Re: [PATCH v7 00/11] extend task comm from 16 to 24

[Petr Mladek <pmladek@suse.com>]

On Mon 2021-11-01 06:04:08, Yafang Shao wrote:
> There're many truncated kthreads in the kernel, which may make trouble
> for the user, for example, the user can't get detailed device
> information from the task comm.
> 
> This patchset tries to improve this problem fundamentally by extending
> the task comm size from 16 to 24, which is a very simple way. 
> 
> In order to do that, we have to do some cleanups first.
> 
> 1. Make the copy of task comm always safe no matter what the task
>    comm size is. For example,
> 
>       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)
> 
>    After this step, the comm size change won't make any trouble to the
>    kernel or the in-tree tools for example perf, BPF programs.
> 
> 2. Cleanup some old hard-coded 16
>    Actually we don't need to convert all of them to TASK_COMM_LEN or
>    TASK_COMM_LEN_16, what we really care about is if the convert can
>    make the code more reasonable or easier to understand. For
>    example, some in-tree tools read the comm from sched:sched_switch
>    tracepoint, as it is derived from the kernel, we'd better make them
>    consistent with the kernel.

The above changes make sense even if we do not extend comm[] array in
task_struct.


> 3. Extend the task comm size from 16 to 24
>    task_struct is growing rather regularly by 8 bytes. This size change
>    should be acceptable. We used to think about extending the size for
>    CONFIG_BASE_FULL only, but that would be a burden for maintenance
>    and introduce code complexity.
> 
> 4. Print a warning if the kthread comm is still truncated.
> 
> 5. What will happen to the out-of-tree tools after this change?
>    If the tool get task comm through kernel API, for example prctl(2),
>    bpf_get_current_comm() and etc, then it doesn't matter how large the
>    user buffer is, because it will always get a string with a nul
>    terminator. While if it gets the task comm through direct string copy,
>    the user tool must make sure the copied string has a nul terminator
>    itself. As TASK_COMM_LEN is not exposed to userspace, there's no
>    reason that it must require a fixed-size task comm.

The amount of code that has to be updated is really high. I am pretty
sure that there are more potential buffer overflows left.

You did not commented on the concerns in the thread
https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@mail.gmail.com/

Several people suggested to use a more conservative approach. I mean
to keep comm[16] as is and add a new pointer to the full name. The buffer
for the long name might be dynamically allocated only when needed.

The pointer might be either in task_struct or struct kthread. It might
be used the same way as the full name stored by workqueue kthreads.

The advantage of the separate pointer:

   + would work for names longer than 32
   + will not open security holes in code

Best Regards,
Petr