linux-kernel - Re: [PATCH] livepatch: fix size mismatch

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Date:   Wed, 23 Jan 2019 15:58:21 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Nicholas Mc Guire <hofrat@...dl.org>
cc:     Josh Poimboeuf <jpoimboe@...hat.com>, Jessica Yu <jeyu@...nel.org>,
        Jiri Kosina <jikos@...nel.org>, Petr Mladek <pmladek@...e.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] livepatch: fix size mismatch

On Wed, 23 Jan 2019, Nicholas Mc Guire wrote:

> kzalloc(sizeof(int)) is called for an int object but then
> passed into klp_shadow_alloc() using the size of the pointer.
> This probably is not a problem as it will fit - but it should
> be cleaned (after all this is reference code). 
> 
> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
> ---
> 
> Reported by coccicheck
> samples/livepatch//livepatch-shadow-fix1.c:97:30-36: ERROR: application of sizeof to pointer
> 
> Patch was compile tested with: x86_64_defconfig + FTRACE=y,
> FUNCTION_TRACER=y, SAMPLES=y, LIVEPATCH=y SAMPLE_LIVEPATCH=m 
> 
> Patch is against 5.0-rc3 (localversion-next is next-20190123)
> 
>  samples/livepatch/livepatch-shadow-fix1.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/samples/livepatch/livepatch-shadow-fix1.c b/samples/livepatch/livepatch-shadow-fix1.c
> index a5a5cac..643ffd5 100644
> --- a/samples/livepatch/livepatch-shadow-fix1.c
> +++ b/samples/livepatch/livepatch-shadow-fix1.c
> @@ -94,7 +94,7 @@ struct dummy *livepatch_fix1_dummy_alloc(void)
>  		return NULL;
>  	}
>  
> -	klp_shadow_alloc(d, SV_LEAK, sizeof(leak), GFP_KERNEL,
> +	klp_shadow_alloc(d, SV_LEAK, sizeof(*leak), GFP_KERNEL,
>  			 shadow_leak_ctor, leak);

I think it is actually fine. We allocate something sizeof(int), but it is 
not important much. The leaked pointer is important. We attach the pointer 
as a shadow variable, so we can free it later. Thus, the size of the 
pointer is important.

Moreover, you're introducing this with the change:

samples/livepatch/livepatch-shadow-fix1.c:97:38: warning: expression using sizeof(void)

Miroslav