SchOS/kernel/sys/sys_sig.c

#include "types.h"
#include "defs.h"
#include "param.h"
#include "proc.h"
#include "x86.h"
#include "syscall.h"
#include "uaccess.h"
#include "errno.h"
#include "posix.h"   /* SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK, signal numbers */

// Linux sigset_t puts signal N at bit N-1; kernel-internal masks
// (pending_signals / signal_mask / deliver_signals) put signal N at bit N.
// Convert at the user boundary — storing user sets raw silently shifted
// every mask by one signal (blocking SIGCHLD actually blocked SIGCONT),
// which let mksh's SIGCHLD handler run inside the very critical sections
// sigprocmask() was guarding, wrecking its job table.
static uint64 sigset_from_user(uint64 uset) { return uset << 1; }
static uint64 sigset_to_user(uint64 kset)   { return kset >> 1; }

addr_t
sys_sigfillset(void)
{
    uint64 *set;
    if (argptr(0, (char**)&set, sizeof(uint64)) < 0)
        return -1;
    if (set) *set = ~0ULL;
    return 0;
}

addr_t
sys_sigprocmask(void)
{
    int how;
    addr_t setp, oldsetp;
    argint(0, &how);
    argaddr(1, &setp);
    argaddr(2, &oldsetp);

    // oldset is optional (NULL = don't return previous mask)
    if (oldsetp) {
        uint64 old = sigset_to_user(myproc()->signal_mask);
        put_user(old, oldsetp);
    }
    // set is optional (NULL = just query)
    if (setp) {
        uint64 uset;
        if (copy_from_user(&uset, setp, sizeof(uset)) == 0) {
            uint64 set = sigset_from_user(uset);
            if (how == SIG_BLOCK)        myproc()->signal_mask |= set;
            else if (how == SIG_UNBLOCK) myproc()->signal_mask &= ~set;
            else                         myproc()->signal_mask  = set;
        }
    }
    return 0;
}

addr_t
sys_sigwait(void)
{
    uint64 *set_ptr;
    int *sig_ptr;
    if (argptr(0, (char**)&set_ptr, sizeof(uint64)) < 0 ||
        argptr(1, (char**)&sig_ptr,  sizeof(int))    < 0)
        return -1;
    uint64 set = sigset_from_user(*set_ptr);
    acquire(&tickslock);
    while (!(myproc()->pending_signals & set)) {
        if (myproc()->killed) { release(&tickslock); return -1; }
        sleep(myproc(), &tickslock);
    }
    for (int i = 0; i < 64; i++) {
        if ((myproc()->pending_signals & set) & (1ULL << i)) {
            myproc()->pending_signals &= ~(1ULL << i);
            *sig_ptr = i;
            break;
        }
    }
    release(&tickslock);
    return 0;
}

// musl's struct sigaction (musl 1.2.5 signal.h) for this target:
// sa_handler (8 bytes) at offset 0, then a 128-byte sigset_t (musl uses
// 128/sizeof(long) = 16 longs regardless of NSIG), sa_flags (4 bytes) +
// padding, then sa_restorer (8 bytes) at offset 144. We only care about
// those two 8-byte fields.
#define SA_HANDLER_OFF 0
#define SA_RESTORER_OFF 144

// rt_sigaction: store the handler + restorer so deliver_signals() (called
// from syscall()'s tail, kernel/sys/syscall.c) can actually jump to user
// code instead of silently doing nothing. sig 0 is invalid (matches
// sigaction(2)); we don't validate against NSIG beyond our own array size.
//
// User accesses go through copy_from_user/put_user (uaccess.h), which are
// page-safe. An earlier version bulk-copied the ~152-byte struct through a
// single uva2ka() translation of its start address; when the user's
// act/oldact pointer landed close enough to a page boundary, the write ran
// past the mapped page into an unrelated physical page, silently crashing
// every freshly exec'd /bin/sh (mksh calls sigaction() during startup).
addr_t
sys_rt_sigaction(void)
{
    int sig;
    addr_t actp, oldactp;
    argint(0, &sig);
    argaddr(1, &actp);
    argaddr(2, &oldactp);
    if (sig <= 0 || sig >= 32) return -1;

    if (oldactp) {
        put_user(myproc()->sig_handler[sig], oldactp + SA_HANDLER_OFF);
        put_user(myproc()->sig_restorer[sig], oldactp + SA_RESTORER_OFF);
    }
    if (actp) {
        copy_from_user(&myproc()->sig_handler[sig], actp + SA_HANDLER_OFF,
                       sizeof(myproc()->sig_handler[sig]));
        copy_from_user(&myproc()->sig_restorer[sig], actp + SA_RESTORER_OFF,
                       sizeof(myproc()->sig_restorer[sig]));
    }
    return 0;
}

