linux-kernel - Re: [PATCH v2] livepatch: Reorder to use before freeing a pointer

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Date:   Mon, 21 Mar 2022 07:05:59 -0700
From:   Tom Rix <trix@...hat.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>, jpoimboe@...hat.com,
        jikos@...nel.org, mbenes@...e.cz, pmladek@...e.com,
        nathan@...nel.org, ndesaulniers@...gle.com
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v2] livepatch: Reorder to use before freeing a pointer


On 3/21/22 6:39 AM, Joe Lawrence wrote:
> On 3/19/22 9:51 PM, trix@...hat.com wrote:
>> From: Tom Rix <trix@...hat.com>
>>
>> Clang static analysis reports this issue
>> livepatch-shadow-fix1.c:113:2: warning: Use of
>>    memory after it is freed
>>    pr_info("%s: dummy @ %p, prevented leak @ %p\n",
>>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> The pointer is freed in the previous statement.
>> Reorder the pr_info to report before the free.
>>
>> Similar issue in livepatch-shadow-fix2.c
>>
>> Signed-off-by: Tom Rix <trix@...hat.com>
>> ---
>> v2: Fix similar issue in livepatch-shadow-fix2.c
>>
>>   samples/livepatch/livepatch-shadow-fix1.c | 2 +-
>>   samples/livepatch/livepatch-shadow-fix2.c | 2 +-
>>   2 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/samples/livepatch/livepatch-shadow-fix1.c b/samples/livepatch/livepatch-shadow-fix1.c
>> index 918ce17b43fda..6701641bf12d4 100644
>> --- a/samples/livepatch/livepatch-shadow-fix1.c
>> +++ b/samples/livepatch/livepatch-shadow-fix1.c
>> @@ -109,9 +109,9 @@ static void livepatch_fix1_dummy_leak_dtor(void *obj, void *shadow_data)
>>   	void *d = obj;
>>   	int **shadow_leak = shadow_data;
>>   
>> -	kfree(*shadow_leak);
>>   	pr_info("%s: dummy @ %p, prevented leak @ %p\n",
>>   			 __func__, d, *shadow_leak);
>> +	kfree(*shadow_leak);
>>   }
>>   
>>   static void livepatch_fix1_dummy_free(struct dummy *d)
>> diff --git a/samples/livepatch/livepatch-shadow-fix2.c b/samples/livepatch/livepatch-shadow-fix2.c
>> index 29fe5cd420472..361046a4f10cf 100644
>> --- a/samples/livepatch/livepatch-shadow-fix2.c
>> +++ b/samples/livepatch/livepatch-shadow-fix2.c
>> @@ -61,9 +61,9 @@ static void livepatch_fix2_dummy_leak_dtor(void *obj, void *shadow_data)
>>   	void *d = obj;
>>   	int **shadow_leak = shadow_data;
>>   
>> -	kfree(*shadow_leak);
>>   	pr_info("%s: dummy @ %p, prevented leak @ %p\n",
>>   			 __func__, d, *shadow_leak);
>> +	kfree(*shadow_leak);
>>   }
>>   
>>   static void livepatch_fix2_dummy_free(struct dummy *d)
>>
> Hi Tom,
>
> Ordering doesn't matter for the example, so let's clean up the static
> analysis.
>
> Acked-by: Joe Lawrence <joe.lawrence@...hat.com>
>
> But for my sanity, isn't this a false positive?  There shouldn't be harm
> in printing the pointer itself, even after what it points to has been
> freed, i.e.
>
> 	int *i = malloc(sizeof(*i));
> 	free(i);
> 	printf("%p\n", i);      << ok
> 	printf("%d\n", *i);     << NOT ok
>
> But I suppose clang doesn't know that the passed pointer isn't getting
> dereferenced by the function, so it throws up a warning?  Just curious
> what your experience has been with respect to these reports.

The analysis it good for static functions, for extern functions it has 
nothing to analyze so a worst case is assumed.

I agree this is likely a false positive.

Tom

>
> Thanks,