gdb: Avoid signed integer overflow when printing source lines

When printing source lines with calls to print_source_lines we need to
pass a start line number and an end line number.  The end line number
is calculated by calling get_lines_to_list and adding this value to
the start line number.  For example this code from list_command:

    print_source_lines (cursal.symtab, first,
                        first + get_lines_to_list (), 0);

The problem is that get_lines_to_list returns a value based on the
GDB setting `set listsize LISTSIZE`.  By default LISTSIZE is 10,
however, its also possible to set LISTSIZE to unlimited, in which
case get_lines_to_list will return INT_MAX.

As the parameter signature for print_source_lines is:

  void print_source_lines (struct symtab *, int, int,
                           print_source_lines_flags);

and `first` in the above code is an `int`, then when LISTSIZE is
`unlimited` the above code will result in signed integer overflow,
which is undefined.

The solution in this patch is a new class source_lines_range that can
be constructed from a single line number and a direction (forward or
backward).  The range is then constructed from the line number and the
value of get_lines_to_list.

gdb/ChangeLog:

	* cli/cli-cmds.c (list_command): Pass a source_lines_range to
	print_source_lines.
	* source.c (print_source_lines_base): Update line number check.
	(print_source_lines): New function.
	(source_lines_range::source_lines_range): New function.
	* source.h (class source_lines_range): New class.
	(print_source_lines): New declaration.

4 files changed, 115 insertions, 26 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 7d6d597..c89c86b 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,13 @@
+2019-01-09  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* cli/cli-cmds.c (list_command): Pass a source_lines_range to
+	print_source_lines.
+	* source.c (print_source_lines_base): Update line number check.
+	(print_source_lines): New function.
+	(source_lines_range::source_lines_range): New function.
+	* source.h (class source_lines_range): New class.
+	(print_source_lines): New declaration.
+
 2019-01-09  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
 
 	* linespec.c (linespec_state_destructor): Free self->canonical_names.
diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
index 9e09c05..57cfad4 100644
--- a/gdb/cli/cli-cmds.c
+++ b/gdb/cli/cli-cmds.c
@@ -895,14 +895,13 @@ list_command (const char *arg, int from_tty)
 	      && get_lines_to_list () == 1 && first > 1)
 	    first -= 1;
 
-	  print_source_lines (cursal.symtab, first,
-			      first + get_lines_to_list (), 0);
+	  print_source_lines (cursal.symtab, source_lines_range (first), 0);
 	}
 
       /* "l" or "l +" lists next ten lines.  */
       else if (arg == NULL || arg[0] == '+')
-	print_source_lines (cursal.symtab, cursal.line,
-			    cursal.line + get_lines_to_list (), 0);
+	print_source_lines (cursal.symtab,
+			    source_lines_range (cursal.line), 0);
 
       /* "l -" lists previous ten lines, the ones before the ten just
 	 listed.  */
@@ -911,10 +910,9 @@ list_command (const char *arg, int from_tty)
 	  if (get_first_line_listed () == 1)
 	    error (_("Already at the start of %s."),
 		   symtab_to_filename_for_display (cursal.symtab));
-	  print_source_lines (cursal.symtab,
-			      std::max (get_first_line_listed ()
-					- get_lines_to_list (), 1),
-			      get_first_line_listed (), 0);
+	  source_lines_range range (get_first_line_listed (),
+				    source_lines_range::BACKWARD);
+	  print_source_lines (cursal.symtab, range, 0);
 	}
 
       return;
@@ -1059,9 +1057,11 @@ list_command (const char *arg, int from_tty)
   if (dummy_beg && sal_end.symtab == 0)
     error (_("No default source file yet.  Do \"help list\"."));
   if (dummy_beg)
-    print_source_lines (sal_end.symtab,
-			std::max (sal_end.line - (get_lines_to_list () - 1), 1),
-			sal_end.line + 1, 0);
+    {
+      source_lines_range range (sal_end.line + 1,
+				source_lines_range::BACKWARD);
+      print_source_lines (sal_end.symtab, range, 0);
+    }
   else if (sal.symtab == 0)
     error (_("No default source file yet.  Do \"help list\"."));
   else if (no_end)
@@ -1074,17 +1074,14 @@ list_command (const char *arg, int from_tty)
 	    first_line = 1;
 	  if (sals.size () > 1)
 	    print_sal_location (sal);
-	  print_source_lines (sal.symtab,
-			      first_line,
-			      first_line + get_lines_to_list (),
-			      0);
+	  print_source_lines (sal.symtab, source_lines_range (first_line), 0);
 	}
     }
+  else if (dummy_end)
+    print_source_lines (sal.symtab, source_lines_range (sal.line), 0);
   else
-    print_source_lines (sal.symtab, sal.line,
-			(dummy_end
-			 ? sal.line + get_lines_to_list ()
-			 : sal_end.line + 1),
+    print_source_lines (sal.symtab,
+			source_lines_range (sal.line, (sal_end.line + 1)),
 			0);
 }
 
diff --git a/gdb/source.c b/gdb/source.c
index f865c8a..1f10379 100644
--- a/gdb/source.c
+++ b/gdb/source.c
@@ -1331,13 +1331,8 @@ print_source_lines_base (struct symtab *s, int line, int stopline,
   last_source_error = 0;
 
