The calls a runtime makes behind libc's back

Node’s npm smoke failed on every Linux leg and passed on both macOS legs, with the same image on both sides:

The path is the mount’s own spelling, and the file is in it. A tracer settles what the error cannot:

Both calls reached the kernel. Neither the interposed statx nor the interposed close ran, which means the runtime did not call them: it asked the kernel directly. Node’s own imports say the same thing from the other side — for the stat family it names xstat, lxstat and syscall, and the dynamic loader never binds statx inside the process.

The reason is a library’s own fast path. libuv probes statx once and, when the kernel answers, uses it for every stat it makes, through the syscall entry rather than through the C library’s wrapper. macOS has no such path, which is why the two platform families disagreed.

The shim interposes the raw entry and routes the numbers a mounted tree can answer: the three stat forms, close, and execve. Every other number forwards verbatim, and so does a routed number whose path is not inside a mount — an ordinary host operation must not change because a package is running.

One detail of the ABI is worth stating, because it looks wrong at first: the interposition is a fixed seven-argument function. Linux caps a syscall’s arguments at six, so that signature is identical to the variadic original for every call the kernel can answer, and a variadic definition is not expressible on stable Rust. The pre-arm arm forwards through a resolved entry, so the interposition cannot call itself while the shim boots.

Two gaps the same afternoon exposed land with it. A stat that honours the no-follow flag has to report a link as a link, because module resolution walks along a path and bin/npm is a link to npm-cli.js. And the directory listing is answered from the mount, because the C library implements it on its own internal calls that no preload can interpose — so readdir answered ENOENT while opendir worked.

One case stays open, and it is named rather than implied. Bun’s bunx is not a link to the interpreter but a full copy of it, exec’d from a child process, and a child is not the process that armed the engine — the backends own worker pools that do not survive a fork. Reaching into the engine from there is the wedge a guard already exists for, and bypassing the guard makes our own regression pin hang until its watchdog kills it.

So the answer for bun is not a longer guard. It is the entry resolving to the interpreter itself: a link in the image, and the driver’s direct form taking it under the preload mechanism as it already does under the exec-cache one. That lands next.

A Linux pair built from the previous release’s assets, in a container, with the new shim in front:

macOS is unchanged by construction: every added interposition is Linux-only, and the shared bodies keep their existing callers. The crate’s tests run on both hosts, including one that pins the child rule — the engine is entered when it is free and answered EAGAIN when it is held, never waited on.