Re: Missed warning (-Wuse-after-free)

[Martin Uecker <uecker@tugraz.at>]

Am Donnerstag, dem 23.02.2023 um 19:21 -0600 schrieb Serge E. Hallyn:
> On Fri, Feb 24, 2023 at 01:02:54AM +0100, Alex Colomar wrote:
> > Hi Martin,
> > 
> > On 2/23/23 20:57, Martin Uecker wrote:
> > > Am Donnerstag, dem 23.02.2023 um 20:23 +0100 schrieb Alex Colomar:
> > > > Hi Martin,
> > > > 
> > > > On 2/17/23 14:48, Martin Uecker wrote:
> > > > > > This new wording doesn't even allow one to use memcmp(3);
> > > > > > just reading the pointer value, however you do it, is UB.
> > > > > 
> > > > > memcmp would not use the pointer value but work
> > > > > on the representation bytes and is still allowed.
> > > > 
> > > > Hmm, interesting.  It's rather unspecified behavior. Still
> > > > unpredictable: (memcmp(&p, &p, sizeof(p) == 0) might evaluate to true or
> > > > false randomly; the compiler may compile out the call to memcmp(3),
> > > > since it knows it won't produce any observable behavior.
> > > > 
> > > > <https://software.codidact.com/posts/287905>
> > > 
> > > No, I think several things get mixed up here.
> > > 
> > > The representation of a pointer that becomes invalid
> > > does not change.
> > > 
> > > So (0 === memcmp(&p, &p, sizeof(p)) always
> > > evaluates to true.
> > > 
> > > Also in general, an unspecified value is simply unspecified
> > > but does not change anymore.
> 
> Right.  p is its own thing - n bytes on the stack containing some value.
> Once it comes into scope, it doesn't change on its own.  And if I do
> free(p) or o = realloc(p), then the value of p itself - the n bytes on
> the stack - does not change.

Yes, but one comment about terminology:. The C standard
differentiates between the representation, i.e. the bytes on
the stack, and the value.  The representation is converted to
a value during lvalue conversion.  For an invalid pointer
the representation is indeterminate because it now does not
point to a valid object anymore.  So it is not possible to
convert the representation to a value during lvalue conversion.
In other words, it does not make sense to speak of the value
of the pointer anymore.

> I realize C11 appears to have changed that.  I fear that in doing so it
> actually risks increasing the confusion about pointers.  IMO it's much
> easier to reason about
> 
> 	o = realloc(p, X);
> 
> (and more baroque constructions) when keeping in mind that o, p, and the
> object pointed to by either one are all different things.
> 

What did change in C11? As far as I know, the pointer model
did not change in C11.

> > > Reading an uninitialized value of automatic storage whose
> > > address was not taken is undefined behavior, so everything
> > > is possible afterwards.
> > > 
> > > An uninitialized variable whose address was taken has a
> > > representation which can represent an unspecified value
> > > or a no-value (trap) representation. Reading the
> > > representation itself is always ok and gives consistent
> > > results. Reading the variable can be undefined behavior
> > > iff it is a trap representation, otherwise you get
> > > the unspecified value which is stored there.
> > > 
> > > At least this is my reading of the C standard. Compilers
> > > are not full conformant.
> > 
> > Does all this imply that the following is well defined behavior (and shall
> > print what one would expect)?
> > 
> >   free(p);
> > 
> >   (void) &p;  // take the address
> >   // or maybe we should (void) memcmp(&p, &p, sizeof(p)); ?
> > 
> >   printf("%p\n", p);  // we took previously its address,
> >                       // so now it has to hold consistently
> >                       // the previous value
> > 
> > 

No, the printf is not well defined, because the lvalue conversion
of the pointer with indeterminate representation may lead to
undefined behavior.


Martin


> > This feels weird.  And a bit of a Schroedinger's pointer.  I'm not entirely
> > convinced, but might be.
> 
> Again, p is just an n byte variable which happens to have (one hopes)
> pointed at a previously malloc'd address.
> 
> And I'd argue that pre-C11, this was not confusing, and would not have
> felt weird to you.
> 
> But I am most grateful to you for having brought this to my attention.
> I may not agree with it and not like it, but it's right there in the
> spec, so time for me to adjust :)
>