Re: [PATCH] http API: fix dangling pointer issue noted by GCC 12.0
On Wed, Jan 26 2022, Junio C Hamano wrote:
Show 37 quoted lines
> Junio C Hamano <gitster@pobox.com> writes:
>
>> I am puzzled by this error. The assignment is the only one that
>> assigns a real pointer to the .finished member, and until
>> finish_active_slot() is called on the slot, the loop would not
>> leave. I would understand the error if slot->finished is used after
>> the function returns to the caller, but I do not think it is the
>> case.
>
> IOW, I am wondering if this is a mistaken compiler that needs to be
> told not to raise a false warning.
>
> If the motivation behind the original "do not get fooled by a reused
> slot still working on somebody else's request---instead return when
> our request is done" was indeed what I speculated, then the pointer
> slot->finished when we leave this function should not matter to
> anybody. Would the following patch make the compiler realize that
> we never smuggle a local variable's address out of this function via
> a pointer in the structure?
>
> http.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git c/http.c w/http.c
> index 229da4d148..85437b1980 100644
> --- c/http.c
> +++ w/http.c
> @@ -1367,6 +1367,9 @@ void run_active_slot(struct active_request_slot *slot)
> select(max_fd+1, &readfds, &writefds, &excfds, &select_timeout);
> }
> }
> +
> + if (slot->finished == &finished)
> + slot->finished = NULL;
> }
>
> static void release_active_slot(struct active_request_slot *slot)
Yes, that does quiet it. The GCC warning is specifically about pointers
that survive the exit of the function. From the commit that added it to
gcc.git:
+ /* The use is one of a dangling pointer if a clobber of the variable
+ [the pointer points to] has not been found before the function exit
+ point. */
Ævar Arnfjörð Bjarmason <avarab@gmail.com>