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SubjectRe: [PATCH v2] module: check if memory leak by module.
On Fri, Mar 31, 2017 at 1:00 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Thu 30-03-17 23:49:52, Joel Fernandes wrote:
>> 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?
>
> Not really. First of all it will be vmalloc() to be tracked in the above
> the example because vmalloc is not inlined. And secondly even if the

vmalloc is not inlined, but __built_in_address(0) will return the
*return address* of vmalloc:

From https://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Return-Address.html :
"The level argument is number of frames to scan up the call stack. A
value of 0 yields the return address of the current function"

> caller of the vmalloc was tracked then it would be hopelessly
> insufficient because you would get coverage of the _direct_ module usage
> of vmalloc rather than anything that the module triggered and that is
> outside of the module. Which means any library function etc...

Yes true, but I think the check is for direct allocations, done by the
module, not indirect ones... it may not be a catch-all issues type of
deal but is still IMO a good check since we already have
va->vm->caller available.

Thanks,
Joel

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