strace

A reimplementation of strace(1) built on Chimera's SystemCalls trait.

The handler runs in Chimera's own address space alongside the guest, so it can read guest pointers (e.g. the file path passed to openat) with a plain dereference. Each syscall is logged to standard error in the same name(args) = ret format strace(1) prints.

Build and run

cargo run --example strace -- <program> [args...]

For the conformance test:

cargo run --example strace -- testing/conformance/entry/a.out hello

Sample output (abridged):

brk(0x0)                                 = 0x55e36bf2c000
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC, 0o0) = 3
fstat(3, 0x7ffe1c8f7a30)                 = 0
mmap(0x0, 98731, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f1d2a5cd000
openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC, 0o0) = 3
...
arch_prctl(0x1002, 0x7f1d2a829740)       = 0
set_tid_address(0x7f1d2a829a10)          = 2039123
exit_group(42)                           = ?

How it works

The example implements the chimera::SystemCalls trait:

impl SystemCalls for Strace {
    fn post_syscall(&mut self, call: &SystemCall) {
        let name = syscall_name(call.number);
        let line = format!("{}({})", name, format_args(name, call));
        match call.result() {
            Some(result) => eprintln!("{:<40} = {}", line, format_ret(result)),
            None => eprintln!("{:<40} = ?", line),
        }
    }
}

pre_syscall() and post_syscall() run for every guest syscall, even when Chimera services it internally. do_syscall() is only used for delegated syscalls and defaults to forwarding them with host_syscall(call).

The Chimera runtime intercepts arch_prctl(ARCH_SET_FS, ...) inside syscall so the guest's TLS setup doesn't disturb the runtime's own FS-base register, handles the mmap family itself so guest mapping bookkeeping stays authoritative, and terminates exit/exit_group itself instead of forwarding them to the host kernel. Logging in post_syscall() keeps those syscalls visible without making them interceptable.