Re: [PATCH v12] mm: slub: move sysfs slab alloc/free interfaces to debugfs

[Faiyaz Mohammed]

On 6/16/2021 4:35 PM, Vlastimil Babka wrote:
> On 6/15/21 5:58 PM, Qian Cai wrote:
>>
>>
>> On 6/11/2021 3:03 PM, Faiyaz Mohammed wrote:
>>> alloc_calls and free_calls implementation in sysfs have two issues,
>>> one is PAGE_SIZE limitation of sysfs and other is it does not adhere
>>> to "one value per file" rule.
>>>
>>> To overcome this issues, move the alloc_calls and free_calls
>>> implementation to debugfs.
>>>
>>> Debugfs cache will be created if SLAB_STORE_USER flag is set.
>>>
>>> Rename the alloc_calls/free_calls to alloc_traces/free_traces,
>>> to be inline with what it does.
>>>
>>> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>>> Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>> Signed-off-by: Faiyaz Mohammed <faiyazm@xxxxxxxxxxxxxx>
>>
>> Reverting this commit on today's linux-next fixed all leaks (hundreds) reported by kmemleak like below,
>>
>> unreferenced object 0xffff00091ae1b540 (size 64):
>>   comm "lsbug", pid 1607, jiffies 4294958291 (age 1476.340s)
>>   hex dump (first 32 bytes):
>>     02 00 00 00 00 00 00 00 6b 6b 6b 6b 6b 6b 6b 6b  ........kkkkkkkk
>>     6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
>>   backtrace:
>>     [<ffff8000106b06b8>] slab_post_alloc_hook+0xa0/0x418
>>     [<ffff8000106b5c7c>] kmem_cache_alloc_trace+0x1e4/0x378
>>     [<ffff8000106b5e40>] slab_debugfs_start+0x30/0x50
>>     slab_debugfs_start at /usr/src/linux-next/mm/slub.c:5831
>>     [<ffff8000107b3dbc>] seq_read_iter+0x214/0xd50
>>     [<ffff8000107b4b84>] seq_read+0x28c/0x418
>>     [<ffff8000109560b4>] full_proxy_read+0xdc/0x148
>>     [<ffff800010738f24>] vfs_read+0x104/0x340
>>     [<ffff800010739ee0>] ksys_read+0xf8/0x1e0
>>     [<ffff80001073a03c>] __arm64_sys_read+0x74/0xa8
>>     [<ffff8000100358d4>] invoke_syscall.constprop.0+0xdc/0x1d8
>>     [<ffff800010035ab4>] do_el0_svc+0xe4/0x298
>>     [<ffff800011138528>] el0_svc+0x20/0x30
>>     [<ffff800011138b08>] el0t_64_sync_handler+0xb0/0xb8
>>     [<ffff80001001259c>] el0t_64_sync+0x178/0x17c
>>
> 
> I think the problem is here:
> 
>>> +static void slab_debugfs_stop(struct seq_file *seq, void *v)
>>> +{
>>> +	kfree(v);
>>> +}
>>> +
>>> +static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
>>> +{
>>> +	loff_t *spos = v;
>>> +	struct loc_track *t = seq->private;
>>> +
>>> +	if (*ppos < t->count) {
>>> +		*ppos = ++*spos;
>>> +		return spos;
>>> +	}
>>> +	*ppos = ++*spos;
>>> +	return NULL;
>>> +}
> 
> If we return NULL, then NULL is passed to slab_debugfs_stop and thus we don't
> kfree ppos. kfree(NULL) is silently ignored.
> 
I think yes, if NULL passed to kfree, it simply do return.
> I think as we have private struct loc_track, we can add a pos field there and
> avoid the kmaloc/kfree altogether.
> 
Hmm, yes we can add pos field "or" we can use argument "v" mean we can
update v with pos in ->next() and use in ->show() to avoid the leak
(kmalloc/kfree).


Thanks and regards,
Mohammed Faiyaz