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

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2023-02-16 at 22:48 -0500, Siddhesh Poyarekar wrote:
> On 2023-02-16 10:15, David Malcolm via Gcc wrote:
> > I'm not convinced that it's useful to the end-user to warn about
> > the
> > "use of q itself" case.
> 
> FWIW, -Wuse-after-free=3 already should do this:
> 
> At level 3, the warning also diagnoses uses of indeterminate pointers
> in 
> equality expressions.  All uses of indeterminate pointers are
> undefined 
> but equality tests sometimes appear after
> calls to "realloc" as an attempt to determine whether the call
> resulted 
> in relocating the object to a different address.  They are diagnosed
> at 
> a separate level to aid legacy code gradually
> transition to safe alternatives.

This is possibly a silly question, but what *are* these safe
alternatives? [1] How does one test to see if an object has been
reallocated?

Dave
[1] Would suggesting "rust" here be too snarky? :-P

>   For example, the equality test in the 
> function below is diagnosed at this level:
> 
> Jakub and I had discussed this in the context of _FORTIFY_SOURCE=3 
> (which is anal about this and can break things) and we got pr#105217,
> but that is also a best-effort thing, not really a guarantee.
> 
> IMO the analyzer should go that extra mile and warn for the use of q 
> itself and maybe deprecate -Wuse-after-free=3 in its favour.
> 
> Sid
>