Re: [PATCH] async_die_is_recursing: fix use of pthread_getspecific for Fedora

From: Carlo Arenas <carenas@gmail.com>
Date: Nov 4, 2021, 02:23 UTC
Message-ID: <CAPUEspjw4zypiywCBRc=y9uC4G5CKTE35GLVf_wDY436oO0C5w@mail.gmail.com>
In-Reply-To: <pull.1072.git.1635990465854.gitgitgadget@gmail.com>

On Wed, Nov 3, 2021 at 6:48 PM Victoria Dye via GitGit

> The fix imitates a workaround added in SELinux [2] by using the pointer to
> `ret` as the second argument to `pthread_getspecific`.

the SELinux workaround uses a valid global pointer though; while this won't

Show 10 quoted lines

> diff --git a/run-command.c b/run-command.c
> index 7ef5cc712a9..a82cf69e7d3 100644
> --- a/run-command.c
> +++ b/run-command.c
> @@ -1099,7 +1099,7 @@ static NORETURN void die_async(const char *err, va_list params)
>  static int async_die_is_recursing(void)
>  {
>         void *ret = pthread_getspecific(async_die_counter);
> -       pthread_setspecific(async_die_counter, (void *)1);
> +       pthread_setspecific(async_die_counter, &ret); /* set to any non-NULL valid pointer */

I guess this would work, since the pointer is never dereferenced, but the use of (void *)1 was hacky, and this warning seems like the right time to make it less so.

Would a dynamically allocated pthread_local variable be a better option, or even a static global, since we don't care about its value so no need to worry about any races?

If neither, would MAP_FAILED also trigger the warning?

Carlo