6260b6. Summary will update on new commits.}

[cubic-dev-ai]

2 issues found across 5 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/syscall/fs.c">

<violation number="1" location="src/syscall/fs.c:804">
P2: New F_SETOWN/F_GETOWN handlers can return success for closed FDs because they skip closed-descriptor validation.</violation>

<violation number="2" location="src/syscall/fs.c:805">
P2: F_SETOWN_EX no-op path skips user-pointer read/validation, so bad guest pointers incorrectly succeed.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P2: F_SETOWN_EX no-op path skips user-pointer read/validation, so bad guest pointers incorrectly succeed.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/syscall/fs.c, line 805:

<comment>F_SETOWN_EX no-op path skips user-pointer read/validation, so bad guest pointers incorrectly succeed.</comment>

<file context>
@@ -801,6 +801,29 @@ int64_t sys_fcntl(guest_t *g, int fd, int cmd, uint64_t arg)
         return 0;
     }
+    case 8:  /* F_SETOWN */
+    case 15: /* F_SETOWN_EX */
+        /* SIGIO/SIGURG delivery owner. nginx's ngx_spawn_process pairs
+         * ioctl(FIOASYNC) with fcntl(F_SETOWN) on the channel socket before
</file context>

[cubic-dev-ai]

P2: New F_SETOWN/F_GETOWN handlers can return success for closed FDs because they skip closed-descriptor validation.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/syscall/fs.c, line 804:

<comment>New F_SETOWN/F_GETOWN handlers can return success for closed FDs because they skip closed-descriptor validation.</comment>

<file context>
@@ -801,6 +801,29 @@ int64_t sys_fcntl(guest_t *g, int fd, int cmd, uint64_t arg)
         host_fd_ref_close(&host_ref);
         return 0;
     }
+    case 8:  /* F_SETOWN */
+    case 15: /* F_SETOWN_EX */
+        /* SIGIO/SIGURG delivery owner. nginx's ngx_spawn_process pairs
</file context>

[jserv]

Rebase latest 'main` branch and resolve conflicts.