Add support for Alpine Linux (#7440)

This change enables build of CoreCLR on Alpine Linux. Here is the list
of changes:
- Disable asserts checking RSP in arbitrary threads against cached stack limit
  for the respective thread. The stack on Alpine obviously grows over the limit
  reported by the pthread functions.
- Disable using XSTATE. This should be re-enabled after MUSL gets the _xstate,
  _fpx_sw_bytes and related data structures added to the signal.h header.
- Disable setting rlimit of RLIMIT_NOFILE to the max value, since it breaks
  debugging for some reason.
- Add skipping over the hardware signal trampoline in the PAL_VirtualUnwind.
  While we were not trying to walk over it in a simple case, in a case where
  an exception was thrown from a catch handler of a hardware exception, we
  still attempted to walk over it and it fails on Alpine.
- Fix detection of Alpine Linux in the PAL's CMakeLists.txt so that it works
  in Docker containers too.
- Modified PAL_VirtualUnwind to make the check for unwinding past the bottom
  of the stack unconditional. We had a long list of platforms where we were
  doing this check and it doesn't hurt to do it on platforms where it is not
  needed. I have done that rather than adding a check for Alpine Linux as
  another platform that needs it.

1 files changed, 31 insertions, 18 deletions

diff --git a/src/pal/src/exception/seh-unwind.cpp b/src/pal/src/exception/seh-unwind.cpp
index 24eebbbf94..6f552bd81c 100644
--- a/src/pal/src/exception/seh-unwind.cpp
+++ b/src/pal/src/exception/seh-unwind.cpp
@@ -221,15 +221,34 @@ static void GetContextPointers(unw_cursor_t *cursor, unw_context_t *unwContext,
 #endif
 }
 
+extern int g_common_signal_handler_context_locvar_offset;
+
 BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextPointers)
 {
     int st;
     unw_context_t unwContext;
     unw_cursor_t cursor;
 
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) || defined(_ARM64_) || defined(_ARM_)
-    DWORD64 curPc;
-#endif
+    DWORD64 curPc = CONTEXTGetPC(context);
+
+#ifndef __APPLE__
+    // Check if the PC is the return address from the SEHProcessException in the common_signal_handler. 
+    // If that's the case, extract its local variable containing the native_context_t of the hardware 
+    // exception and return that. This skips the hardware signal handler trampoline that the libunwind 
+    // cannot cross on some systems.
+    if ((void*)curPc == g_SEHProcessExceptionReturnAddress)
+    {
+        ULONG contextFlags = CONTEXT_CONTROL | CONTEXT_INTEGER | CONTEXT_FLOATING_POINT | CONTEXT_EXCEPTION_ACTIVE;
+
+    #if defined(_AMD64_)
+        contextFlags |= CONTEXT_XSTATE;
+    #endif
+        size_t nativeContext = *(size_t*)(CONTEXTGetFP(context) + g_common_signal_handler_context_locvar_offset);
+        CONTEXTFromNativeContext((const native_context_t *)nativeContext, context, contextFlags);
+
+        return TRUE;
+    }
+#endif 
 
     if ((context->ContextFlags & CONTEXT_EXCEPTION_ACTIVE) != 0)
     {
@@ -240,7 +259,7 @@ BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextP
         // So we compensate it by incrementing the PC before passing it to the unwinder.
         // Without it, the unwinder would not find unwind info if the hardware exception
         // happened in the first instruction of a function.
-        CONTEXTSetPC(context, CONTEXTGetPC(context) + 1);
+        CONTEXTSetPC(context, curPc + 1);
     }
 
 #if !UNWIND_CONTEXT_IS_UCONTEXT_T
@@ -264,18 +283,6 @@ BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextP
     WinContextToUnwindCursor(context, &cursor);
 #endif
 
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__)  || defined(_ARM64_) || defined(_ARM_)
-    // FreeBSD, NetBSD and OSX appear to do two different things when unwinding
-    // 1: If it reaches where it cannot unwind anymore, say a 
-    // managed frame.  It wil return 0, but also update the $pc
-    // 2: If it unwinds all the way to _start it will return
-    // 0 from the step, but $pc will stay the same.
-    // The behaviour of libunwind from nongnu.org is to null the PC
-    // So we bank the original PC here, so we can compare it after
-    // the step
-    curPc = CONTEXTGetPC(context);
-#endif
-
     st = unw_step(&cursor);
     if (st < 0)
     {
@@ -303,12 +310,18 @@ BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextP
     // Update the passed in windows context to reflect the unwind
     //
     UnwindContextToWinContext(&cursor, context);
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__)  || defined(_ARM64_) || defined(_ARM_)
+
+    // FreeBSD, NetBSD, OSX and Alpine appear to do two different things when unwinding
+    // 1: If it reaches where it cannot unwind anymore, say a 
+    // managed frame.  It will return 0, but also update the $pc
+    // 2: If it unwinds all the way to _start it will return
+    // 0 from the step, but $pc will stay the same.
+    // So we detect that here and set the $pc to NULL in that case.
+    // This is the default behavior of the libunwind on Linux.
     if (st == 0 && CONTEXTGetPC(context) == curPc)
     {
         CONTEXTSetPC(context, 0);
     }
-#endif
 
     if (contextPointers != NULL)
     {