Re: [PATCH v2] module: check if memory leak by module.

[Joel Fernandes <joelaf@google.com>]

Hi Michal,

On Wed, Mar 29, 2017 at 3:43 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Wed 29-03-17 09:23:32, Vaneet Narang wrote:
>> Hi,
>>
>> >> Hmm, how can you track _all_ vmalloc allocations done on behalf of the
>> >> module? It is quite some time since I've checked kernel/module.c but
>> >> from my vague understading your check is basically only about statically
>> >> vmalloced areas by module loader. Is that correct? If yes then is this
>> >> actually useful? Were there any bugs in the loader code recently? What
>> >> led you to prepare this patch? All this should be part of the changelog!
>>
>> First of all there is no issue in kernel/module.c. This patch add functionality
>> to detect scenario where some kernel module does some memory allocation but gets
>> unloaded without doing vfree. For example
>> static int kernel_init(void)
>> {
>>         char * ptr = vmalloc(400 * 1024);
>>         return 0;
>> }
>
> How can you track that allocation back to the module? Does this patch
> actually works at all? Also why would be vmalloc more important than
> kmalloc allocations?

Doesn't the patch use caller's (in this case, the module is the
caller) text address for tracking this? vma->vm->caller should track
the caller doing the allocation?

>From the code:
vmalloc -> __vmalloc_node_flags

In __vmalloc_node_flags:
        return __vmalloc_node(size, 1, flags, PAGE_KERNEL,
                                        node, __builtin_return_address(0));

Since __vmalloc_node_flags is marked as inline, I believe the
__builtin_return_address(0) will return the return address of the
original vmalloc() call which is in the module calling vmalloc.

Regards,
Joel

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