linux-kernel - Re: [PATCH RFC 0/2] Dynamically allocate memory to store task's full name

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Message-ID: <24766119-3aa2-fe88-878a-90a2794cca8f@igalia.com>
Date: Tue, 18 Mar 2025 23:36:40 +0530
From: Bhupesh Sharma <bhsharma@...lia.com>
To: Kees Cook <kees@...nel.org>
Cc: Andres Rodriguez <andresx7@...il.com>, Bhupesh <bhupesh@...lia.com>,
 akpm@...ux-foundation.org, kernel-dev@...lia.com,
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Subject: Re: [PATCH RFC 0/2] Dynamically allocate memory to store task's full
 name

Hi Kees,

On 3/18/25 9:21 PM, Kees Cook wrote:
> On Tue, Mar 18, 2025 at 04:49:28PM +0530, Bhupesh Sharma wrote:
>> On 3/15/25 1:13 PM, Andres Rodriguez wrote:
>>> On 3/14/25 14:25, Kees Cook wrote:
>>>> On Fri, Mar 14, 2025 at 10:57:13AM +0530, Bhupesh wrote:
>>>>> While working with user-space debugging tools which work especially
>>>>> on linux gaming platforms, I found that the task name is truncated due
>>>>> to the limitation of TASK_COMM_LEN.
>>>>>
>>>>> For example, currently running 'ps', the task->comm value of a long
>>>>> task name is truncated due to the limitation of TASK_COMM_LEN.
>>>>>       create_very_lon
>>>>>
>>>>> This leads to the names passed from userland via pthread_setname_np()
>>>>> being truncated.
>>>> So there have been long discussions about "comm", and it mainly boils
>>>> down to "leave it alone". For the /proc-scraping tools like "ps" and
>>>> "top", they check both "comm" and "cmdline", depending on mode. The more
>>>> useful (and already untruncated) stuff is in "cmdline", so I suspect it
>>>> may make more sense to have pthread_setname_np() interact with that
>>>> instead. Also TASK_COMM_LEN is basically considered userspace ABI at
>>>> this point and we can't sanely change its length without breaking the
>>>> world.
>>>>
>>> Completely agree that comm is best left untouched. TASK_COMM_LEN is
>>> embedded into the kernel and the pthread ABI changes here should be
>>> avoided.
>>>
>> So, basically my approach _does not_ touch TASK_COMM_LEN at all. The normal
>> 'TASK_COMM_LEN' 16byte design remains untouched.
>> Which means that all the legacy / existing ABi which uses 'task->comm' and
>> hence are designed / written to handle 'TASK_COMM_LEN' 16-byte name,
>> continue to work as before using '/proc/$pid/task/$tid/comm'.
>>
>> This change-set only adds a _parallel_ dynamically allocated
>> 'task->full_name' which can be used by interested users via
>> '/proc/$pid/task/$tid/full_name'.
> I don't want to add this to all processes at exec time as the existing
> solution works for those processes: read /proc/$pid/cmdline.
>
> That said, adding another pointer to task_struct isn't to bad I guess,
> and it could be updated by later calls. Maybe by default it just points
> to "comm".
Sure.

>
>> I am fine with adding either '/proc/$pid/task/$tid/full_name' or
>> '/proc/$pid/task/$tid/debug_name' (actually both of these achieve the same).
>> The new / modified users (especially the debug applications you listed
>> above) can switch easily to using '/proc/$pid/task/$tid/full_name' instead
>> of ''/proc/$pid/task/$tid/comm'
>>
>> AFAIK we already achieved for the kthreads using d6986ce24fc00 ("kthread:
>> dynamically allocate memory to store kthread's full name"), which adds
>> 'full_name' in parallel to 'comm' for kthread names.
> If we do this for task_struct, we should remove "full_name" from kthread
> and generalize it for all processes.
>
Got it. Ok, let me rework the series so that we have a unified 
'full_name' inside 'task_struct' and have kthread use it as well.

I will send a v2 accordingly.

Thanks,
Bhupesh