On Tue, Nov 2, 2021 at 12:02 AM Petr Mladek <pmladek@xxxxxxxx> wrote: > > 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. > Thanks for the summary. So with Stenven's new reply[1], the opinion in common is storing long names into a separate place. And no one is against it now. [1]. https://lore.kernel.org/lkml/20211101120636.3cfc5afa@xxxxxxxxxxxxxxxxxx/ > > > 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. > XFS may create many kthreads with longer names, especially if there're many partitions in the disk. For example, xfs-reclaim/sd{a, b, c, ...} xfs-blockgc/sd{a, b, c, ...} xfs-inodegc/sd{a, b, c, ...} They are supposed to be created at boot time, and shouldn't be heavily created and destroyed. > > > > 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. > Right. The userspace processes can be obtained from /proc/*/cmdline. > > > > 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. > So if no one against, I will do it in two steps, 1. Send the task comm cleanups in a separate patchset named "task comm cleanups" This patchset includes patch #1, #2, #4, #5, #6, #7 and #9. Cleaning them up can make it less error prone, and it will be helpful if we want to extend task comm in the future :) 2. Keep the current comm[16] as-is and introduce a separate pointer to store kthread's long name Now we only care about kthread, so we can put the pointer into a kthread specific struct. For example in the struct kthread, or in kthread->data (which may conflict with workqueue). And then dynamically allocate a longer name if it is truncated, for example, __kthread_create_on_node len = vsnprintf(name, sizeof(name), namefmt, args); if (len >= TASK_COMM_LEN) { /* create a longer name */ } And then we modify proc_task_name(), so the user can get kthread's longer name via /proc/[pid]/comm. And then free the allocated memory when the kthread is destroyed. -- Thanks Yafang
