[Ada] Integer overflow in SS_Allocate

This patch imposes a new check and rewrites existing ones to ensure operations
involving SS_Ptr do not cause an Integer overflow. The Default_Sec_Stack_Size
function was removed in the process to simplify System.Parameter.

SS_Ptr was derived from the integer System.Parameters.Size_Type to ease the
creation of objects of type SS_Stack by the binder and imposes a maximum
secondary stack size of 2GB. In most cases, the user will not hit this limit as
they cannot specify task stack sizes of more than 2GB via the Storage_Size and
Secondary_Stack_Size pragmas. Additionally, most operating systems limit the
primary stack size to less than 2GB, with defaults under 10MB. Linux is the
rare exception where the user can unbound the primary stack.

Executing the following:
gnatmake -q overflow
./overflow

must yield:

raised STORAGE_ERROR : s-secsta.adb:140 explicit raise

--  overflow.adb:

with String_Pack;

procedure Overflow is
begin
   null;
end Overflow;

-- string_pack.ads:

package String_Pack is
   function Return_Big_String return String;
end String_Pack;

-- string_pack.adb:

with Ada.Strings.Fixed; use Ada.Strings.Fixed;

package body String_Pack is
   function Return_Big_String return String is
   begin
      return Integer'Last * "P";
   end Return_Big_String;

   S : String := Return_Big_String;

end String_Pack;

2018-01-11  Patrick Bernardi  <bernardi@adacore.com>

gcc/ada/

	* libgnat/s-parame*.adb, libgnat/s-parame*.ads: Remove unneeded
	Default_Sec_Stack_Size.
	* libgnat/s-secsta.adb (SS_Allocate): Handle the fixed secondary stack
	limit check so that the integer index does not overflow. Check the
	dynamic stack allocation does not cause the secondary stack pointer to
	overflow.
	(SS_Info): Align colons.
	(SS_Init): Cover the case when bootstraping with an old compiler that
	does not set Default_SS_Size.

From-SVN: r256492

8 files changed, 52 insertions, 95 deletions

diff --git a/gcc/ada/libgnat/s-parame.adb b/gcc/ada/libgnat/s-parame.adb
index 359edac..0f4d45f 100644
--- a/gcc/ada/libgnat/s-parame.adb
+++ b/gcc/ada/libgnat/s-parame.adb
@@ -50,34 +50,6 @@ package body System.Parameters is
       end if;
    end Adjust_Storage_Size;
 
-   ----------------------------
-   -- Default_Sec_Stack_Size --
-   ----------------------------
-
-   function Default_Sec_Stack_Size return Size_Type is
-      Default_SS_Size : Integer;
-      pragma Import (C, Default_SS_Size,
-                     "__gnat_default_ss_size");
-   begin
-      --  There are two situations where the default secondary stack size is
-      --  set to zero:
-      --
-      --    * The user sets it to zero erroneously thinking it will disable
-      --      the secondary stack.
-      --
-      --    * Or more likely, we are building with an old compiler and
-      --      Default_SS_Size is never set.
-      --
-      --  In both case set the default secondary stack size to the run-time
-      --  default.
-
-      if Default_SS_Size > 0 then
-         return Size_Type (Default_SS_Size);
-      else
-         return Runtime_Default_Sec_Stack_Size;
-      end if;
-   end Default_Sec_Stack_Size;
-
    ------------------------
    -- Default_Stack_Size --
    ------------------------
diff --git a/gcc/ada/libgnat/s-parame.ads b/gcc/ada/libgnat/s-parame.ads
index 60a5e99..6cad3e7 100644
--- a/gcc/ada/libgnat/s-parame.ads
+++ b/gcc/ada/libgnat/s-parame.ads
@@ -93,10 +93,6 @@ package System.Parameters is
    --  The run-time chosen default size for secondary stacks that may be
    --  overriden by the user with the use of binder -D switch.
 
-   function Default_Sec_Stack_Size return Size_Type;
-   --  The default initial size for secondary stacks that reflects any user
-   --  specified default via the binder -D switch.
-
    Sec_Stack_Dynamic : constant Boolean := True;
    --  Indicates if secondary stacks can grow and shrink at run-time. If False,
    --  the size of a secondary stack is fixed at the point of its creation.
diff --git a/gcc/ada/libgnat/s-parame__ae653.ads b/gcc/ada/libgnat/s-parame__ae653.ads
index 42d438e..68da0c9 100644
--- a/gcc/ada/libgnat/s-parame__ae653.ads
+++ b/gcc/ada/libgnat/s-parame__ae653.ads
@@ -93,10 +93,6 @@ package System.Parameters is
    --  The run-time chosen default size for secondary stacks that may be
    --  overriden by the user with the use of binder -D switch.
 
