Re: Possible memory leaks in proc_sysctl.c

[Eric W. Biederman]

Catalin Marinas <catalin.marinas@xxxxxxx> writes:

> On Wed, Apr 18, 2012 at 03:52:58PM +0100, Eric W. Biederman wrote:
>> Catalin Marinas <catalin.marinas@xxxxxxx> writes:
>> > On Wed, Apr 18, 2012 at 02:22:09PM +0100, Eric W. Biederman wrote:
>> >> Catalin Marinas <catalin.marinas@xxxxxxx> 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).

I will take a look when I merge the patch.

Would something like kmemleak_ignore() be better?  What I want is
kmemleak_this_is_a_deliberate_leak_so_shut_up(), but the API doesn't
seem to exactly include that function.  I'm not certain what the proper
name is as I haven't worked much with kmemleak.

Eric

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