linux-kernel - Re: [PATCH timers/core] posix-cpu-timers: Put the task_struct in posix_cpu_timers

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Message-ID: <87lfogfax5.fsf@x220.int.ebiederm.org>
Date:   Wed, 04 Mar 2020 08:32:22 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Qian Cai <cai@....pw>, oleg@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH timers/core] posix-cpu-timers: Put the task_struct in posix_cpu_timers_create

Thomas Gleixner <tglx@...utronix.de> writes:

> ebiederm@...ssion.com (Eric W. Biederman) writes:
>
>> Qian Cai <cai@....pw> writes:
>>> The recent commit removed put_task_struct() in posix_cpu_timer_del()
>>> results in many memory leaks like this,
>>>
>>> unreferenced object 0xc0000016d9b44480 (size 8192):
>>>   comm "timer_create01", pid 57749, jiffies 4295163733 (age 6159.670s)
>>>   hex dump (first 32 bytes):
>>>     02 00 00 00 00 00 00 00 10 00 00 00 00 00 00 00  ................
>>>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>>   backtrace:
>>>     [<0000000056aca129>] copy_process+0x26c/0x18e0
>>>     alloc_task_struct_node at kernel/fork.c:169
>>>     (inlined by) dup_task_struct at kernel/fork.c:877
>>>     (inlined by) copy_process at kernel/fork.c:1929
>>>     [<00000000bdbbf9f8>] _do_fork+0xac/0xb20
>>>     [<00000000dcb1c445>] __do_sys_clone+0x98/0xe0
>>>     __do_sys_clone at kernel/fork.c:2591
>>>     [<000000006c059205>] ppc_clone+0x8/0xc
>>>     ppc_clone at arch/powerpc/kernel/entry_64.S:479
>>>
>>
>> I forgot that get_task_for_clock called by posix_cpu_timer_create
>> returns a reference to a task_struct.  Put that reference
>> to avoid the leak.
>
> I took the liberty to fold this back into the affected commit and add a
> comment why this put_task_struct() is actually required.

Good enough.

We should be able to use rcu and remove the reference entirely.
But that is a change for another day.

Eric