Re: [PATCH] module: Fix missing to hold module_mutex lock in module_kallsyms_lookup_name

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 28, 2015 at 10:01:35AM +0930, Rusty Russell wrote:
> Peter?

> module: weaken locking assertion for oops path.
> 
> We don't actually hold the module_mutex when calling find_module_all
> from module_kallsyms_lookup_name: that's because it's used by the oops
> code and we don't want to deadlock.
> 
> However, access to the list read-only is safe if preempt is disabled,
> so we can weaken the assertion.  Keep a strong version for external
> callers though.
> 
> Fixes: 0be964be0d45 ("module: Sanitize RCU usage and locking")
> Reported-by: He Kuang <hekuang@huawei.com>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 4d2b82e610e2..b86b7bf1be38 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -602,13 +602,16 @@ const struct kernel_symbol *find_symbol(const char *name,
>  }
>  EXPORT_SYMBOL_GPL(find_symbol);
>  
> -/* Search for module by name: must hold module_mutex. */
> +/*
> + * Search for module by name: must hold module_mutex (or preempt disabled
> + * for read-only access).
> + */
>  static struct module *find_module_all(const char *name, size_t len,
>  				      bool even_unformed)
>  {
>  	struct module *mod;
>  
> -	module_assert_mutex();
> +	module_assert_mutex_or_preempt();

Yeah, that should be fine indeed, I went by that comment you just
expanded.

The operation itself does indeed not modify data at all, so the
preempt_disable is perfectly adequate.

Thanks!

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>