   /* If the user requested a sequence of lines that seems to go backward
-     (from high to low line numbers) then we don't print anything.
-     The use of '- 1' here instead of '<=' is currently critical, we rely
-     on the undefined wrap around behaviour of 'int' for stopline.  When
-     the use has done: 'set listsize unlimited' then stopline can overflow
-     and appear as MIN_INT.  This is a long-standing bug that needs
-     fixing.  */
-  if (stopline - 1 < line)
+     (from high to low line numbers) then we don't print anything.  */
+  if (stopline <= line)
     return;
 
   std::string lines;
@@ -1399,6 +1394,18 @@ print_source_lines (struct symtab *s, int line, int stopline,
 {
   print_source_lines_base (s, line, stopline, flags);
 }
+
+/* See source.h.  */
+
+void
+print_source_lines (struct symtab *s, source_lines_range line_range,
+		    print_source_lines_flags flags)
+{
+  print_source_lines_base (s, line_range.startline (),
+			   line_range.stopline (), flags);
+}
+
+
 
 /* Print info on range of pc's in a specified line.  */
 
@@ -1822,6 +1829,33 @@ set_substitute_path_command (const char *args, int from_tty)
   forget_cached_source_info ();
 }
 
+/* See source.h.  */
+
+source_lines_range::source_lines_range (int startline,
+					source_lines_range::direction dir)
+{
+  if (dir == source_lines_range::FORWARD)
+    {
+      LONGEST end = static_cast <LONGEST> (startline) + get_lines_to_list ();
+
+      if (end > INT_MAX)
+	end = INT_MAX;
+
+      m_startline = startline;
+      m_stopline = static_cast <int> (end);
+    }
+  else
+    {
+      LONGEST start = static_cast <LONGEST> (startline) - get_lines_to_list ();
+
+      if (start < 1)
+	start = 1;
+
+      m_startline = static_cast <int> (start);
+      m_stopline = startline;
+    }
+}
+
 
 void
 _initialize_source (void)
diff --git a/gdb/source.h b/gdb/source.h
index fcd83da..f1b5f6e 100644
--- a/gdb/source.h
+++ b/gdb/source.h
@@ -157,6 +157,54 @@ DEF_ENUM_FLAGS_TYPE (enum print_source_lines_flag, print_source_lines_flags);
 extern void print_source_lines (struct symtab *s, int line, int stopline,
 				print_source_lines_flags flags);
 
+/* Wrap up the logic to build a line number range for passing to
+   print_source_lines when using get_lines_to_list.  An instance of this
+   class can be built from a single line number and a direction (forward or
+   backward) the range is then computed using get_lines_to_list.  */
+class source_lines_range
+{
+public:
+  /* When constructing the range from a single line number, does the line
+     range extend forward, or backward.  */
+  enum direction
+  {
+   FORWARD,
+   BACKWARD
+  };
+
+  /* Construct a SOURCE_LINES_RANGE starting at STARTLINE and extending in
+   direction DIR.  The number of lines is from GET_LINES_TO_LIST.  If the
+   direction is backward then the start is actually (STARTLINE -
+   GET_LINES_TO_LIST).  There is also logic in place to ensure the start
+   is always 1 or more, and the end will be at most INT_MAX.  */
+  explicit source_lines_range (int startline, direction dir = FORWARD);
+
+  /* Construct a SOURCE_LINES_RANGE from STARTLINE to STOPLINE.  */
+  explicit source_lines_range (int startline, int stopline)
+    : m_startline (startline),
+      m_stopline (stopline)
+  { /* Nothing.  */ }
+
+  /* Return the line to start listing from.  */
+  int startline () const
+  { return m_startline; }
+
+  /* Return the line after the last line that should be listed.  */
+  int stopline () const
+  { return m_stopline; }
+
+private:
+
+  /* The start and end of the range.  */
+  int m_startline;
+  int m_stopline;
+};
+
+/* Variation of previous print_source_lines that takes a range instead of a
+   start and end line number.  */
+extern void print_source_lines (struct symtab *s, source_lines_range r,
+				print_source_lines_flags flags);
+
 /* Forget line positions and file names for the symtabs in a
    particular objfile.  */
 extern void forget_cached_source_info_for_objfile (struct objfile *);