-   function Default_Sec_Stack_Size return Size_Type;
-   --  The default size for secondary stacks that reflects any user specified
-   --  default via the binder -D switch.
-
    Sec_Stack_Dynamic : constant Boolean := False;
    --  Indicates if secondary stacks can grow and shrink at run-time. If False,
    --  the size of a secondary stack is fixed at the point of its creation.
diff --git a/gcc/ada/libgnat/s-parame__hpux.ads b/gcc/ada/libgnat/s-parame__hpux.ads
index 846b1655..e8ced87 100644
--- a/gcc/ada/libgnat/s-parame__hpux.ads
+++ b/gcc/ada/libgnat/s-parame__hpux.ads
@@ -91,10 +91,6 @@ package System.Parameters is
    --  The run-time chosen default size for secondary stacks that may be
    --  overriden by the user with the use of binder -D switch.
 
-   function Default_Sec_Stack_Size return Size_Type;
-   --  The default initial size for secondary stacks that reflects any user
-   --  specified default via the binder -D switch.
-
    Sec_Stack_Dynamic : constant Boolean := True;
    --  Indicates if secondary stacks can grow and shrink at run-time. If False,
    --  the size of a secondary stack is fixed at the point of its creation.
diff --git a/gcc/ada/libgnat/s-parame__rtems.adb b/gcc/ada/libgnat/s-parame__rtems.adb
index 5a19c43..551eb36 100644
--- a/gcc/ada/libgnat/s-parame__rtems.adb
+++ b/gcc/ada/libgnat/s-parame__rtems.adb
@@ -56,18 +56,6 @@ package body System.Parameters is
       end if;
    end Adjust_Storage_Size;
 
-   ----------------------------
-   -- Default_Sec_Stack_Size --
-   ----------------------------
-
-   function Default_Sec_Stack_Size return Size_Type is
-      Default_SS_Size : Integer;
-      pragma Import (C, Default_SS_Size,
-                     "__gnat_default_ss_size");
-   begin
-      return Size_Type (Default_SS_Size);
-   end Default_Sec_Stack_Size;
-
    ------------------------
    -- Default_Stack_Size --
    ------------------------
diff --git a/gcc/ada/libgnat/s-parame__vxworks.adb b/gcc/ada/libgnat/s-parame__vxworks.adb
index 97d74b6..325aa2e 100644
--- a/gcc/ada/libgnat/s-parame__vxworks.adb
+++ b/gcc/ada/libgnat/s-parame__vxworks.adb
@@ -48,18 +48,6 @@ package body System.Parameters is
       end if;
    end Adjust_Storage_Size;
 
-   ----------------------------
-   -- Default_Sec_Stack_Size --
-   ----------------------------
-
-   function Default_Sec_Stack_Size return Size_Type is
-      Default_SS_Size : Integer;
-      pragma Import (C, Default_SS_Size,
-                     "__gnat_default_ss_size");
-   begin
-      return Size_Type (Default_SS_Size);
-   end Default_Sec_Stack_Size;
-
    ------------------------
    -- Default_Stack_Size --
    ------------------------
diff --git a/gcc/ada/libgnat/s-parame__vxworks.ads b/gcc/ada/libgnat/s-parame__vxworks.ads
index e395e01..ab5a085 100644
--- a/gcc/ada/libgnat/s-parame__vxworks.ads
+++ b/gcc/ada/libgnat/s-parame__vxworks.ads
@@ -93,10 +93,6 @@ package System.Parameters is
    --  The run-time chosen default size for secondary stacks that may be
    --  overriden by the user with the use of binder -D switch.
 
-   function Default_Sec_Stack_Size return Size_Type;
-   --  The default initial size for secondary stacks that reflects any user
-   --  specified default via the binder -D switch.
-
    Sec_Stack_Dynamic : constant Boolean := True;
    --  Indicates if secondary stacks can grow and shrink at run-time. If False,
    --  the size of a secondary stack is fixed at the point of its creation.
diff --git a/gcc/ada/libgnat/s-secsta.adb b/gcc/ada/libgnat/s-secsta.adb
index b39cf0d..84d6095 100644
--- a/gcc/ada/libgnat/s-secsta.adb
+++ b/gcc/ada/libgnat/s-secsta.adb
@@ -52,27 +52,40 @@ package body System.Secondary_Stack is
      (Addr         : out Address;
       Storage_Size : SSE.Storage_Count)
    is
+      use type System.Storage_Elements.Storage_Count;
+
       Max_Align   : constant SS_Ptr := SS_Ptr (Standard'Maximum_Alignment);
-      Mem_Request : constant SS_Ptr :=
-                      ((SS_Ptr (Storage_Size) + Max_Align - 1) / Max_Align) *
-                        Max_Align;
-      --  Round up Storage_Size to the nearest multiple of the max alignment
-      --  value for the target. This ensures efficient stack access.
+      Mem_Request : SS_Ptr;
 