// rt_sigreturn: entered via `syscall` (rax=15) from musl's __restore_rt
// trampoline, which is exactly what we set as the manufactured return
// address in deliver_signals() below — the handler function's own `ret`
// lands here once it's done. Restore the trapframe deliver_signals()
// stashed before redirecting to the handler, so syscall()'s normal
// sysretq path resumes the interrupted code exactly where it left off.
addr_t
sys_rt_sigreturn(void)
{
    addr_t saved_rax = myproc()->saved_sigtf.rax;
    *myproc()->tf = myproc()->saved_sigtf;
    fxrstor(myproc()->sigfpu);      // what the interrupted code had
    myproc()->in_signal = 0;
    // If this handler interrupted a sigsuspend(), put its pre-suspend
    // signal mask back now that the handler has finished.
    if (myproc()->sigsusp_active) {
        myproc()->signal_mask = myproc()->sigsusp_oldmask;
        myproc()->sigsusp_active = 0;
    }
    // syscall() does `tf->rax = <our return value>` right after this
    // returns — hand back the already-restored rax so that assignment is
    // a harmless no-op instead of clobbering what we just restored.
    return saved_rax;
}

// Called from syscall()'s tail (kernel/sys/syscall.c), right before
// returning to user mode, for every syscall a process makes (this is the
// only "check point" we have without also hooking the interrupt-return
// path in trap.c — sufficient for what actually needed this: a process
// blocked in a syscall, like sigsuspend(), waiting for its own next
// syscall return to notice a signal arrived).
//
// If there's a pending, unblocked signal with a registered handler,
// redirect tf to it: rdi = signal number (SysV ABI first arg), rip =
// handler.
//
// The return address (what the handler's own `ret` lands on) can't be
// sig_restorer[sig] — this musl build has no SA_RESTORER defined at all
// (checked: not defined anywhere in musl 1.2.5), so sigaction() never
// fills it in; it's always 0. Pushing 0 as a return address is exactly
// what crashed every mksh instance the first time a signal was ever
// actually delivered (handler ran, `ret` jumped to address 0). Instead,
// inject a tiny trampoline directly onto the user's own stack — 7 bytes
// of machine code for `mov eax, 15; syscall` (rt_sigreturn) — below the
// current stack pointer (clear of the SysV red zone) and point the
// return address at that, the standard way a kernel supplies sigreturn
// when the C library doesn't hand it one itself.
void
deliver_signals(struct trapframe *tf)
{
    if (myproc()->in_signal) return; // no nested delivery
    uint64 deliverable = myproc()->pending_signals & ~myproc()->signal_mask;
    if (!deliverable) return;

    for (int sig = 1; sig < 32; sig++) {
        if (!(deliverable & (1ULL << sig))) continue;

        // SIG_DFL is 0 and SIG_IGN is 1 -- neither is an address to jump to.
        //
        // Only the 0 case was checked here. A process that had asked to
        // ignore a signal got its handler "called" at address 1, which is a
        // page fault at rip=0x1 and a dead process. Nothing sent SIGPIPE
        // until pipes learned to, at which point every shell that ignores
        // SIGPIPE -- which is every shell -- died the first time a pipeline
        // ended early. `cat big | head -c 5` was enough.
        //
        // The default action for a signal with no handler has already been
        // applied by sigsend(); all that is left here is to stop it being
        // pending, which the old `continue` never did, so an ignored signal
        // was re-examined on the way out of every syscall forever after.
        if (myproc()->sig_handler[sig] <= 1) {
            myproc()->pending_signals &= ~(1ULL << sig);
            continue;
        }

        uint64 tramp_addr = (tf->rsp - 256) & ~0xFULL;
        uint8 *tramp = (uint8*)uva2ka(myproc()->pgdir, (char*)tramp_addr);
        if (!tramp) continue; // can't deliver safely; try another pending signal

        uint64 sp = tramp_addr - 8;
        uint64 *retaddr = (uint64*)uva2ka(myproc()->pgdir, (char*)sp);
        if (!retaddr) continue;

