Improve stack overflow recovery

It's now possible to use sigaltstack() to recover from stack overflows
on Windows. Several bugs in sigaltstack() have been fixed, for all our
supported platforms. There's a newer better example showing how to use
this, along with three independent unit tests just to further showcase
the various techniques.

libc/runtime/isstackoverflow.c

@@ -19,45 +19,19 @@
 #include "libc/calls/struct/siginfo.h"
 #include "libc/calls/struct/ucontext.internal.h"
 #include "libc/calls/ucontext.h"
-#include "libc/intrin/kprintf.h"
+#include "libc/macros.internal.h"
 #include "libc/runtime/runtime.h"
 #include "libc/sysv/consts/auxv.h"
 #include "libc/sysv/consts/sig.h"
-#include "libc/thread/posixthread.internal.h"
-#include "libc/thread/thread.h"
-#include "libc/thread/tls.h"
 
 /**
- * Returns true if signal is likely a stack overflow.
+ * Returns true if signal is most likely a stack overflow.
  */
-char __is_stack_overflow(siginfo_t *si, void *ucontext) {
-
-  // check if signal has the information we need
-  ucontext_t *uc = ucontext;
-  if (!si) return false;
-  if (!uc) return false;
+char __is_stack_overflow(siginfo_t *si, void *arg) {
+  ucontext_t *uc = arg;
+  if (!si || !uc) return false;
   if (si->si_signo != SIGSEGV && si->si_signo != SIGBUS) return false;
-
-  // with threads we know exactly where the guard page is
-  int pagesz = getauxval(AT_PAGESZ);
-  uintptr_t addr = (uintptr_t)si->si_addr;
-  struct PosixThread *pt = _pthread_self();
-  if (pt->attr.__stacksize) {
-    uintptr_t stack = (uintptr_t)pt->attr.__stackaddr;
-    uintptr_t guard = pt->attr.__guardsize;
-    uintptr_t bot, top;
-    if (guard) {
-      bot = stack;
-      top = bot + guard;
-    } else {
-      bot = stack - pagesz;
-      top = stack;
-    }
-    return addr >= bot && addr < top;
-  }
-
-  // it's easy to guess with the main stack
-  // even though it's hard to know its exact boundaries
-  uintptr_t sp = uc->uc_mcontext.SP;
-  return addr <= sp && addr >= sp - pagesz;
+  intptr_t sp = uc->uc_mcontext.SP;
+  intptr_t fp = (intptr_t)si->si_addr;
+  return ABS(fp - sp) < getauxval(AT_PAGESZ);
 }

libc/runtime/mmap.c

@@ -28,6 +28,7 @@
 #include "libc/intrin/bsr.h"
 #include "libc/intrin/describeflags.internal.h"
 #include "libc/intrin/directmap.internal.h"
+#include "libc/intrin/kprintf.h"
 #include "libc/intrin/likely.h"
 #include "libc/intrin/safemacros.internal.h"
 #include "libc/intrin/strace.internal.h"
@@ -37,6 +38,9 @@
 #include "libc/log/libfatal.internal.h"
 #include "libc/log/log.h"
 #include "libc/macros.internal.h"
+#include "libc/nt/enum/memflags.h"
+#include "libc/nt/enum/pageflags.h"
+#include "libc/nt/memory.h"
 #include "libc/nt/process.h"
 #include "libc/nt/runtime.h"
 #include "libc/nt/struct/processmemorycounters.h"
@@ -396,9 +400,15 @@ inline void *__mmap_unlocked(void *addr, size_t size, int prot, int flags,
                     kAsanMmapSizeOverrun);
     }
     if (needguard) {
-      unassert(!mprotect(p, pagesize, PROT_NONE));
-      if (IsAsan()) {
-        __asan_poison(p, pagesize, kAsanStackOverflow);
+      if (!IsWindows()) {
+        unassert(!mprotect(p, pagesize, PROT_NONE));
+        if (IsAsan()) {
+          __asan_poison(p, pagesize, kAsanStackOverflow);
+        }
+      } else {
+        uint32_t oldattr;
+        unassert(VirtualProtect(p, pagesize, kNtPageReadwrite | kNtPageGuard,
+                                &oldattr));
       }
     }
   }

libc/runtime/winmain.greg.c

@@ -169,6 +169,9 @@ static abi wontreturn void WinInit(const char16_t *cmdline) {
     uint32_t old;
     __imp_VirtualProtect((void *)stackaddr, stacksize, kNtPageReadwrite, &old);
   }
+  uint32_t oldattr;
+  __imp_VirtualProtect((void *)stackaddr, GetGuardSize(),
+                       kNtPageReadwrite | kNtPageGuard, &oldattr);
   _mmi.p[0].x = stackaddr >> 16;
   _mmi.p[0].y = (stackaddr >> 16) + ((stacksize - 1) >> 16);
   _mmi.p[0].prot = prot;