feat: extend shell with execute

src/sigTrampoline.S

@@ -0,0 +1,41 @@
+#include "syscall.h"
+
+/*
+ * Shared signal-delivery trampoline (see signal.h/signal.c). This is NOT
+ * entered like a normal function call from C -- signal.c's
+ * sigConsumePending rewrites a thread's own saved trapframe so that
+ * iretq lands ring 3 execution directly at sigTrampolineStart, with %rax
+ * already holding the handler's address and %rdi already holding the
+ * signal number (the handler's sole SysV argument), exactly as if the
+ * kernel itself had performed `call *%rax` with that argument set up.
+ *
+ * Position independent by construction: every instruction here is either
+ * PC-relative (the final backward jmp) or references no address at all
+ * (call through a register, an immediate syscall number, int $0x80) -- so
+ * sigInstallTrampoline can copy these bytes to any physical frame and map
+ * that frame at any virtual address (sigTrampolineVirtualAddr) and it
+ * still runs correctly, unlike this same code linked and executed in
+ * place inside the kernel's own (very different) virtual address range.
+ */
+.section .text
+.code64
+
+.global sigTrampolineStart
+.global sigTrampolineEnd
+
+sigTrampolineStart:
+    call *%rax
+    movl $sysSigReturn, %eax
+    int $0x80
+
+/* Only reached if sysSigReturn somehow returned instead of restoring the
+   pre-signal context and resuming it (a kernel bug, not a state a correct
+   sysOnSigReturn can produce) -- park here rather than run off into
+   whatever garbage happens to follow this page. */
+sigTrampolineHang:
+    hlt
+    jmp sigTrampolineHang
+
+sigTrampolineEnd:
+
+.section .note.GNU-stack, "", @progbits