[v2] nsswitch: do not reload if "/" changes

[Florian Weimer]

* DJ Delorie via Libc-alpha:

> +  /* Before we reload, verify that "/" hasn't changed.  We assume that
> +     errors here are very unlikely, but the chance that we're entering
> +     a container is also very unlikely, so we err on the side of both
> +     very unlikely things not happening at the same time.  */
> +  if (__stat64 ("/", &str) == 0)
> +    {
> +      if (local->root_ino != 0
> +	  && (str.st_ino != local->root_ino
> +	      ||  str.st_dev != local->root_dev))
> +	{
> +	  /* Change detected; disable reloading.  */
> +	  atomic_store_release (&local->data.reload_disabled, 1);
> +	  __libc_lock_unlock (local->lock);
> +	  __nss_module_disable_loading ();
> +	  return true;
> +	}
> +      local->root_ino = str.st_ino;
> +      local->root_dev = str.st_dev;
> +    }

I still think you should disable (re)loading if __stat64 fails.

>    __libc_lock_unlock (local->lock);
>  
>    /* Avoid overwriting the global configuration until we have loaded
> diff --git a/nss/nss_module.c b/nss/nss_module.c
> index 1a9359930d..6c5f341f3e 100644
> --- a/nss/nss_module.c
> +++ b/nss/nss_module.c
> @@ -349,6 +349,19 @@ __nss_disable_nscd (void (*cb) (size_t, struct traced_file *))
>  }
>  #endif
>  
> +/* Block attempts to dlopen any module we haven't already opened.  */
> +void
> +__nss_module_disable_loading (void)
> +{
> +  __libc_lock_lock (nss_module_list_lock);
> +
> +  for (struct nss_module *p = nss_module_list; p != NULL; p = p->next)
> +    if (p->state == nss_module_uninitialized)
> +      p->state = nss_module_failed;
> +
> +  __libc_lock_unlock (nss_module_list_lock);
> +}

Maybe call __nss_module_disable_loading after releasing local->lock?
Although there should not be a risk of deadlocks in the current code
because there aren't any relevant function calls while
nss_module_list_lock is locked.

Thanks,
Florian
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