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

[Michal Hocko <mhocko@kernel.org>]

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
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...
-- 
Michal Hocko
SUSE Labs

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