From 973c4d3882d8f51c7b80fbcaa8b51d5278e89f5a Mon Sep 17 00:00:00 2001
From: Zoltan Varga <vargaz@gmail.com>
Date: Tue, 22 Oct 2019 06:31:23 -0400
Subject: [PATCH] [jit] Avoid running mono_handle_native_crash () on the
 altstack, it can't produce a backtrace. AMD64 only for now. (mono/mono#17466)

* [jit] Avoid running mono_handle_native_crash () on the altstack, it can't produce a backtrace. AMD64 only for now.

* Fix checks so stack overflows work again.


Commit migrated from https://github.com/mono/mono/commit/b96260e603fe0188e54019bfde8ca00bc2b5d78c
---
 src/mono/mono/mini/exceptions-amd64.c | 12 +++++++++---
 src/mono/mono/mini/mini-runtime.c     | 14 ++++++++++----
 src/mono/mono/mini/mini-runtime.h     |  3 +++
 3 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/src/mono/mono/mini/exceptions-amd64.c b/src/mono/mono/mini/exceptions-amd64.c
index b274cc4..c301b90 100644
--- a/src/mono/mono/mini/exceptions-amd64.c
+++ b/src/mono/mono/mini/exceptions-amd64.c
@@ -867,12 +867,14 @@ prepare_for_guard_pages (MonoContext *mctx)
 }
 
 static void
-altstack_handle_and_restore (MonoContext *ctx, MonoObject *obj, gboolean stack_ovf)
+altstack_handle_and_restore (MonoContext *ctx, MonoObject *obj, guint32 flags)
 {
 	MonoContext mctx;
 	MonoJitInfo *ji = mini_jit_info_table_find (mono_domain_get (), MONO_CONTEXT_GET_IP (ctx), NULL);
+	gboolean stack_ovf = (flags & 1) != 0;
+	gboolean nullref = (flags & 2) != 0;
 
-	if (!ji)
+	if (!ji || (!stack_ovf && !nullref))
 		mono_handle_native_crash ("SIGSEGV", ctx, NULL);
 
 	mctx = *ctx;
@@ -894,6 +896,10 @@ mono_arch_handle_altstack_exception (void *sigctx, MONO_SIG_HANDLER_INFO_TYPE *s
 	gpointer *sp;
 	int frame_size;
 	MonoContext *copied_ctx;
+	gboolean nullref = TRUE;
+
+	if (!mono_is_addr_implicit_null_check (fault_addr))
+		nullref = FALSE;
 
 	if (stack_ovf)
 		exc = mono_domain_get ()->stack_overflow_ex;
@@ -920,7 +926,7 @@ mono_arch_handle_altstack_exception (void *sigctx, MONO_SIG_HANDLER_INFO_TYPE *s
 	UCONTEXT_REG_RSP (sigctx) = (unsigned long)(sp - 1);
 	UCONTEXT_REG_RDI (sigctx) = (unsigned long)(copied_ctx);
 	UCONTEXT_REG_RSI (sigctx) = (guint64)exc;
-	UCONTEXT_REG_RDX (sigctx) = stack_ovf;
+	UCONTEXT_REG_RDX (sigctx) = (stack_ovf ? 1 : 0) | (nullref ? 2 : 0);
 #endif
 }
 
diff --git a/src/mono/mono/mini/mini-runtime.c b/src/mono/mono/mini/mini-runtime.c
index 0089cfb..f05af55 100644
--- a/src/mono/mono/mini/mini-runtime.c
+++ b/src/mono/mono/mini/mini-runtime.c
@@ -3274,8 +3274,8 @@ MONO_SIG_HANDLER_FUNC (, mono_sigill_signal_handler)
 
 #endif
 
-static gboolean
-is_addr_implicit_null_check (void *addr)
+gboolean
+mono_is_addr_implicit_null_check (void *addr)
 {
 	/* implicit null checks are only expected to work on the first page. larger
 	 * offsets are expected to have an explicit null check */
@@ -3365,11 +3365,17 @@ MONO_SIG_HANDLER_FUNC (, mono_sigsegv_signal_handler)
 		if (!ji && mono_chain_signal (MONO_SIG_HANDLER_PARAMS))
 			return;
 
-		if (is_addr_implicit_null_check (info->si_addr)) {
+#ifdef TARGET_AMD64
+		/* exceptions-amd64.c handles the check itself */
+		mono_arch_handle_altstack_exception (ctx, info, info->si_addr, FALSE);
+#else
+		if (mono_is_addr_implicit_null_check (info->si_addr)) {
 			mono_arch_handle_altstack_exception (ctx, info, info->si_addr, FALSE);
 		} else {
+			// FIXME: This shouldn't run on the altstack
 			mono_handle_native_crash ("SIGSEGV", &mctx, info);
 		}
+#endif
 	}
 #else
 
@@ -3385,7 +3391,7 @@ MONO_SIG_HANDLER_FUNC (, mono_sigsegv_signal_handler)
 		}
 	}
 
-	if (is_addr_implicit_null_check (fault_addr)) {
+	if (mono_is_addr_implicit_null_check (fault_addr)) {
 		mono_arch_handle_exception (ctx, NULL);
 	} else {
 		mono_handle_native_crash ("SIGSEGV", &mctx, (MONO_SIG_HANDLER_INFO_TYPE*)info);
diff --git a/src/mono/mono/mini/mini-runtime.h b/src/mono/mono/mini/mini-runtime.h
index 8cb8213..3981e77 100644
--- a/src/mono/mono/mini/mini-runtime.h
+++ b/src/mono/mono/mini/mini-runtime.h
@@ -558,6 +558,9 @@ mono_dump_native_crash_info (const char *signal, MonoContext *mctx, MONO_SIG_HAN
 void
 mono_post_native_crash_handler (const char *signal, MonoContext *mctx, MONO_SIG_HANDLER_INFO_TYPE *info, gboolean crash_chaining);
 
+gboolean
+mono_is_addr_implicit_null_check (void *addr);
+
 /*
  * Signal handling
  */
-- 
2.7.4