On Thu, Jul 02, 2020 at 06:00:17PM -0700, Linus Torvalds wrote:
If somebody is interested in looking into things like that, it might be a good idea to have kernel threads with that counter incremented by default.
With 67 kthreads on a booted system, this patch does not immediately blow up... And it likely needs some beautification. (Note that current_cred_*() calls current_cred() under the hood, so AFAICT, only current_cred() needs coverage.)
diff --git a/include/linux/cred.h b/include/linux/cred.h index 18639c069263..a624847cb0ce 100644 --- a/include/linux/cred.h +++ b/include/linux/cred.h @@ -295,7 +295,10 @@ static inline void put_cred(const struct cred *_cred) * since nobody else can modify it. */ #define current_cred() \ - rcu_dereference_protected(current->cred, 1) +({ \ + WARN_ON_ONCE(current->warn_on_current_cred); \ + rcu_dereference_protected(current->cred, 1); \ +})
/** * current_real_cred - Access the current task's objective credentials diff --git a/include/linux/sched.h b/include/linux/sched.h index b62e6aaf28f0..21ab1b81aa40 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -652,6 +652,7 @@ struct task_struct { /* Per task flags (PF_*), defined further below: */ unsigned int flags; unsigned int ptrace; + unsigned int warn_on_current_cred;
#ifdef CONFIG_SMP struct llist_node wake_entry; diff --git a/kernel/fork.c b/kernel/fork.c index 142b23645d82..2e181b9bfd3f 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2527,8 +2527,12 @@ pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags) .stack = (unsigned long)fn, .stack_size = (unsigned long)arg, }; + pid_t pid;
- return _do_fork(&args); + pid = _do_fork(&args); + if (pid == 0) + current->warn_on_current_cred = 1; + return pid; }
#ifdef __ARCH_WANT_SYS_FORK