kernel: Reenable userspace stack tracing, but hidden behind a config flag

Sometimes this is needed for userspace program debugging (such as ports),
but sometimes it can crash, so we leave it off by default.

kernel/config.cmake.template

@@ -15,3 +15,7 @@
 # control characters, leading/trailing spaces, problematic characters and invalid UTF-8). Keep in mind that this restriction
 # is only enforced when creating files; existing files with such illegal filenames are parsed correctly and fully usable.
 # target_compile_definitions(moon PRIVATE MOON_DISABLE_FILENAME_RESTRICTIONS)
+
+# Uncomment the line below to make the kernel also calculate stack traces for userspace addresses on program crashes. 
+# This can aid in debugging, but makes the kernel more unstable as stack tracing will access arbitrary userspace memory.
+# target_compile_definitions(moon PRIVATE MOON_ENABLE_USERSPACE_STACK_TRACES)

kernel/src/arch/x86_64/CPU.cpp

@@ -326,7 +326,10 @@ namespace CPU
     static void backtrace_impl(u64 base_pointer, void (*callback)(u64, void*), void* arg)
     {
         StackFrame* current_frame = (StackFrame*)base_pointer;
-        while (current_frame && (u64)current_frame >= 0xFFFF'FFFF'8000'0000 &&
+        while (current_frame &&
+#ifndef MOON_ENABLE_USERSPACE_STACK_TRACES
+               (u64)current_frame >= 0xFFFF'FFFF'8000'0000 &&
+#endif
                MemoryManager::validate_access(current_frame, sizeof(*current_frame), MemoryManager::DEFAULT_ACCESS) &&
                current_frame->instruction)
         {

libc/include/bits/setjmp-types.h

@@ -5,6 +5,7 @@
 
 #include <bits/fixed-size-types.h>
 #include <bits/platform.h>
+#include <bits/signal.h>
 
 #ifndef _SETJMP_H
 #error "Never use bits/setjmp-types.h directly; include setjmp.h instead."
@@ -12,7 +13,13 @@
 
 #ifdef __libc_arch_x86_64
 typedef __u64_t jmp_buf[8];
-typedef __u64_t sigjmp_buf[8];
+typedef struct
+{
+    jmp_buf buf;
+    sigset_t set;
+    int saved;
+} __sigjmp_buf_tag;
+typedef __sigjmp_buf_tag sigjmp_buf[1];
 #else
 #error "Unsupported architecture."
 #endif

libc/include/setjmp.h

@@ -14,13 +14,13 @@ extern "C"
     /* Saves the current execution state in env. */
     int setjmp(jmp_buf env);
 
-    /* Right now, does the exact same thing as setjmp() (savesigs is ignored), since signals are not implemented. */
+    /* Saves the current execution state (and optionally, the current signal mask) in env. */
     int sigsetjmp(sigjmp_buf env, int savesigs);
 
     /* Restores the execution state saved in env by a setjmp() call. */
     __noreturn void longjmp(jmp_buf env, int val);
 
-    /* Right now, does the exact same as longjmp(), since signals are not implemented. */
+    /* Restores the execution state saved in env by a sigsetjmp() call. */
     __noreturn void siglongjmp(sigjmp_buf env, int val);
 
 #ifdef __cplusplus

libc/src/setjmp.cpp

@@ -1,14 +1,17 @@
 #include <setjmp.h>
+#include <signal.h>
 
 extern "C"
 {
-    int sigsetjmp(sigjmp_buf env, int)
+    int sigsetjmp(sigjmp_buf env, int savesigs)
     {
-        return setjmp(env);
+        if (savesigs) env->saved = 1, sigprocmask(0, nullptr, &env->set);
+        return setjmp(env->buf);
     }
 
     __noreturn void siglongjmp(sigjmp_buf env, int val)
     {
-        longjmp(env, val);
+        if (env->saved) sigprocmask(SIG_SETMASK, &env->set, nullptr);
+        longjmp(env->buf, val);
     }
 }