Re: Possible memory leaks in proc_sysctl.c

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Apr 18, 2012 at 03:52:58PM +0100, Eric W. Biederman wrote:
> Catalin Marinas <catalin.marinas@arm.com> writes:
> > On Wed, Apr 18, 2012 at 02:22:09PM +0100, Eric W. Biederman wrote:
> >> Catalin Marinas <catalin.marinas@arm.com> writes:
> >> > Following your commit f728019bb (sysctl: register only tables of sysctl
> >> > files), I get several kmemleak reports. They all seem to be header
> >> > allocations with kzalloc() in __register_sysctl_table() and
> >> > __register_sysctl_paths(). The patch isn't simple to quickly figure out
> >> > what may be wrong.
> >> 
> >> Due to a change in the data structure places where we register the
> >> sysctl permanently and ignore the result from the register_sysctl_...
> >> family of functions now report this leak.
> >
> > But is the header (or subheader, basically any pointer inside the
> > kmalloc'ed object) never referenced from anywhere? I'm just trying to
> > understand why kmemleak reports it as it seems that the header object is
> > inserted in a ctl_dir.
> 
> It is never reference from anywhere because we never free the structure.
> The job of the header is to be the structure that tells us how to free
> things.
> 
> I see a couple of things going on.
> - For compatibility the header that is returned is a dummy that just
>   points to the real headers.
> 
> - Even without the compatibility we can get the same symptom if
>   we register an empty directory.
> 
> So simply saying kmemleak shut up this is deliberate in these few cases
> where we don't intend to unregister the structure and have a deliberate
> leak seems the clean and maintainable way to go.

OK, I got it now, sounds fair. But please add a comment to the
kmemleak_not_leak() annotations so that others know it's a deliberate
leak (rather than a false positive).

Also the patch should include the linux/kmemleak.h file for the
kmemleak_not_leak() prototype as header changes in the future may cause
problems (I think the one you posted did not include it).

Thanks.

-- 
Catalin