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digitalmars.D.bugs - [Issue 2544] New: implicit const casting rules allow violations of const-safety

reply d-bugmail puremagic.com writes:
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           Summary: implicit const casting rules allow violations of const-
                    safety
           Product: D
           Version: 2.022
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Keywords: spec
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: lat7h virginia.edu


The const system allows const views of mutable data; however, when used with
enough levels of indirection, accidental mutable access of const data is also
possible.

The smallest example I have found is

----
const(real)[] constants = [3.14159265358979323844L, 2.71828182845904523536L];
real[][][] unconsted = [[[]]];        // create mutable data
const(real)[][][] unsafe = unconsted; // and a partially-constant view of it
unsafe[0] = [constants];              // place const data in the const view
unconsted[0][0][0] = 3.14L;           // simplify pi using the mutable view
----

This is obviously contrived, but several of these layers of indirection can be
achieved (less succinctly but more commonly) using ref parameters to methods
instead.

I think that it suffices to require most intermediate levels of const-ness to
be illegal; you can either have the original const-ness or a more-const formal
with at most (I think) 2 levels of mutable indirection remaining: 
    const(T[])[][] assigned from T[][][] is OK, 
    const(T)[][][] assigned from T[][][] is not OK.
I have not been able to prove two levels is safe, but I have also not been able
to construct a counterexample.


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Dec 27 2008
next sibling parent d-bugmail puremagic.com writes:
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I included an unnecessary level of indirection before.  A simpler example:
----
const(int)[] answers = [42];       // create const data
int[][] unconsted = [[]];          // create mutable data
const(int)[][] unsafe = unconsted; // and a partially-constant view of it
unsafe[0] = answers;               // place const data in the const view
unconsted[0][0] = 43;              // change const data
----
Thus, while "const(T)[] = T[]" is safe, "const(T)[][] = T[][]" is not; only a
single level of implicit indirection in the cast preserves constness.


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Jan 01 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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smjg iname.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |smjg iname.com





I've thought quite a bit about this and come up with what seems to be a
solution, which also deals with another problem that's been around for years. 
I've posted it on the newsgroup, as it's rather long for here.

"Possible D2 solution to the upcasting array problem, and a related problem
with const and nested arrays"
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=81566


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Jan 02 2009
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maxmo pochta.ru changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |DUPLICATE







*** This bug has been marked as a duplicate of 2095 ***


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Feb 18 2009
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Stewart Gordon <smjg iname.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla digitalmars.com



*** Issue 3621 has been marked as a duplicate of this issue. ***

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Dec 17 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Steven Schveighoffer <schveiguy yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
                 CC|                            |schveiguy yahoo.com
         Resolution|DUPLICATE                   |



05:11:24 PST ---
I'm going to reopen this.  Although it is technically a subset of bug 2095, it
is not a duplicate.  It is possible to fix this bug without fixing 2095.

It appears that a fix was inserted for 3621, repeated here for consistency:

http://www.dsource.org/projects/dmd/changeset/299

When the compiler is released with this fix, then this bug can be closed.

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Dec 17 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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 I'm going to reopen this.  Although it is technically a subset of bug 2095, it
 is not a duplicate.  It is possible to fix this bug without fixing 2095.
 
 It appears that a fix was inserted for 3621, repeated here for consistency:
 
 http://www.dsource.org/projects/dmd/changeset/299
 
 When the compiler is released with this fix, then this bug can be closed.


Does this change cover all cases discussed in the thread linked to in comment
2?

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Dec 17 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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05:31:50 PST ---


 
 Does this change cover all cases discussed in the thread linked to in comment
 2?


From looking at the code changes, it does not fix casts from a derived class
array to a base class array.  However, that issue is captured in bug 2095.  I
don't think this means 2095 is not an important bug to fix, but it might be
that this bug is *really* simple to fix, and 2095 is not (Walter would know
better).  I'd rather have this part of it fixed than neither part fixed.

In any case, the example in comment 1 and the original description should
theoretically be fixed by the change, so we can at least close *this* bug.  I
agree that something needs to be done about casting a derived class array to a
base class array, and it seems like the solution proposed in the thread would
work.

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20:19:39 PDT ---
It appears that the offending code still compiles in dmd 2.041.  I can't
remember why I thought the given changeset should fix the problem, maybe it was
fixed and then regressed.  In any case, it's still broken.

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Mar 16 2010
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10:36:19 PDT ---


 Although it is technically a subset of bug 2095


To be precise, it's theoretically a subset of 2095, but technically they are
different :)

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Apr 14 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Stewart Gordon <smjg iname.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         OS/Version|Linux                       |All



Since when has this issue been Linux-specific?

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Apr 14 2010
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Leandro Lucarella <llucax gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|D1 & D2                     |2.022



PDT ---
I'm sorry about the last version change, I accidentally changed it thinking I
was editing bug 3463 instead (I hate bugzilla for automatically moving to the
next bug report when committing changes).

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Jul 20 2010
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12:15:53 PST ---
Seems to be a regression, since according to bug 2056, this code didn't compile
in dmd 2.013.

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Nov 18 2010
prev sibling parent d-bugmail puremagic.com writes:
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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
                 CC|                            |yebblies gmail.com
         Resolution|                            |DUPLICATE



As far as I can tell, all of these cases are fixed by my patch to 4251.
It uses references as the highest indirection instead of arrays, but exposes
the same bug.

*** This issue has been marked as a duplicate of issue 4251 ***

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Jun 15 2011