[Oberon] Serious type loophole in type case statements and a possible fix

[Andreas Pirklbauer]

This case below is now also caught by the modified compiler at [*]. A case
variable of a type case statement can now only be one of the following:

  1. Local variables (pointer or record)
  2. Pointer value parameters
  3. Record VAR parameters                                            
 
-ap


[*] http://github.com/andreaspirklbauer/Oberon-type-case-statement-without-loopholes


> When the compiler automatically generates a type guard (ORG.TypeTest)
> for all assignments to p in the CASE, shouldn’t that > catch your case?
> Br Jörg

> Am 30.10.2020 um 23:05 schrieb Michael Schierl <schierlm at gmx.de>: 

> 
> Hello Andreas, 
> 
> 
>> Am 30.10.2020 um 14:05 schrieb Andreas Pirklbauer: 
>> The official Oberon-07 compiler, as published at www.projectoberon.com, 
>> contains a serious type loophole in type case statements 
>> 
> 
> [...] 
> 
> 
>> but continues to ALLOW… 
>> 
>> 1. Local variables or parameters (value or VAR) parameters as case variables 
> 
> I think this can be exploited: 
> 
> MODULE TestB; 
> IMPORT Texts, Oberon; 
> 
> TYPE R0 = RECORD END; 
> R1 = RECORD (R0) fld1: SET END; 
> R2 = RECORD (R0) fld2: INTEGER END; 
> P0 = POINTER TO R0; 
> P1 = POINTER TO R1; 
> P2 = POINTER TO R2; 
> 
> VAR p0: P0; p1: P1; p2: P2; W: Texts.Writer; 
> 
> PROCEDURE checkA(VAR p: P0); 
> BEGIN 
> CASE p OF P1: 
> p0 := p2; 
> p.fld1 := {4,1} 
> END; 
> Texts.WriteInt(W, p2.fld2, 4); (*18*) 
> Texts.Append(Oberon.Log, W.buf) 
> END checkA; 
> 
> PROCEDURE Go*; 
> BEGIN p0 := p1; checkA(p0) 
> END Go; 
> 
> BEGIN NEW(p1); NEW(p2); Texts.OpenWriter(W) 
> END TestB. 
> 
> 
> Using a real (but harmless) type confusion from SET to INTEGER to prove 
> the point better :-) 
> 
> This can obviously can be more convoluted/sophisticated, and I can't 
> think of an easy way to fix it without disallowing POINTER VAR 
> parameters altogether. 
> 
> 
> Regards, 
> 
> 
> Michael 
> --