[PATCH] SEH: Reimplement the .seh_scope directive

[Julian Waters <tanksherman27@gmail.com>]

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The .seh_scope directive was removed a little over a decade ago due to
being too Microsoft specific. This has proven to be a mistake as there is
now no easy way for reusable inline assembly to express Structured
Exception Handling scopes for Microsoft Windows targets. This patch
reimplements a simpler version of .seh_scope, with the proper semantics in
place

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From 18da45e966a7f1cc91782517e3c992cbf52cd974 Mon Sep 17 00:00:00 2001
From: TheShermanTanker <tanksherman27@gmail.com>
Date: Fri, 7 Jul 2023 16:23:07 +0800
Subject: [PATCH] Reimplement the .seh_scope directive

---
 gas/config/obj-coff-seh.c | 100 +++++++++++++++++++++++++++++++++++---
 gas/config/obj-coff-seh.h |  16 +++++-
 2 files changed, 108 insertions(+), 8 deletions(-)

diff --git a/gas/config/obj-coff-seh.c b/gas/config/obj-coff-seh.c
index 7b4486a348f..661c3d61137 100644
--- a/gas/config/obj-coff-seh.c
+++ b/gas/config/obj-coff-seh.c
@@ -388,6 +388,69 @@ obj_coff_seh_handlerdata (int what ATTRIBUTE_UNUSED)
   switch_xdata (seh_ctx_cur->subsection + 1, seh_ctx_cur->code_seg);
 }
 
+static void
+obj_coff_seh_scope (int)
+{
+  if (!verify_context_and_target (".seh_scope", seh_kind_x64))
+    return;
+
+  seh_ctx_cur->scopes += 1;
+
+  expressionS addrs[4];
+
+  for (unsigned short i = 0; i < 4; i += 1) {
+    while (*input_line_pointer == ' ' || *input_line_pointer == '\t' || *input_line_pointer == ',') {
+      input_line_pointer += 1;
+    }
+
+    if (*input_line_pointer == NULL || *input_line_pointer == '\n') {
+        as_fatal (_(".seh_scope requires four symbol names.\n"));
+        demand_empty_rest_of_line ();
+        return;
+    }
+
+    expressionS exp;
+
+    if (*input_line_pointer == '@' && i > 1) {
+      char* name;
+      char end;
+      end = get_symbol_name (&name);
+      if (strcasecmp (name, "@null") == 0) {
+        exp.X_op = O_constant;
+        exp.X_add_number = 0;
+      } else {
+        as_fatal (_(".seh_scope encountered an unknown constant\n"));
+        demand_empty_rest_of_line ();
+        return;
+      }
+      restore_line_pointer (end);
+    } else {
+      expression(&exp);
+      exp.X_op = O_symbol_rva;
+    }
+
+    addrs[i] = exp;
+  }
+
+  seh_scope scope;
+
+  scope.start = addrs[0];
+  scope.end = addrs[1];
+  scope.filter = addrs[2];
+  scope.handler = addrs[3];
+
+  if (seh_ctx_cur->scopeinfo == NULL) {
+    seh_ctx_cur->scopeinfo = XCNEW (seh_scope);
+  } else {
+    seh_scope* newarray = XCNEWVEC (seh_scope, seh_ctx_cur->scopes);
+    memcpy (newarray, seh_ctx_cur->scopeinfo, (seh_ctx_cur->scopes - 1) * sizeof (seh_scope));
+    XDELETEVEC (seh_ctx_cur->scopeinfo);
+    seh_ctx_cur->scopeinfo = newarray;
+  }
+
+  memcpy (seh_ctx_cur->scopeinfo + (seh_ctx_cur->scopes - 1), &scope, sizeof (seh_scope));
+}
+
 /* Mark end of current context.  */
 
 static void
@@ -902,6 +965,25 @@ seh_x64_write_function_xdata (seh_context *c)
   /* Handler data will be tacked in here by subsections.  */
 }
 
+static void
+seh_x64_maybe_emit_scope_info (seh_context *c)
+{
+
+  if (c->scopeinfo != NULL) {
+    out_four (c->scopes);
+    for (unsigned short i = 0; i < c->scopes; i += 1) {
+      seh_scope scope = *(c->scopeinfo + i);
+      emit_expr (&(scope.start), 4);
+      emit_expr (&(scope.end), 4);
+      emit_expr (&(scope.filter), 4);
+      emit_expr (&(scope.handler), 4);
+    }
+
+    XDELETEVEC (c->scopeinfo);
+  }
+
+}
+
 /* Write out xdata for one function.  */
 
 static void
@@ -918,6 +1000,10 @@ write_function_xdata (seh_context *c)
 
   seh_x64_write_function_xdata (c);
 
+  switch_xdata (c->subsection + 1, c->code_seg);
+
+  seh_x64_maybe_emit_scope_info (c);
+
   subseg_set (save_seg, save_subseg);
 }
 
diff --git a/gas/config/obj-coff-seh.h b/gas/config/obj-coff-seh.h
index 8d77bacff71..f83e99c3c65 100644
--- a/gas/config/obj-coff-seh.h
+++ b/gas/config/obj-coff-seh.h
@@ -75,7 +75,8 @@
 	{"seh_no32", obj_coff_seh_32, 0}, \
 	{"seh_handler", obj_coff_seh_handler, 0}, \
 	{"seh_code", obj_coff_seh_code, 0}, \
-	{"seh_handlerdata", obj_coff_seh_handlerdata, 0},
+	{"seh_handlerdata", obj_coff_seh_handlerdata, 0}, \
+	{"seh_scope", obj_coff_seh_scope, 0},
 
 /* Type definitions.  */
 
@@ -87,6 +88,14 @@ typedef struct seh_prologue_element
   symbolS *pc_addr;
 } seh_prologue_element;
 
+typedef struct seh_scope
+{
+  expressionS start;
+  expressionS end;
+  expressionS filter;
+  expressionS handler;
+} seh_scope;
+
 typedef struct seh_context
 {
   struct seh_context *next;
@@ -128,6 +137,10 @@ typedef struct seh_context
   int elems_count;
   int elems_max;
   seh_prologue_element *elems;
+
+  /* Information for .seh_scope on x64 */
+  unsigned int scopes;
+  seh_scope *scopeinfo;
 } seh_context;
 
 typedef enum seh_kind {
@@ -151,6 +164,7 @@ static void obj_coff_seh_proc  (int);
 static void obj_coff_seh_handler (int);
 static void obj_coff_seh_handlerdata (int);
 static void obj_coff_seh_code (int);
+static void obj_coff_seh_scope (int);
 
 #define UNDSEC bfd_und_section_ptr
 
-- 
2.35.1.windows.2