Date:   Tue, 26 Oct 2021 10:22:54 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Valentin Schneider <valentin.schneider@....com>,
        Qiang Zhang <qiang.zhang@...driver.com>,
        robdclark <robdclark@...omium.org>,
        christian <christian@...uner.io>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        dennis.dalessandro@...nelisnetworks.com,
        mike.marciniszyn@...nelisnetworks.com, dledford@...hat.com,
        jgg@...pe.ca, linux-rdma@...r.kernel.org,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "linux-perf-use." <linux-perf-users@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel test robot <oliver.sang@...el.com>,
        kbuild test robot <lkp@...el.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH v6 11/12] sched.h: extend task comm from 16 to 24

On Tue, Oct 26, 2021 at 5:30 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Mon, Oct 25, 2021 at 08:33:14AM +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-
> >     xfs-reclaim/sdf
> >
> > 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-
> >     ...
> >
> > We should improve this problem fundamentally by extending comm size to
> > 24 bytes. task_struct is growing rather regularly by 8 bytes.
> >
> > After this change, the truncated kthreads listed above will be
> > displayed as:
> >
> >     rcu_tasks_kthread
> >     rcu_tasks_rude_kthread
> >     rcu_tasks_trace_kthread
> >     poll_mpt3sas0_statu
> >     ext4-rsv-conversion
> >     xfs-reclaim/sdf1
> >     xfs-blockgc/sdf1
> >     xfs-inodegc/sdf1
> >     audit_send_reply
> >     ecryptfs-kthread
> >     vfio-irqfd-cleanup
> >     jbd2/nvme0n1p2-8
> >
> > As we have converted all the unsafe copy of task comm to the safe one,
> > this change won't make any trouble to the kernel or the in-tree tools.
> > The safe one and unsafe one of comm copy as follows,
> >
> >   Unsafe                 Safe
> >   strlcpy                strscpy_pad
> >   strncpy                strscpy_pad
> >   bpf_probe_read_kernel  bpf_probe_read_kernel_str
> >                          bpf_core_read_str
> >                          bpf_get_current_comm
> >                          perf_event__prepare_comm
> >                          prctl(2)
> >
> > Regarding the possible risk it may take to the out-of-tree user tools, if
> > the user tools get the task comm through kernel API like prctl(2),
> > bpf_get_current_comm() and etc, the tools still work well after this
> > change. While If the user tools get the task comm through direct string
> > copy, it must make sure the copied string should be with a nul terminator.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
> > Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Al Viro <viro@...iv.linux.org.uk>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Petr Mladek <pmladek@...e.com>
> > ---
> >  include/linux/sched.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 124538db792c..490d12eabe44 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -279,7 +279,7 @@ struct task_group;
> >   * BPF programs.
> >   */
> >  enum {
> > -     TASK_COMM_LEN = 16,
> > +     TASK_COMM_LEN = 24,
> >  };
>
> I suspect this should be kept in sync with the tools/ copy of sched.h
> (i.e. we may need to keep the TASK_COMM_LEN_16 around in the kernel tree
> too.)
>

Sure. I will change it.

> >
> >  extern void scheduler_tick(void);
> > --
> > 2.17.1
> >
>
> --
> Kees Cook



-- 
Thanks
Yafang

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