On Mon 2021-11-01 22:34:30, Yafang Shao wrote: > 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: > > > 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/ Steven was against switching task->comm[16] into a dynamically allocated pointer. But he was not against storing longer names separately. > [2]. https://lore.kernel.org/all/202110251406.56F87A3522@keescook/ Honestly, I am a bit confused by Kees' answer. IMHO, he agreed that switching task->comm[16] into a pointer was not worth it. But I am not sure what he meant by "Agreed -- this is a small change for what is already an "uncommon" corner case." > > 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/ IMHO, Al suggested to store the long name separately and return it by proc_task_name() when available. > [4]. https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@xxxxxxxxxxxxxx/ Alexei used dentry->d_iname as an exaxmple. struct dentry uses d_iname[DNAME_INLINE_LEN] for short names. And dynamically allocated d_name for long names, see *__d_alloc() implementation. > > 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 allocation will be done only when needed. IMHO, the performance is important only for userspace processes. I am not aware of any kernel subsystem that would heavily create and destroy kthreads. > > 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. Is it really needed for userspace processes? For example, ps shows the information from /proc/*/cmdline instead. > > 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() It should not be a problem if we do it only when necessary and only for kthreads. Best Regards, Petr
