Subject: Re: mmotm 2020-03-03-22-28 uploaded (warning: objtool:)

From: Walter Wu <[email-masked]>
Date: Wed, 11 Mar 2020 09:33:38 +0800

On Thu, 2020-03-05 at 10:54 +0100, Peter Zijlstra wrote:
> On Thu, Mar 05, 2020 at 05:16:22PM +0800, Walter Wu wrote:
> > On Thu, 2020-03-05 at 09:18 +0100, Peter Zijlstra wrote:
> > > On Thu, Mar 05, 2020 at 09:17:17AM +0100, Peter Zijlstra wrote:
> > > > On Wed, Mar 04, 2020 at 09:34:49AM -0800, Randy Dunlap wrote:
> > > 
> > > > > mm/kasan/common.o: warning: objtool: kasan_report()+0x13: call to report_enabled() with UACCESS enabled
> > > > 
> > > > I used next/master instead, and found the below broken commit
> > > > responsible for this.
> > > 
> > > > @@ -634,12 +637,20 @@ void kasan_free_shadow(const struct vm_struct *vm)
> > > >  #endif
> > > >  
> > > >  extern void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip);
> > > > +extern bool report_enabled(void);
> > > >  
> > > > -void kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
> > > > +bool kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
> > > >  {
> > > > -	unsigned long flags = user_access_save();
> > > > +	unsigned long flags;
> > > > +
> > > > +	if (likely(!report_enabled()))
> > > > +		return false;
> > > 
> > > This adds an explicit call before the user_access_save() and that is a
> > > straight on bug.
> > > 
> > Hi Peter,
> > 
> > Thanks for your help. Unfortunately, I don't reproduce it in our
> > environment, so I have asked Stephen, if I can reproduce it, then we
> > will send new patch.
> 
> The patch is trivial; and all you need is an x86_64 (cross) compiler to
> reproduce.
> 
> 
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index ad2dc0c9cc17..2906358e42f0 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -618,16 +618,17 @@ extern bool report_enabled(void);
>  
>  bool kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
>  {
> -	unsigned long flags;
> +	unsigned long flags = user_access_save();
> +	bool ret = false;
>  
> -	if (likely(!report_enabled()))
> -		return false;
> +	if (likely(report_enabled())) {
> +		__kasan_report(addr, size, is_write, ip);
> +		ret = true;
> +	}
>  
> -	flags = user_access_save();
> -	__kasan_report(addr, size, is_write, ip);
>  	user_access_restore(flags);
>  
> -	return true;
> +	return ret;
>  }
>  
>  #ifdef CONFIG_MEMORY_HOTPLUG

Reviewed-and-tested-by: Walter Wu <[email-masked]>

It need newer GCC compiler(>7.4) enough to reproduce.
Thanks.

Walter