Re: [PATCH v6 11/12] sched.h: extend task comm from 16 to 24

[Kees Cook]

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.)

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

-- 
Kees Cook