Re: [PATCH v2 2/3] kernel-shark: Fix potential memory leak in libkshark-collection

["Yordan Karadzhov (VMware)" <y.karadz@gmail.com>]

On 27.11.19 г. 20:54 ч., Steven Rostedt wrote:
> On Wed, 23 Oct 2019 15:21:44 +0300
> "Yordan Karadzhov (VMware)" <y.karadz@gmail.com> wrote:
> 
>> When searching for the entry, do not loop over the original list of
>> requests. Use a copy instead. If we loop over the original list and
>> no entry is found in the first element of the list, later the memory
>> used for this first element will leak.
>>
>> Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
>> ---
>>   kernel-shark/src/libkshark-collection.c | 14 ++++++--------
>>   1 file changed, 6 insertions(+), 8 deletions(-)
>>
>> diff --git a/kernel-shark/src/libkshark-collection.c b/kernel-shark/src/libkshark-collection.c
>> index 02a014e..95fdbab 100644
>> --- a/kernel-shark/src/libkshark-collection.c
>> +++ b/kernel-shark/src/libkshark-collection.c
>> @@ -622,6 +622,7 @@ kshark_get_collection_entry_front(struct kshark_entry_request **req,
>>   				  ssize_t *index)
>>   {
>>   	const struct kshark_entry *entry = NULL;
>> +	struct kshark_entry_request *list;
> 
> Hi Yordan,
> 
> I was looking at this patch in more detail, and I'm thinking that we
> don't need to pass in the address of the req pointer, but just the req
> pointer itself. The only place that I see the req pointer being
> modified is the failure case in map_collection_request_init() where it
> does:
> 
> 	kshark_free_entry_request(*req);
> 	*req = NULL;
> 
> But all callers do that free anyway.
>

Yes, because the caller is expected to do 
kshark_free_entry_request(*req) at the end, here we have to set the 
original pointer to NULL. Otherwise we will get double free error.
I think this is what I have been trying to fix, when I introduced the 
memory leak.

And yes, I agree with you that carrying the address of the pointer 
through all these functions is a bit ugly.

Thanks!
Yordan

> Maybe I'm missing something, but why are we passing in the pointer to
> the pointer of req, and not just the req pointer itself? I don't see a
> need to modify the pointer.
> 
> Before this patch, *req is modified, but after this patch, it is not.
> If you pass in just "struct kshark_entry_request *req" then you don't
> even need to have the "list" variable, you could just use "req" because
> that would be a copy of the pointer.
> 
> -- Steve
> 
> 
> 
>>   	int req_count;
>>   
>>   	/*
>> @@ -638,12 +639,10 @@ kshark_get_collection_entry_front(struct kshark_entry_request **req,
>>   	 * Loop over the list of redefined requests and search until you find
>>   	 * the first matching entry.
>>   	 */
>> -	while (*req) {
>> -		entry = kshark_get_entry_front(*req, data, index);
>> +	for (list = *req; list; list = list->next) {
>> +		entry = kshark_get_entry_front(list, data, index);
>>   		if (entry)
>>   			break;
>> -
>> -		*req = (*req)->next;
>>   	}
>>   
>>   	return entry;
>> @@ -680,6 +679,7 @@ kshark_get_collection_entry_back(struct kshark_entry_request **req,
>>   				 ssize_t *index)
>>   {
>>   	const struct kshark_entry *entry = NULL;
>> +	struct kshark_entry_request *list;
>>   	int req_count;
>>   
>>   	/*
>> @@ -695,12 +695,10 @@ kshark_get_collection_entry_back(struct kshark_entry_request **req,
>>   	 * Loop over the list of redefined requests and search until you find
>>   	 * the first matching entry.
>>   	 */
>> -	while (*req) {
>> -		entry = kshark_get_entry_back(*req, data, index);
>> +	for (list = *req; list; list = list->next) {
>> +		entry = kshark_get_entry_back(list, data, index);
>>   		if (entry)
>>   			break;
>> -
>> -		*req = (*req)->next;
>>   	}
>>   
>>   	return entry;
>