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SubjectRe: [PATCH v2] livepatch: Reorder to use before freeing a pointer
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On 3/21/22 6:39 AM, Joe Lawrence wrote:
> On 3/19/22 9:51 PM, trix@redhat.com wrote:
>> From: Tom Rix <trix@redhat.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@redhat.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@redhat.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,

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