On Mon, Nov 1, 2021 at 10:07 PM Petr Mladek <pmladek@xxxxxxxx> wrote: > > 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@xxxxxxxxxxxxxx/ > I thought Steven[1] and Kees[2] have already clearly explained why we do it like that, so I didn't give any more words on it. [1]. https://lore.kernel.org/all/20211025170503.59830a43@xxxxxxxxxxxxxxxxxx/ [2]. https://lore.kernel.org/all/202110251406.56F87A3522@keescook/ > Several people suggested to use a more conservative approach. Yes, they are Al[3] and Alexei[4]. [3]. https://lore.kernel.org/lkml/YVkmaSUxbg%2FJtBHb@xxxxxxxxxxxxxxxxxxxxx/ [4]. https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@xxxxxxxxxxxxxx/ > 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. > That would add a new allocation in the fork() for the threads with a long name. I'm not sure if it is worth it. > 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. > If we decide to do it like that, I think we'd better add it in task_struct, then it will work for all tasks. > The advantage of the separate pointer: > > + would work for names longer than 32 > + will not open security holes in code > Yes, those are the advantages. And the disadvantage of it is: - new allocation in fork() -- Thanks Yafang
