Display ExceptionRecord for $_siginfo

Uses the $_siginfo convenience variable to show the last exception.

The type looks like this:

(gdb) pt $_siginfo
type = struct EXCEPTION_RECORD {
    DWORD ExceptionCode;
    DWORD ExceptionFlags;
    struct EXCEPTION_RECORD *ExceptionRecord;
    PVOID ExceptionAddress;
    DWORD NumberParameters;
    ULONG_PTR ExceptionInformation[15];
}

EXCEPTION_RECORD is documented at [1].

Example:

Program received signal SIGSEGV, Segmentation fault.
main () at crasher.c:4
4         *(int*)0x123 = 0;
(gdb) p $_siginfo
$1 = {
  ExceptionCode = 3221225477,
  ExceptionFlags = 0,
  ExceptionRecord = 0x0,
  ExceptionAddress = 0x401632 <main+18>,
  NumberParameters = 2,
  ExceptionInformation = {1, 291, 0 <repeats 13 times>}
}
(gdb) p/x $_siginfo.ExceptionCode
$2 = 0xc0000005
(gdb) p/x $_siginfo.ExceptionInformation[1]
$3 = 0x123

And 0xc0000005 is the value of EXCEPTION_ACCESS_VIOLATION.

[1] https://docs.microsoft.com/en-us/windows/win32/api/winnt/ns-winnt-exception_record

gdb/ChangeLog:

2020-02-09  Hannes Domani  <ssbssa@yahoo.de>

	* NEWS: Mention $_siginfo support for Windows.
	* windows-nat.c (handle_exception): Set siginfo_er.
	(windows_nat_target::mourn_inferior): Reset siginfo_er.
	(windows_xfer_siginfo): New function.
	(windows_nat_target::xfer_partial): Call windows_xfer_siginfo.
	* windows-tdep.c (struct windows_gdbarch_data): New struct.
	(init_windows_gdbarch_data): New function.
	(get_windows_gdbarch_data): New function.
	(windows_get_siginfo_type): New function.
	(windows_init_abi): Register windows_get_siginfo_type.
	(_initialize_windows_tdep): Register init_windows_gdbarch_data.

gdbserver/ChangeLog:

2020-02-09  Hannes Domani  <ssbssa@yahoo.de>

	* win32-low.c (win32_clear_inferiors): Reset siginfo_er.
	(handle_exception): Set siginfo_er.
	(win32_xfer_siginfo): New function.

1 files changed, 31 insertions, 0 deletions

diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index 366c98f..76fcdd6 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -236,6 +236,7 @@ static DEBUG_EVENT current_event;	/* The current debug event from
 					   WaitForDebugEvent */
 static HANDLE current_process_handle;	/* Currently executing process */
 static windows_thread_info *current_thread;	/* Info on currently selected thread */
+static EXCEPTION_RECORD siginfo_er;	/* Contents of $_siginfo */
 
 /* Counts of things.  */
 static int exception_count = 0;
@@ -1167,6 +1168,8 @@ handle_exception (struct target_waitstatus *ourstatus)
   DWORD code = rec->ExceptionCode;
   handle_exception_result result = HANDLE_EXCEPTION_HANDLED;
 
+  memcpy (&siginfo_er, rec, sizeof siginfo_er);
+
   ourstatus->kind = TARGET_WAITKIND_STOPPED;
 
   /* Record the context of the current thread.  */
@@ -2863,6 +2866,7 @@ windows_nat_target::mourn_inferior ()
       CHECK (CloseHandle (current_process_handle));
       open_process_used = 0;
     }
+  siginfo_er.ExceptionCode = 0;
   inf_child_target::mourn_inferior ();
 }
 
@@ -2996,6 +3000,30 @@ windows_xfer_shared_libraries (struct target_ops *ops,
   return len != 0 ? TARGET_XFER_OK : TARGET_XFER_EOF;
 }
 
+/* Helper for windows_nat_target::xfer_partial that handles signal info.  */
+
+static enum target_xfer_status
+windows_xfer_siginfo (gdb_byte *readbuf, ULONGEST offset, ULONGEST len,
+		      ULONGEST *xfered_len)
+{
+  if (siginfo_er.ExceptionCode == 0)
+    return TARGET_XFER_E_IO;
+
+  if (readbuf == nullptr)
+    return TARGET_XFER_E_IO;
+
+  if (offset > sizeof (siginfo_er))
+    return TARGET_XFER_E_IO;
+
+  if (offset + len > sizeof (siginfo_er))
+    len = sizeof (siginfo_er) - offset;
+
+  memcpy (readbuf, (char *) &siginfo_er + offset, len);
+  *xfered_len = len;
+
+  return TARGET_XFER_OK;
+}
+
 enum target_xfer_status
 windows_nat_target::xfer_partial (enum target_object object,
 				  const char *annex, gdb_byte *readbuf,
@@ -3011,6 +3039,9 @@ windows_nat_target::xfer_partial (enum target_object object,
       return windows_xfer_shared_libraries (this, object, annex, readbuf,
 					    writebuf, offset, len, xfered_len);
 
+    case TARGET_OBJECT_SIGNAL_INFO:
+      return windows_xfer_siginfo (readbuf, offset, len, xfered_len);
+
     default:
       if (beneath () == NULL)
 	{