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Date:   Thu, 7 Oct 2021 07:56:57 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Yafang Shao <laoar.shao@gmail.com>
Cc:     pmladek@suse.com, viro@zeniv.linux.org.uk,
        akpm@linux-foundation.org, peterz@infradead.org,
        valentin.schneider@arm.com, mathieu.desnoyers@efficios.com,
        qiang.zhang@windriver.com, robdclark@chromium.org,
        christian@brauner.io, dietmar.eggemann@arm.com, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/4] sched.h: extend task comm from 16 to 24 for
 CONFIG_BASE_FULL
Message-ID: <202110070755.60DFB87711@keescook>
References: <20211007120752.5195-1-laoar.shao@gmail.com>
 <20211007120752.5195-4-laoar.shao@gmail.com>
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On Thu, Oct 07, 2021 at 12:07:51PM +0000, Yafang Shao wrote:
> When I was implementing a new per-cpu kthread cfs_migration, I found the
> comm of it "cfs_migration/%u" is truncated due to the limitation of
> TASK_COMM_LEN. For example, the comm of the percpu thread on CPU10~19 are
> all with the same name "cfs_migration/1", which will confuse the user. This
> issue is not critical, because we can get the corresponding CPU from the
> task's Cpus_allowed. But for kthreads correspoinding to other hardware
> devices, it is not easy to get the detailed device info from task comm,
> for example,
> 
>     jbd2/nvme0n1p2-
>     nvidia-modeset/
> 
> We can also shorten the name to work around this problem, but I find
> there are so many truncated kthreads:
> 
>     rcu_tasks_kthre
>     rcu_tasks_rude_
>     rcu_tasks_trace
>     poll_mpt3sas0_s
>     ext4-rsv-conver
>     xfs-reclaim/sd{a, b, c, ...}
>     xfs-blockgc/sd{a, b, c, ...}
>     xfs-inodegc/sd{a, b, c, ...}
>     audit_send_repl
>     ecryptfs-kthrea
>     vfio-irqfd-clea
>     jbd2/nvme0n1p2-
>     ...
> 
> Besides the in-tree kthreads listed above, the out-of-tree kthreads may
> also be truncated:
> 
>     rtase_work_queu
>     nvidia-modeset/
>     UVM global queu
>     UVM deferred re
>     ...
> 
> We should improve this problem fundamentally.
> 
> This patch extends the size of task comm to 24 bytes, which is the
> same length with workqueue's, for the CONFIG_BASE_FULL case. And for the
> CONFIG_BASE_SMALL case, the size of task comm is still kept as 16 bytes.
> 
> Suggested-by: Petr Mladek <pmladek@suse.com>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>

This, as expected, adds 8 bytes to task_struct, which is reasonable. I
don't see any easy places to consolidate other members to make this a
"free" change, but I did just remove 64 bytes from task_struct[1], so
this is okay. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

[1] https://lore.kernel.org/lkml/20210924025450.4138503-1-keescook@chromium.org/

-- 
Kees Cook