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Launch /bin/sh as interactive login shell from init #918

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Merged
dburkart merged 2 commits into from
May 6, 2026
Merged

Launch /bin/sh as interactive login shell from init #918

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418ecd2
Launch /bin/sh as interactive login shell from init
claude May 6, 2026
80b902b
fix: update ext2 image hash for init binary change
claude May 6, 2026
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161 changes: 91 additions & 70 deletions base/init/src/main.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,9 @@
//! 2. Calls fork(). The child calls execve() to load the hello binary,
//! which prints "hello: hello from execed child" and exits(0).
//! 3. The parent calls wait4() to collect the child's exit status, then
//! prints "init: fork+exec+wait ok" and loops forever.
//! prints "init: fork+exec+wait ok".
//! 4. Fork+exec `/bin/sh` as an interactive login shell with a minimal
//! environment (HOME, PATH, TERM, PS1). Respawns the shell if it exits.
//!
//! Syscall ABI (Linux x86_64 convention used by the vibix kernel):
//! - rax = syscall number
Expand Down Expand Up @@ -111,6 +113,9 @@ const SH_PATH: &[u8] = b"/bin/sh\0";
/// Smoke-test marker — emitted before exec-ing /bin/sh.
const SH_LAUNCH_MSG: &[u8] = b"init: launching /bin/sh\n";

/// Diagnostic — emitted when /bin/sh exits and init is about to respawn it.
const SH_RESPAWN_MSG: &[u8] = b"init: respawning /bin/sh\n";

#[no_mangle]
pub extern "C" fn _start() -> ! {
// Pre-write diagnostic marker — see #478. Emitted on fd=2 so it
Expand Down Expand Up @@ -223,55 +228,23 @@ pub extern "C" fn _start() -> ! {
}
}

// Launch /bin/sh — the POSIX shell. This is only meaningful when
// the ext2 rootfs is mounted (root=/dev/vda), which places the sh
// binary at /bin/sh. When booting from the ISO-only path (no ext2),
// execve will fail and the child exits with status 1; init continues
// to its idle loop regardless.
write(1, SH_LAUNCH_MSG);

let sh_fork_ret: i64;
unsafe {
core::arch::asm!(
"syscall",
inlateout("rax") 57u64 => sh_fork_ret,
lateout("rcx") _,
lateout("rdx") _,
lateout("rdi") _,
lateout("rsi") _,
lateout("r8") _,
lateout("r9") _,
lateout("r10") _,
lateout("r11") _,
options(nostack, preserves_flags),
);
}
// Launch /bin/sh as an interactive login shell. Respawn if it exits
// so the system stays usable. This is only meaningful when the ext2
// rootfs is mounted (root=/dev/vda), which places the sh binary at
// /bin/sh. When booting from the ISO-only path (no ext2), execve
// will fail and the child exits with status 1; init respawns it
// indefinitely (harmless — the system idles on wait4 each iteration).
loop {
write(1, SH_LAUNCH_MSG);

if sh_fork_ret == 0 {
// Child: exec /bin/sh.
unsafe {
core::arch::asm!(
"syscall",
inlateout("rax") 59u64 => _, // execve
inlateout("rdi") SH_PATH.as_ptr() as u64 => _, // path
inlateout("rsi") 0u64 => _, // argv (NULL = empty)
inlateout("rdx") 0u64 => _, // envp (NULL = empty)
lateout("rcx") _,
lateout("r8") _,
lateout("r9") _,
lateout("r10") _,
lateout("r11") _,
options(nostack, preserves_flags),
);
}
// execve only returns on failure — exit with status 1.
let sh_fork_ret: i64;
unsafe {
core::arch::asm!(
"syscall",
inlateout("rax") 60u64 => _, // exit
inlateout("rdi") 1u64 => _, // status 1 (exec failed)
inlateout("rax") 57u64 => sh_fork_ret,
lateout("rcx") _,
lateout("rdx") _,
lateout("rdi") _,
lateout("rsi") _,
lateout("r8") _,
lateout("r9") _,
Expand All @@ -280,35 +253,83 @@ pub extern "C" fn _start() -> ! {
options(nostack, preserves_flags),
);
}
loop {
core::hint::spin_loop();
}
}

// Parent: wait for the shell to exit, then idle.
if sh_fork_ret > 0 {
let sh_pid = sh_fork_ret as u64;
let mut sh_wstatus: i32 = 0;
unsafe {
core::arch::asm!(
"syscall",
inlateout("rax") 61u64 => _, // wait4
inlateout("rdi") sh_pid => _, // pid
inlateout("rsi") &mut sh_wstatus as *mut i32 as u64 => _, // *wstatus
inlateout("rdx") 0u64 => _, // options
inlateout("r10") 0u64 => _, // rusage
lateout("rcx") _,
lateout("r8") _,
lateout("r9") _,
lateout("r11") _,
options(nostack),
);
if sh_fork_ret == 0 {
// Child: exec /bin/sh with argv and envp.
//
// argv: ["sh", NULL]
// envp: ["HOME=/", "PATH=/bin:/usr/bin", "TERM=vt100",
// "PS1=vibix$ ", NULL]
//
// Each string is a null-terminated byte literal. The pointer
// arrays are stack-allocated and null-terminated per the
// execve(2) contract.
let arg0: *const u8 = b"sh\0".as_ptr();
let argv: [*const u8; 2] = [arg0, core::ptr::null()];

let env_home: *const u8 = b"HOME=/\0".as_ptr();
let env_path: *const u8 = b"PATH=/bin:/usr/bin\0".as_ptr();
let env_term: *const u8 = b"TERM=vt100\0".as_ptr();
let env_ps1: *const u8 = b"PS1=vibix$ \0".as_ptr();
let envp: [*const u8; 5] = [env_home, env_path, env_term, env_ps1, core::ptr::null()];

unsafe {
core::arch::asm!(
"syscall",
inlateout("rax") 59u64 => _, // execve
inlateout("rdi") SH_PATH.as_ptr() as u64 => _, // path
inlateout("rsi") argv.as_ptr() as u64 => _, // argv
inlateout("rdx") envp.as_ptr() as u64 => _, // envp
lateout("rcx") _,
lateout("r8") _,
lateout("r9") _,
lateout("r10") _,
lateout("r11") _,
options(nostack, preserves_flags),
);
}
// execve only returns on failure — exit with status 1.
unsafe {
core::arch::asm!(
"syscall",
inlateout("rax") 60u64 => _, // exit
inlateout("rdi") 1u64 => _, // status 1 (exec failed)
lateout("rcx") _,
lateout("rdx") _,
lateout("rsi") _,
lateout("r8") _,
lateout("r9") _,
lateout("r10") _,
lateout("r11") _,
options(nostack, preserves_flags),
);
}
loop {
core::hint::spin_loop();
}
}
}

// Loop forever.
loop {
core::hint::spin_loop();
// Parent: wait for the shell to exit, then respawn.
if sh_fork_ret > 0 {
let sh_pid = sh_fork_ret as u64;
let mut sh_wstatus: i32 = 0;
unsafe {
core::arch::asm!(
"syscall",
inlateout("rax") 61u64 => _, // wait4
inlateout("rdi") sh_pid => _, // pid
inlateout("rsi") &mut sh_wstatus as *mut i32 as u64 => _, // *wstatus
inlateout("rdx") 0u64 => _, // options
inlateout("r10") 0u64 => _, // rusage
lateout("rcx") _,
lateout("r8") _,
lateout("r9") _,
lateout("r11") _,
options(nostack),
);
}
write(1, SH_RESPAWN_MSG);
}
}
}

Expand Down
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