Re: [PATCH v2 1/2] add -Wuse-after-free

[Jeff Law <jeffreyalaw@gmail.com>]

On 1/11/2022 3:40 PM, Jason Merrill wrote:
> On 11/30/21 17:32, Martin Sebor via Gcc-patches wrote:
>> Attached is a revised patch with the following changes based
>> on your comments:
>>
>> 1) Set and use statement uids to determine which statement
>>     precedes which in the same basic block.
>> 2) Avoid testing flag_isolate_erroneous_paths_dereference.
>> 3) Use post-dominance to decide whether to use the "maybe"
>>     phrasing vs a definite form.
>>
>> David raised (and in our offline discussion today reiterated)
>> an objection to the default setting of the option being
>> the strictest.  I have not changed that in this revision.
>> See my rationale for this choice in my reply below:
>> https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583176.html
>
> In the latest C2x draft I see in the list of undefined behavior
>
> "The value of a pointer that refers to space deallocated by a call to 
> the free or realloc function is used (7.22.3)."
>
> So the case that would be technically undefined would be comparing the 
> reallocated pointer to the old pointer which has been deallocated.
>
> The C++ draft is more nuanced: it says, "When the end of the duration 
> of a region of storage is reached, the values of all pointers 
> representing the address of any part of that region of storage become 
> invalid pointer values (6.8.3).  Indirection through an invalid 
> pointer value and passing an invalid pointer value to a deallocation 
> function have undefined behavior.  Any other use of an invalid pointer 
> value has implementation-defined behavior."
>
> So the case above is implementation-defined in C++, not undefined.
>
> Let's put =2 in -Wall for now.
>
> With that change, this and the -Wdangling-pointer patch are OK on 
> Friday afternoon if there are no other comments before then.
THanks for picking this up.  I've been busier than expected the last 
several weeks.

jeff