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

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Message-ID: <20200305081842.GB2619@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Mar 2020 09:18:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     akpm@...ux-foundation.org, broonie@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-next@...r.kernel.org, mhocko@...e.cz,
        mm-commits@...r.kernel.org, sfr@...b.auug.org.au,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        walter-zh.wu@...iatek.com, dvyukov@...gle.com
Subject: Re: mmotm 2020-03-03-22-28 uploaded (warning: objtool:)

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.

> +
> +	flags = user_access_save();
>  	__kasan_report(addr, size, is_write, ip);
>  	user_access_restore(flags);
> +
> +	return true;
>  }