        // The return trampoline: `mov eax, <rt_sigreturn> ; syscall`.
        //
        // **Which number depends on the process's ABI**, and it used to be
        // hardcoded to 15. Fifteen is Linux's rt_sigreturn, which is right for
        // anything musl-linked -- every port, and the shells. It is *native*
        // syscall 15 for a first-party binary, and native 15 is `open`. So a
        // handler in any cmd/ program ran, returned into the trampoline, called
        // open() with the handler's leftover registers as arguments, got
        // -ENOENT, and carried on executing the bytes after the trampoline --
        // which are stack, not code.
        //
        // See table.inc: SYSCALL(97, 15, rt_sigreturn). syscall_map is the
        // thing that says which numbering this process speaks.
        uint32 sret = myproc()->syscall_map ? 15 : SYS_rt_sigreturn;
        tramp[0] = 0xb8;                            // mov eax, imm32
        tramp[1] = (uint8)(sret      );
        tramp[2] = (uint8)(sret >>  8);
        tramp[3] = (uint8)(sret >> 16);
        tramp[4] = (uint8)(sret >> 24);
        tramp[5] = 0x0f; tramp[6] = 0x05;           // syscall
        *retaddr = tramp_addr;

        myproc()->pending_signals &= ~(1ULL << sig);
        myproc()->saved_sigtf = *tf;
        // The handler is free to use %xmm; the code it interrupted had values
        // there. Put back by rt_sigreturn.
        fxsave(myproc()->sigfpu);
        myproc()->in_signal = 1;

        tf->rsp = sp;
        tf->rdi = (uint64)sig;
        // Both return paths must point at the handler: iretq-based returns
        // use tf->rip, but syscall_trapret exits via sysretq, which takes
        // the user RIP from RCX and *discards* tf->rip (trapasm.S pops rcx
        // from the frame, then `addq $40` throws rip/cs/rflags away). With
        // only tf->rip set, the "delivered" handler never actually ran on
        // the syscall path: user code resumed at the old rcx with the
        // shifted rsp, and its ret fell into the sigreturn trampoline,
        // silently undoing everything. exec() handles this same quirk by
        // setting tf->rcx = elf.entry.
        tf->rip = myproc()->sig_handler[sig];
        tf->rcx = myproc()->sig_handler[sig];
        return;
    }
}

// sigaltstack: set/get alternate signal stack.
// We don't deliver signals, so just store/return zeros.
addr_t sys_sigaltstack(void) { return 0; }

// rt_sigpending: write the set of pending signals to user buffer.
addr_t sys_rt_sigpending(void)
{
    addr_t setp;
    argaddr(0, &setp);
    if (setp) {
        uint64 pend = sigset_to_user(myproc()->pending_signals);
        put_user(pend, setp);
    }
    return 0;
}

// rt_sigtimedwait: wait for a signal from the set; returns -EAGAIN immediately.
addr_t sys_rt_sigtimedwait(void) { return -EAGAIN; }

// rt_sigsuspend: atomically replace the signal mask and suspend until a
// signal not blocked by the *temporary* mask is pending, then return -EINTR.
//
// The atomic mask swap is the entire point of sigsuspend(2): mksh blocks
// SIGCHLD while it inspects its job table, then calls sigsuspend() with the
// old (SIGCHLD-unblocked) mask to wait for a child. The previous stub
// ignored the mask argument and just slept one tick — SIGCHLD stayed
// blocked, deliver_signals() could never fire the handler, and the shell
// hung forever after running any external command (unreaped zombie child,
// shell parked in sigsuspend: the "can't type after a command" hang).
//
// The temporary mask must still be in effect when deliver_signals() runs at
// syscall()'s tail (that's what allows the delivery), so the old mask can't
// be restored here — sys_rt_sigreturn() restores it once the handler
// finishes, via sigsusp_oldmask/sigsusp_active.
addr_t sys_rt_sigsuspend(void)
{
    addr_t maskp;
    uint64 newmask;
    argaddr(0, &maskp);

    uint64 oldmask = myproc()->signal_mask;
    if (maskp && copy_from_user(&newmask, maskp, sizeof(newmask)) == 0)
        myproc()->signal_mask = sigset_from_user(newmask);

    acquire(&tickslock);
    while (!(myproc()->pending_signals & ~myproc()->signal_mask) && !myproc()->killed)
        sleep(&ticks, &tickslock);
    release(&tickslock);

    // Will any pending signal actually reach a handler? If so, the mask is
    // restored by sigreturn after the handler runs; otherwise (no handler
    // installed, or we were killed) restore it right now.
    uint64 deliverable = myproc()->pending_signals & ~myproc()->signal_mask;
    int will_deliver = 0;
    for (int sig = 1; sig < 32; sig++)
        if ((deliverable & (1ULL << sig)) && myproc()->sig_handler[sig])
            { will_deliver = 1; break; }

    if (will_deliver && !myproc()->in_signal) {
        myproc()->sigsusp_oldmask = oldmask;
        myproc()->sigsusp_active = 1;
    } else {
        myproc()->signal_mask = oldmask;
    }
    return -EINTR;
}