-      Stack : constant SS_Stack_Ptr := SSL.Get_Sec_Stack.all;
+      Stack       : constant SS_Stack_Ptr := SSL.Get_Sec_Stack.all;
    begin
+      --  Round up Storage_Size to the nearest multiple of the max alignment
+      --  value for the target. This ensures efficient stack access. First
+      --  perform a check to ensure that the rounding operation does not
+      --  overflow SS_Ptr.
+
+      if SSE.Storage_Count (SS_Ptr'Last) - Standard'Maximum_Alignment <
+        Storage_Size
+      then
+         raise Storage_Error;
+      end if;
+
+      Mem_Request := ((SS_Ptr (Storage_Size) + Max_Align - 1) / Max_Align) *
+                       Max_Align;
+
       --  Case of fixed secondary stack
 
       if not SP.Sec_Stack_Dynamic then
          --  Check if max stack usage is increasing
 
-         if Stack.Top + Mem_Request > Stack.Max then
+         if Stack.Max - Stack.Top - Mem_Request < 0 then
 
             --  If so, check if the stack is exceeded, noting Stack.Top points
             --  to the first free byte (so the value of Stack.Top on a fully
-            --  allocated stack will be Stack.Size + 1).
+            --  allocated stack will be Stack.Size + 1). The comparison is
+            --  formed to prevent integer overflows.
 
-            if Stack.Top + Mem_Request > Stack.Size + 1 then
+            if Stack.Size - Stack.Top - Mem_Request < -1 then
                raise Storage_Error;
             end if;
 
@@ -90,8 +103,8 @@ package body System.Secondary_Stack is
 
       else
          declare
-            Chunk : Chunk_Ptr;
-
+            Chunk                : Chunk_Ptr;
+            Chunk_Size           : SS_Ptr;
             To_Be_Released_Chunk : Chunk_Ptr;
 
          begin
@@ -108,9 +121,8 @@ package body System.Secondary_Stack is
             --  sufficient, if not, go to the next one and eventually create
             --  the necessary room.
 
-            while Chunk.Last - Stack.Top + 1 < Mem_Request loop
+            while Chunk.Last - Stack.Top - Mem_Request < -1 loop
                if Chunk.Next /= null then
-
                   --  Release unused non-first empty chunk
 
                   if Chunk.Prev /= null and then Chunk.First = Stack.Top then
@@ -121,24 +133,29 @@ package body System.Secondary_Stack is
                      Free (To_Be_Released_Chunk);
                   end if;
 
-               --  Create new chunk of default size unless it is not sufficient
-               --  to satisfy the current request.
+               --  Create a new chunk
 
-               elsif Mem_Request <= Stack.Size then
-                  Chunk.Next :=
-                    new Chunk_Id
-                      (First => Chunk.Last + 1,
-                       Last  => Chunk.Last + SS_Ptr (Stack.Size));
+               else
+                  --  The new chunk should be no smaller than the default
+                  --  chunk size to minimize the amount of secondary stack
+                  --  management.
+
+                  if Mem_Request <= Stack.Size then
+                     Chunk_Size := Stack.Size;
+                  else
+                     Chunk_Size := Mem_Request;
+                  end if;
 
-                  Chunk.Next.Prev := Chunk;
+                  --  Check that the indexing limits are not exceeded
 
-               --  Otherwise create new chunk of requested size
+                  if SS_Ptr'Last - Chunk.Last - Chunk_Size < 0 then
+                     raise Storage_Error;
+                  end if;
 
-               else
                   Chunk.Next :=
                     new Chunk_Id
                       (First => Chunk.Last + 1,
-                       Last  => Chunk.Last + Mem_Request);
+                       Last  => Chunk.Last + Chunk_Size);
 
                   Chunk.Next.Prev := Chunk;
                end if;
@@ -267,10 +284,10 @@ package body System.Secondary_Stack is
                       & SS_Ptr'Image (Stack.Top - 1)
                       & " bytes");
 
-            Put_Line ("  Number of Chunks       : "
+            Put_Line ("  Number of Chunks        : "
                       & Integer'Image (Nb_Chunks));
 
-            Put_Line ("  Default size of Chunks : "
+            Put_Line ("  Default size of Chunks  : "
                       & SP.Size_Type'Image (Stack.Size));
          end;
       end if;
@@ -300,7 +317,15 @@ package body System.Secondary_Stack is
 
       if Stack = null then
          if Size = Unspecified_Size then
-            Stack_Size := Default_Sec_Stack_Size;
+            --  Cover the case when bootstraping with an old compiler that does
+            --  not set Default_SS_Size.
+
+            if Default_SS_Size > 0 then
+               Stack_Size := Default_SS_Size;
+            else
+               Stack_Size := Runtime_Default_Sec_Stack_Size;
+            end if;
+
          else
             Stack_Size := Size;
          end if;