On Tue, Oct 26, 2021 at 5:30 AM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
> > Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Petr Mladek <pmladek@xxxxxxxx>
> > ---
> > 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
